Bulletin


STATE OF ILLINOIS

DEPARTMENT OF REGISTRATION AND EDUCATION

DIVISION OF THE

NATURAL HISTORY SURVEY

Stephen A. Forbes, Chief

Vol. XVIII. BULLETIN Article III.

Epidemic Diseases of Fruit Trees

in Illinois 1922-1928

BY

L. R. TEHON

AND

GILBERT L. STOUT

PRINTED BY AUTHORITY OF THE STATE OF ILLINOIS

URBANA, ILLINOIS

1930



STATE OF ILLINOIS

DEPARTMENT OF REGISTRATION AND EDUCATION

M. F. Walsh, Director

BOARD OF NATURAL RESOURCES AND CONSERVATION

M. F. Walsh, Chairman

WiLUAM Trelease, BioloQy

Henry C. Cowles, Forestry

Edson S. Bastin, Geology

William A. Notes, Chemistry

John W. Alvord, Engineering

Charles M. Thompson, Representing

the President of the University of

Illinois

STATE NATURAL HISTORY SURVEY DIVISION

Stephen A. Forbes, Chief

H. C. Oesterling. Editor

Journal Printing Compan y,

Springfield, Illinois.

193

(33069—2M)



^tPpVn Alfred ^atbsB

Mav 29, ia44-iiarrl| 13. 1930

This article completes the Eighteenth Volume of

the Bulletin of the Illinois State Natural History

Survey published under the direction of Stephen Alfred

Forbes. Volumes I to XII, inclusive, were issued under

the title of the Bulletin of the Illinois State Laboratory

of Natural History and Volume XIII to date as the

Bulletin of the Illinois State Natural History Survey.

Stephen Alfred Forbes. was the founder of the Illi-

nois State Laboratory of Natural History and served as

its Director from 1877 to 1917. He was appointed State

Entomologist in 1882 and continued in this capacity

until 1917, when he became Chief of the Illinois State

Natural History Survey, a division of the State De-

partment of Eegistration and Education, under the

Civil Administrative Code merging the State Labora-

tory and the Ofiice of the State Entomologist.

In addition to the Bulletin series there have been

issued under his authorship or direction three Final

Reports on the Natural History Survey of Illinois, two

volumes dealing with birds and one with fishes ; the last

eighteen of the twenty-nine Reports of the State Ento-

mologist on the Noxious and Beneficial Insects of the

State of Illinois; and numerous miscellaneous papers

dealing with various phases of natural science.



CONTENTS

Page

Introduction - 415

General methods 416

Analysis of data 420

Limitations of data 426

Fruit tree distribution 427

Apple diseases 429

Scab 430

Blotch 439

Black-rot 446

Fire-blight 450

Rust 453

Summary of apple disease data 456

Pear diseases 469

Pear-blight 469

Peach diseases 472

Brown-rot 472

Bacterial shot-hole 475

Leaf-curl 477

Scab 479

Summary of peach disease data 481

Cherry and plum diseases 487

Data on cherry and plum diseases 487

Summary of data 492

Recapitulation 500



Volume XVIII Article III

EPIDEMIC DISEASES OE ERUIT
TREES IN ILLINOIS

1922-1928

Fj. R. Tehox and Gilbert T^. Stout

In a wide geographical sense the diseases of fruit-trees, like those of

other crops, are distributed in strict accordance with climates; and in

each climatic region the particular diseases which find conditions suit-

able for their existence exhibit notable yearly variations both in pre-

valence and in severity. It is well understood that these annual fluctu-

ations occur simultaneously with, or in some cases subsequent to, certain

kinds of variations in weather; and it is generally known also that the

degree of success attending the application of sprays to control disease

depends to a very high degree upon the prevailing weather conditions,

partly because of the great influence they have upon the growth and

development of the parasitic fungi which cause diseases and partly be-

cause they determine the seasonal progress in flowering, leafing, and

fruiting of the trees upon which sprays are applied.

The plant pathologist who has dealt with fruit diseases in a limited

region for a number of years is able to draw upon his experience to the

extent that he can. upon the arrival of very unusual weather, foresee the

kind of variation in disease that will occur. He may then recommend
the modifications of spray-schedules and other control operations along

lines likely to reduce the menace of a disease attack which would, under

the usual program of control, be certain to result in excessive injury and

loss to the season's crop. But it is a fact that forecasts of this kind,

which now can be made only from long personal experience, are

relatively rare and can be only relatively accurate.

The natural laws which govern both the major and minor varia-

tions of disease epidemics are as yet very poorly or not at all understood.

No extensive studies have been made of diseases as they occur on fruit

trees, except with such specific intentions as determining the effects of

sprays or ascertaining the life histories of fungi. Knowledge of plant

diseases and of their causes, apart from ancient superstitions and medi-

eval theories, has been acquired so recently that there is no body of

historical information available upon which either the investigator or

the fruit grower can draw for dependable guidance in forecasting the

extent or destructiveness of an impending epidemic. The efficient use of

fungicides is limited by the same circumstances, the application of dusts

and sprays being governed mainly by the phenomena exhibited by the

fruit-tree which is to be protected rather than by those exhibited by the

parasites which are to be destroyed.

[415]



416 Ilunois Natural History Sirvey Bulletin

Nevertheless, the pressure exerted by the rapidly advancing de-

mands of the modern agriculturalists who are engaged in the production

of the fruit crops, as well as the industrialists who furnish the fungi-

cides for the control processes, for more accurate directions and predic-

tions, makes it necessary to have available a fund of concrete, accurate

data from which to draw conclusions which, in older sciences, might be

deduced from historically recorded experiences.

The practical results to be derived from the accumulation of a body

of exact information on plant disease epidemics are at least two-fold.

First, it is required that the manifestations of each disease shall be

analyzed in such a way as to permit of measurement, with the natural

consequence that there will be developed for the purpose accurate and

uniform scales applicable to disease as a whole and not limited to those

phases resulting in economic loss; and the present tendency to confuse

crop loss with disease intensity will disappear. Second, the exactness of

the accumulated data will make possible much finer comparisons than

can now be made; and the effectiveness of fungicides will be increased,

thereby, because of the certainty of their being applied more and more
in accordance with the life-phenomena of the fungi they are intended to

combat.

Since midsummer of 1921, the Natural History Survey has been

engaged in studying the annual prevalence and intensity of diseases at-

tacking the crops of Illinois. The purpose of the study has been to

observe and record yearly the crop disease conditions, making the obser-

vations with such exactness and at the same time so extensively that the

data and records obtained could be used subsequently to determine with

accuracy the real relations existing between variations in weather and
variations in plant disease epidemics. The diseases of cereals, of fruits,

and of forage, vegetable, and other crops have been given time and atten-

tion each season, and two extensive reports—one on the distribution of

diseases, the other on survey methods for diseases of cereal crops—have

been prepared and printed.

It is our purpose, now, to present in some detail an account of the

survey methods that have been developed and used in dealing with fruit-

tree diseases and to state the results that have been obtained through
their use. These methods are similar in certain respects to those de-

scribed for cereal diseases in a former publication. Consequently this

discussion has been made as brief as possible. For simpler and more
detailed explanations of many of the processes used, the reader may
refer to the more comprehensive discussion given in Article 1, Volume 1?

of the Survey's Bulletin.

General Methods

The aim of the fruit disease survey, as it has been conducted thus

far, has been to "sample" the disease conditions prevailing in each of a

considerable number of scattered localities each year and then, suppos-

ing these samples to be typical of the conditions prevailing in the

regions from which they were taken, to subject them to arithmetical

analysis and summarization in order to obtain numerical expressions of

the general and special characteristics of each year's disease epidemics.



Diseases of Fkuit Trees, 1922-1928 417

In obtaining the samples, various numbers of observers have

traveled the country roads of the State each year since 1921 and have

examined small and large plantings of fruit trees in accordance with

prepared instructions. In finding and selecting orchards for examina-

tion, the Farm Advisers of many County Farm Bureaus have given

cordial assistance.

During the season of 1922, the four observers then employed were

required to collect a specimen of each disease found in each orchard and
to make certain specific notes regarding each disease. The specimens

were intended primarily to serve as a means of confirming the observer's

diagnoses, but they were expected also to serve both as substantiating

records and as reference material from which additional facts could be

obtained by subsequent examination.

For the convenience of the observers, but especially to assure

reasonable uniformity in the making of the required notes, small mimeo-
graphed blanks were provided, one of which was to be filled out as com-
pletely as possible for each disease found in each orchard examined.

These blanks, with their confirmatory specimens, have been held as a

permanent record of the season's work. The following example shows

the form of the blank, in Eoman type, and, in italics, the notes secured

in an unusually well completed instance.

Collector's No. 1220 Accession No. 15,635

Crop: Apple. Variety: Ben Davis. Developmental state:

Disease: Scab. Cause: Venturia inaequalis. Type of injury: Fruit d

leaves.

Town: Frederich. County: Schuyler. Date: July 15, 1922.

Date first observed: Late May. Source of infection:

Control measures: Lime-sulj)hur. Calyx spray only.

Size of field: 200 trees. Infection: in field, 100%; on plant: 60% of

fruit.*

Reduction of photosynthetic surface: Leaf specimen typical.

Past prevalence: Present every year and usually severe.

Usual damage: Considerable. Estimated damage: 5% of fruit ruined.

Association with other diseases: Blotch, blight, frog-eye.

Weather relations: Dry and hot. Additional notes: *Leaves shore

average of 7.5% of area occupied by scab lesions.

Although the topics of importance in a fruit disease survey are in-

cluded in this blank, the small spaces do not provide sufficient room for

definite detail in the notes and a larger blank had to be devised. After

several trials, the form shown on the opposite page was developed. These

note sheets, 81/2 x 11 inches, are printed, and are gummed into pads of

a hundred. They are carried by the observer, and one sheet is required

to be filled out for each disease found in each orchard. As may be seen

from the example, the information they give is essentially the same as



418 Illinois Nati kal Histoky Sikvey Bulletin

was required bv the smaller blank first used; but the larger form has

tlie two advantages of allowing fuller notes on most points and of per-

mitting complete statements of counts made by the observer.

1.928

PLANT DISEASE RECORD BLANK

Crop Apple. Disease Scab (Endostigme inaequalis)

County Cumberland. Locality Ncoga.

Variety of crop Staynian's Winesap. Size of field 1,000 trees.

State of development Trees 26 years old. Season's fruit 2" in diameter.

„ ^ , I Applied X—spray. i Kind used Oii! {dormant) and
Control measures )

^v r .j
(

\ Not applied \ lime-sulphur.

When used: Dormant spray; cluster-bud spray omitted; Ji applications of

lime-sulphtir. Thorough applications with considerable spray burn on

the leaves.

INFECTION: Percentage of trees diseased 100. Leaf infection 3.28% of area.

Scale classes:

Data: Fruits examined 673 ^^— 5—10—1.5—20—30—',5— 60.

Fruits infected 146
^^«** frequency:

125+ 33+ 7+ 4+ 3+ 2+ 1+ 2=177.

Per cent, infected 21.7 ^^^^ ^^^^^^^

.

0+165+70+60+60+60+45+ 120=580.

Type of injury Abundant but light infection on fruit. Light infection on

29% of the leaves.

Estimated damage {Not determinable at date of this report.)

Date first observed May 2Dth. Source of infection Probably old leaves.

Past history Orchard clean before 1927. Increased infection thi^ year may
be due in part to omission of cluster-bud spray.

Association with other diseases Black-rot, blotch, fire-blight, and, rust also

present in this orchard. See particular reports for each.

Additional notes (weather, phenology, etc.) Spring was cool, the month of

June exceptionally wet, and July hot.

Date of observation August 1, 1928. Specimen No. S-379; Ace. No. 21545.

Observer G. L. Stout.

The blank is designed to record information regarding one disease

only. This is seen to be practically a necessity when it is considered that

a very considerable number of orchards may furnish data on each of

several diseases in a given season and that, for convenience in study, the

numerous records pertaining to one disease must be kept together. In
addition to the geographical situation of the orchard, the pertinent facts



DisKASEs OF Fruit Trees, 1922-1928 419

given on each blank include a statement of the fruit variety, the number
of trees in the plantation, their age, and. the stage in seasonal develop-
ment of the fruit or other structures at the time of examination, as well

as information regarding the application of control measures, all of

which may be essential in interpreting the data given in the other spaces.

The tree is taken as the unit of survey; and it is our custom, in

examining large plantations, to prepare separate reports for each horti-

cultural variety, thereby securing accurate information relating to the
differences in predisposition to disease attack exhibited by the more com-
mon varieties of each kind of fruit grown in the State.

The data taken on the season's manifestation of disease is concerned
particularly with two distinct phases : first, the prevalence of the disease,

which we express as the percentage of trees suffering from attack in

diseased orchards and second, the intensity of the attack, which we de-

termine as (a) the percentage of fruits attacked; (b) the amount of

woody growth diseased or destro3^ed; and (c) the destructive effect on
leaves, expressed as destroyed or occupied leaf area. Some diseases do
not, of course, exhil)it all these phases, and from them the data which
can be acquired are limited to the types of disease they produce.

The observer, in all the processes of gathering data, must consider

only the disease. The commercial effects of disease, seen as reductions

in the grade of fruit or in lowered yields, are secondary things, well

recognized by the fruit grower but quite apt to be misleading in a study
of the' epidemiology of disease.

In gathering these data, the observer is re(]uired first to determine

the number of trees that have become diseased in the plantation in

which he is working, and this determination he expresses as a percentage

on the record sheet. When the disease is a prevalent one and the variety

susceptible, this percentage, or prevalence "index," may be ascertained

by a brief examination ; but with rarer diseases, exceptionally good con-

trol, or resistant varieties, more care is recpiired.

To determine the prevalence of fruit infection on the diseased trees,

it has been found necessary to make a definite count of the observable

fruit on each tree, as the tree is examined, and to distinguish between

the fruits that are infected and those that are not- infected, as the count

is in progress. On the sample record, it is indicated that 673 applet

were seen, of v>hich 1-fG were seen to be infected by, and 527 free of,

scab ; and from these data it is considered that infection had occurred

on 21.7 per cent of the fruit.

As the observer conducts his examination, he also gathers a random
sample of leaves, which, to conclude his examination, he compares with

a set of standard, measured diagrams, or scales, with which he is pro-

vided, and thereby secures an estimate of the amount of disease present

on the leaves. The manner in which one such a diagram is used is

described in the discussion of apple scab. The only explanation that

needs to be made here is that, in obtaining the data shown in the sample

record, 177 leaves were scaled, 125 of them being either free of infection

or only very slightly diseased, while various numbers of the remaining 52

were diseased to the extent indicated by the scale classes under which
the numbers are placed. The average amount of leaf area occupied by



420 Illinois Natural History Survey Bulletis:

scab lesions in this sample was 3.28 per cent. This figure we regard a^

an "index'' expressing the intensity of the scab attack on leaves in the

orchard.

When the record blanks have been completed, the collector sends

them, often with substantiating specimens, to headquarters, where, as

they accumulate, they are assorted by crops and by diseases, those for

each disease of each crop being assigned a special folder for each year.

The substantiating specimens are subjected to careful identification,

often by microscopic examination and occasionally by culturing on
artificial media, and then, designated by newly assigned accession num-
bers, they are preserved in the Natural History Survey's mycological

collection for permanent reference.

Analysis of Data

At the close of each season's work, it is necessary to analyze and
summarize the accumulated records, and in order that the results ob-

tained from data gathered in one season may be comparable with those

of other seasons, this must be done in a uniform manner. In general,

the process is the same as that outlined for cereal diseases in a pre-

vious article; but there are slight differences which require some ex-

planation.

The current season's record sheets for each disease, already assorted,

are now arranged alphabetically by counties and numbered in sequence,

and the data which they furnish are assembled in a series of tabulations,

only one of which has been used in preparing data for this discussion.

It is made up in the manner illustrated by the very much abbreviated
example shown in Table I.

One line across this table carries all the data on prevalence and
intensity given by one record sheet and shows, besides, a number of

simple computations made from them. The number of the record sheet

appears in column 1 and serves as a reference to the place in the folder

from which the data are taken. Column 2 shows the name of the county
in which the examined orchard was located. As the record sheets have
been assorted by counties, all the records for each county represented
in the season's accuinulation are grouped on consecutive lines, and
column 3 then shows not only the number of trees represented by the

individual field-record sheets but also the total number of trees repre-

sented by the examinations conducted in each county and in the state.

Column 4 shows the percentage of diseased trees in every infected

orchard examined, as given by the individual records, and also gives

weighted averages of these percentages for each county and for all

records taken during the season. The weighted averages are obtained
with the aid of the figures which appear in column 5, which we have
termed "prevalence products." They are secured by multiplying each
entry in column 3 by its corresponding entry in column 4. The pur-
pose of this is to give a proper relative M^eight, in terms of the particular

number of trees to which each percentage applies, to each of the pre-

valence percentages, so that when all the data obtained in one county,
or in the State, are brought to an average, the observations made in small
and possibly uncared-for orchards will not unduly influence those made



Diseases of Fruit Trees, 1922-1928 421

in plantations of 500 to 1000 or more trees. As an example, suppose

the short list of observed prevalence percentages given for Calhoun
County in the sample table be added and divided by 4. The sum is

300.4 and the quotient is 75.1 per cent. But by weighting the preva-

lence percentages observed in each orchard in terms of the number of

trees in each, the proper relative importance is given to each of the

observations, and the prevalence index, equivalent to the previous

quotient, is determined as 68.8 per cent. Since the difference between

the apparent average of 75.1 per cent and the weighted average of

G8.8 per cent amounts, in this one instance, to 6.3 per cent, it may

Table I

Excerpts From the Summary of Field Notes on the Prevalexci; a.vd

Intensity of the Scab Disease of Apples in 1928



422 Ii.i.ixois Natiral History Survky Bui.i.ktin

ment of seasonal intensity would include twig infection and perhaps

blossom blight.

The observed percentages of diseased fruit recorded on the record

sheets and entered in column 6 apply to diseased trees only. In order

to secure an infection index for all trees, it is necessary to compute a

"prevalence-intensity product'' by multiplying each prevalence product

in column 5 by its corresponding intensity percentage in column 6. This

is entered in column 7. In effect it gives a proper relative weight to

each recorded intensity observation, with the result that the intensity

indexes for counties and for the entire set of data are considerably more
accurate.

The observations on leaf intensity are entered in column 8. These

figures, like those for fruit, intensity, apply only to diseased trees, and
prevalence-intensity products must be computed for them also, and in

the manner just explained for the fruit intensity data.

When all the data given by the field-record sheets have been trans-

ferred to the table and the proper prevalence and prevalence-intensity

products computed, indexes of prevalence and of intensity applicable to

the entire set of observations can be secured by finding the totals of

columns 3, 5, 7 and 9, and then dividing each of the three latter sums
by the first.

The indexes Avhich have been obtained by the completion of this

table express concretely the prevalence of the disease and the intensity

of each phase of its attack, as exemplified in the orchards subjected

to examination in each county in Avhich observations were made and in

all the orchards in which data were taken.

If these orchards have been numerous enough and their distribution

sufficiently wide and representative, the final indexes of the table will

be statistical expressions of the average disease conditions prevailing in

the orchards of the State for the year; but as it usually is impossible

to examine so large a number of orchards carefully, additional treatment
is needed in order to obtain satisfactory indexes from the data secured.

The results computed from observations made in one county usually

vary to a greater or less extent from those of an adjacent county; and
this variation, though influenced by the number of orchards actually

examined as well as by errors inevitable in random sampling, often ex-

presses real differences in the disease epidemics, due to the predominani
use of varieties differing in their predisposition to disease, to cultural

practices, or to the extent to which effective control measures are em-
ployed. The disparity between indexes obtained for any two counties

nevertheless applies directly to the majority of orchards of those coun-
ties; but since counties seldom have the same number of trees, the

average disease condition for a two-county area is necessarily different

from that shown either by the sum of the orchard examinations or by an
average of the indexes of the two counties.

A proper adjustment of values may be obtained, however, by weight-
ing the indexes for each of the counties in proportion to the number
of trees in the respective counties. This process is, of course, necessary
for all the counties in which data have been taken, and a separate

tabulation with accompanying computations can be made.

I



Diseases of Fruit Trees, 1922-1928 423

The number of trees per county, the most important item in this

step, is not directly available and so must often be estimated. The
agricultural reports of the Bureau of the Census give the number of

trees only for the years 1919 and 1924. For the intervening years, the

number of trees per county may be estimated closely by securing the

difference between the given years, dividing this amount into 5 equal

parts (unless there is reason for adjusting the years otherwise), and
according as the change was either an increase or a decrease, adding or

substracting one of the 5 parts for each year subsequent to 1919.

As an example, the number of apple trees in Bond County in 1919
is given in the Census Reports as 87,346. In 1924, this number had
fallen to 59,483—a decrease of 27,863 trees. During this five-year period

the average yearly decrease was 5572.6 trees, and the number of trees

presumed to each year was : 81,773 in 1920, 76,200 in 1921, 70,628 in

1922, and 65,055 in 1923.

Subsequent to the 1925 agricultural census, data of this kind are

not available and will not be again until the results of the 1930 census

are made public.

The employment of data of this kind may be illustrated by using

the indexes obtained in the foregoing sample table and weighting them
in sample Table II, below, in terms of the total number of trees per

county.

Table II

An Illustration of the Adjustment of County Prevalence and Intensity
IN Terms of the Total Number of Trees Each Represents



424 Ir.i.iNois Naitkal History Survey Bulletin

lowered from 0.92 to 0.27. These differences, though much greater in

the two brief examples than in the actual extended tabulations, neverthe-

less serve to illustrate how desirable it is to take into account the entire

number of trees per county, the disease condition of which is represented

by the individual observations, or samples, tabulated from the field

record-sheets. It is, of course, apparent that the indexes with which

Table I closes apply only to the orchards which have furnished data,

while in Table II the entire number of trees in the territory where

sample orchards were examined is taken into account, with the result

that the final indexes of Table II apply to all these trees, whether they

are inside or outside of the sample orchards.

Geographically, cropping in Illinois follows rather well defined

plans. This has been recognized in the preparation of agricultural

statistics to the extent that nine districts have been delimited. A con-

siderable inequality exists with respect to the number of trees in these

statistical districts, but this may not be represented adequately in the

field examinations. To overcome this discrepancy, as well as to counter-

balance the high amount of disease usually found in small-orchard

regions, where spraying is not so generally or carefully done, so that

the data taken there do not outweigh data taken in regions of dense

fruit-tree populations, where commercial production is the aim and
sprays are applied with care and success, a third tabulation of data and
computation of indexes is often necessary. For statistical purposes, the

State is divided from north to south into four sections of about equal

length, and through these sections run longitudinal lines separating the

north section into two approximately equal portions, the next section

into three portions, and each of the two southern sections into two
portions. The lines of division have been made to conform to county

boundaries. Observations made in any one of these districts, after hav-

ing been properly weighted in Tables I and II, may be extended to

apply statistically to the entire fruit tree population of the district, and
the calculation of the weighted district values gives a series of indexes

showing the prevalence of a disease and the intensity of its several types

of infection throughout the State. This process of equalization, de-

scribed fully in our account of cereal diseases, is essentially as illustrated

in Table III.

As a matter of practice, we have not yet been able to make con-

sistent use of Tables II and III for our Illinois data. The making of

these analyses is, as is of course apparent, entirely dependent upon hav-

ing complete agricultural statistics at hand ; but these statistics, provided

at present only by the Bureau of the Census, are not issued concurrently

with the gathering of our data. They are made available once in five

years, and it is only after they have been made available that the fruit

disease data collected during the preceding five years can be considered

in all ways. The result is that, since no new statistical reports of a

suitable character have been at hand since 1925, the observations we have
made during the last four years can be made to apply only to the par-

ticular orchards in which they were taken ; and because the years of this

period number more than half of the years included by our survey, all

the data presented on subsequent pages have been made to conform to

this limiting condition.



Di,sEASE,s OF Fruit Tree,s, 1922-1928 42c



426 Illinois Natikal Histouy Survky Buli.ktin

Limitations of Data

The data upon which the discussions in this paper are based have

been taken during the growing seasons of the past seven years. No two

of these seasons have been alike, either with respect to disease phenomena
or with respect to the number, experience, and capabilities of the ob-

servers who gathered the data. It has been necessary, also, for us to

conduct fruit-disease and cereal-disease surveys simultaneously; and

there has been, as a C()nse(juence, a certain amount of interference by

each of these surveys in accomplishing the objects of the other.

Up to the time the survey was undertaken, no similar investigation

of crop diseases had ever been made. There were no methods of pro-

cedure at hand which could be adapted to our use, nor was there any

adefiuate body of experience upon which to draw for guidance. It is true

that limited surveys had been made in various places, but these were

concerned with one disease of one crop, were made for an immediate

purpose, and had in view the object of determining either the extent of

distribution or, vaguely, the immediate threat of the disease. The ex-

perience of entomologists was pretty largely invalidated by the inherent

differences in size, habits, and methods of reproduction that exist be-

tween insect pests and parasitic fungi.

We were therefore faced by the necessity of accomplishing simul-

taneously two tasks—developing methods and gathering data. Our first

Avork was quite as crude as the methods first conceived; but with im-

provement in method the value and reliability of the data have im-

proved also; and in this connection it has been an especially fortunate

circumstance that during the first years abundant specimens were gath-

ered and preserved as records to which Ave have since been able to turn

for re-examination with great benefit to the quality of our early records.

The investigation of crop diseases in the open eventually resolves

itself into a statistical problem, and in this respect the diseases of fruit-

trees present an especially difficult task. Practically all of the cereals

and most of the vegetables come from herbs that are limited to one year

of life, and their diseases, at least so far as seasonal epidemics are con-

cerned, are limited in the main to some particular plant part, such as

stem, leaf, flower, or seed. Fruit-trees, on the other hand, are woody
perennials, and each of their serious diseases is able to attack several

parts of the tree. The destructive effect of a cereal disease usually may
be measured directly in terms of the one part of the plant that is at-

tacked; but a fruit-tree disease, which may begin the season by destroy-

ing the blossoms and continue by killing the new growth, injuring the
leaves, and marring or rotting the fruit, often exhibits seasonal varia-

tions in the intensity of each of these types of attack, the sum of which
gives it its destructive effect. Each effect must therefore be measured
separately. For certain diseases, one or more types of attack are so rare

or do so little injury that they can be ignored, while certain other types
of attack—notably injury to the fruit—vary so much in severity that
it is impossible, in a general survey, to do more than to note the amount
of diseased fruit.

Moreover, it is permissible to take cognizance only of the disease,

as it is expressed in its effect on the tree. To attempt to express th(!



I

Diseases of Fruit Trees, 1922-1928 427

disease attack in terms of a possible commercial loss alone is not within
the province of such a survey, as the actual loss of fruit, or the reduction
in commercial value as it aifects the crop, may be greater in the year

of a mild epidemic than in the year of a heavy epidemic, according to

the time of the disease attack, the subsequent conditions of the market,
or the prices offered.

There has been a very considerable yearly variation both in the man-
ner in which we have collected data and in the amount of data secured.

In the first season, that of 1922, four observers were active, and the

largest quantity of data that we have been able to obtain in any one
year was taken. The progress of the survey that season was materially

advanced by the cooperation of the County Farm Advisers, who willingly

helped in locating orchards and very often furnished transportation to'

the field men. In the seasons of 1923 and 1924 there were but two ob-

servers; in 1925 the number was reduced to one; and since then there

have been two each season.

Coincident with the development of more painstaking and accurate

methods of surveying, there has been a decrease in the number of

orchards examined and in the amount of data taken in a season; but

this is more than balanced by the increased dependability of the records.

Fkuit-tkee Distkibutiox

The number of fruit trees of various kinds grown in each of the

several sections of Illinois has an important bearing upon the amount
and kind of data that can be obtained from each section in the course

of a survey such as ours; and the cultural and disease-prevention prac-

tices, which are naturally much more adequate where fruit production

is a major interest, as well as the location of orchards with respect to

the geographical range of one or more diseases, strongly influence the

character of the data as well as its significance when subjected to statis-

tical analysis.

As an aid in understanding the data presented on later pages, there-

fore, the reader should have in the background of his thoughts a general

picture of the fruit-tree population of Illinois. The latest available

figures^ indicate a total of a little over 12,821,000 fruit trees, of which

about 6,764,000 are apples, 4,139,000 are peaches, 767,000 pears, 754,000

cherries, and 395,000 plums and prunes.

The location of these trees is such as to permit a rough grouping

of the counties of the State, as shown in Figure 1, into three divisions,

in each of which the number of trees, and the consequent intensity of

fruit production, is essentially uniform. Throughout the northern half

of the State orcharding is a minor interest, and the orchards are, for

the most part, small and scattered. The total number of trees in this

section is about 2,799,000—approximately 22 per cent of those in the

State—and the average density of the tree population is about 89 to

the square mile. In the central section fruit growing is a' more im-

portant interest; and though it is true that the orchards of this region

are also mostly small and scattered, there is a scattering of large

1 The 1925 agricultural census furnishes data for apples, peaches, pears, plums
and prunes but the figures of the 1920 census have to be used for cherries.

—

2



428 Ii.u.Nois Natl KAL Histouy Survey Bulletin

oivliards, and Calhoun County bears the distinction of being the most

inii)ortant apjjle-jjroducing county in the State. The number of fruit

trees in this middle section is a little over 3,764,000—about 29 per cent

of those in the State—and the tree population, about 2G2 per square

mile, is nearly three times as dense as in the northern section. In the

southern section, fruit production is an important commercial occupa-

tion. Here the orchards are large and frequent, and generally they re-

ceive good care. The number of trees in this section, a little over 6,268,-

000. amounts to 49 per cent of all those in the State, and the tree

population, which reaches the high average of 614 per square mile, is

Fig. 1. Thk fruit-growing regions of Illinois

In the northern half of the state fruit production is not generally carried
on as a commercial enterprise. The apple is the most common tree; but with
the exception of a few large plantations it and the other fruit trees are
grown in small farm orchards. The southern half of the state has two fruit

regions which are distinguished chiefly by the density of fruit tree popula-
tion, but in both regions commercial production of apples and peaches is the
common practice. Further discussion is given in the text.



Diseases of Fruit Trees, 1922-1928 429

about seven times as dense as that of the northern section and 2.3 times
as dense as that of the central section.

The order of importance in which the various kinds of fruits stand
also varies from section to section, as may be seen in Figure 1. In the
northern and central regions, apples rank first and peaches second, while
in the southern section this order is reversed. In the north apples are

generally the only trees planted for a commercial crop, while peach,
cherry, plum, and pear trees, numbering in their aggregate only about
four-fifths of the total of apple trees, are seldom found in extensive

plantings.

In the central section, apples and peaches both furnish commercial
crops, and there are occasional pear plantations. Plums and cherries,

also, are of more importance than in the north, though the plantings are
generally small and scattered.

In the south, there has recently been a very decided stimulation in

the planting of peach trees, and they now outnumber the apple trees by
more than 200,000. The most intensive pear-producing regions of the

State are also in this section, notably in Marion, Union, and Pulaski
counties, and plums and cherries together, which are only one-third as

abundant as pears, constitute only 3 per cent of the entire number of

fruit trees, although when the areas of the several sections are considered
they are of nearly the same importance as in the central section and are

even more important than in the north.

In addition to these general features, each section has, of course,

certain particular characteristics. In the north, commercial apple and
peach production has been developed especially in Peoria, Tazewell, Ful-
ton, and Bureau counties. Similar regions of especially intense produc-
tion include Adams, Pike, Calhoun, and Jersey counties in the central

section and Marion, Jefferson, Johnson, Williamson, Union, Jackson,

and Pulaski counties in the southern section. The other counties in each
section are fairly uniform. From north to south, through the State as

a whole, there is, however, a gradual but very evident increase in the

number of fruit trees per county.

APPLE DISEASES

The diseases attacking apple trees in Illinois can be classified rather

conveniently as being either annual or permanent in their nature. Those
that are classed as annual appear on the trees season after season but
leave the trees more or less free from infection during longer or shorter

periods of each year, at least in the case of one or more of their modes
of attack. They are, therefore, the diseases which properly may be

called epidemic.

Permanent diseases, on the other hand, stay Avith the trees they

attack and continue to enlarge their destructive effects from year to

year, though they are, of course, quiescent during the winter season.

Represented in Illinois by the nail-head, fire-blight, bitter-rot, blotch

and black-rot cankers, they are diseases of the larger, older, woody parts

of the tree; and in those parts they persist, unless adequately treated,

from the time of infection to the ultimate death of the tree. It is

probable that their injuriousness varies from year to year; but their



430 Illinois Natural History Survey Bulletin-

effects are very difficult to measure and the results of sucli measurements

so uncertain that in the main they are excluded I'roni consideration

at this time.

The annual, or epidemic, diseases, though in many instances re-

sulting from the same fungi or bacteria as cause the permanent diseases,

generally attack the leaves, the twigs, the flowers, or the fruits; and

though they may arise directly from infective material furnished by

their permanent forms, their effects are usually limited to a single grow-

ing season. Each year brings a new attack, which varies from the attacks

of every other year both in prevalence and in severity.

Apple Scab

Caused by Endostigme inaequalis (Cke.) Sydow

The scab disease of apple, the most widely distributed and most

serious apple disease in North America, occurs throughout Illinois. It

may attack the blossoms, the young t^v'igs, the leaves, and the fruit,

though generally only the leaves and fruit are injured to an appreciable

extent. In Illinois, all four of these types of infection are known to

occur; but the twig and blossom infections are generally so mild that

they need not be given consideration. Leaf and fruit infection is usually

severe in all parts of the State; but as a rule the injury is greatest in

the north and is replaced to a certain extent in the south by blotch

injury.

The effect of scab upon either the fruit or the leaves depends to a

considerable extent upon the time at which infection takes place. When
it occurs on young apples it often results in the production of small,

misshapen, badly scarred, or cracked fruit that is of little value. Certain

varieties of apples, however, have the ability to resist scab attacks, and
the crops taken from these varieties, even in bad scab years, usually

show only a very small percentage of scabby fruit, much of Avhich is

of a salable grade due to the healing, or "corking over," of the scab

spots during the growth of the fruit.

Our comparisons of the year-to-year fluctuation in the amount of

fruit infected with scab has therefore been limited to estimates of the

percentage of apples bearing scab spots, and no consideration has been

given to the very apparent differences in the degree of commercial
damage to infected fruit resulting from the size or number or age of the

scab lesions. From the connnercial point of view, this would be, of

course, an improper procedure ; but since the epidemiologA^ of the disease

is our primary concern, the results of infection when stated in commer-
cial terms do not measure adequately the variations in disease, though
such an expression would indicate the relative predisposition of apple

varieties to disease or the efficacy of spray schedules in preventing or

halting the advance of an epidemic.

A significant refinement of the method Ave have used would be

to make a careful count of the yield at picking time, in order to deter-

mine the ultimate percentage of infected fruits ; and if at the same time
a further careful grading could be performed with the infected fruits,

measuring the quantity and size of the lesions, a still more accurate

index of fruit disease would be obtained. But to do this has been



Diseases of Fruit Trees, 1922-1928 • 431

beyond our means, and we have had to limit our collection of fruit-

infection data to the routine orchard examinations made while the

fruit was still on the trees.

Scab on apple leaves may, under various conditions, result in a

variety of types of injury to the tree, ranging from the mere discoloring

of a few leaves to nearly complete defoliation. Such grossly visible evi-

dences of the severity of the attack depend, however, to a very consid-

erable degree upon the resistance of varieties, upon particular local

conditions of soil and Aveather, and upon other factors that concern the

tree as a whole more than the disease. Because of this, it is necessary

in the measurement of a season's epidemic to ignore these very easily

observed conditions and to place emphasis instead upon the actual extent

of the leaf infection. Observations made with this limitation will serve

in comparing the actual year-by-year success of the disease, and those

influences which heighten the effect of the disease upon the tree will

not enter into the problem.

For the purpose of making such leaf-infection measurements, we
have developed the scales shown in Figures 2 and 3. From among a

large number of infected leaves, readily distinguishable grades of disease

have been selected, and careful measurements of them have been made
by using an instrument called a planimeter, thus determining, first, the

area of the entire leaf surface and, second, the exact area occupied by

the scab infections. From these two measurements, the percentage of

the area of each leaf occupied by scab lesions has been easily computed.

In finally selecting the examples chosen to make up the scale, it

was desired, first, to have percentage values for each unit which could

be handled by the observer with a minimum of trouble and, second, to

have an interval between the increasing grades of infection sufficiently

large to permit easy and certain separation of the leaves examined in

the field into the grades represented by the standard.

If the reader were to measure each of the samples of the scale, he

would find no one of them to be of exactly the value given in the dia-

gram ; but the variation in each case is small. If exactness of measure-

ment had been the only aim in developing the scales, it would have

been an easy task to make the standards conform exactly to the per-

centages given for them by making slight changes; but each of the

standards is a careful tracing of an actual leaf with its actual infection,

and this adds considerably to ease in usage, while the reproduction of

the leaf in diagrammatic black and white masks its reality sufficiently

to avoid confusion.

The two scales for measuring leaf infection by the scab disease are

necessary in order to measure the two types of scab infection. For

reasons which are not well understood, the fungus which causes the

scab disease produces two distinct types of lesions on the leaves. These

types, shown in Figure 4, are usually described as "diffuse" and "spot"

infections. The latter is more commonly met with in Illinois, but the

diffuse type is also abundant; and on a given leaf the lesions on the

upper surface may be of one kind and those on the lower surface of the

other kind. As may readily be seen in the illustration, the two are so

different in appearance that a scale made for one would not fit the other.



43: Illinois Natural History Survey Bulletin

^%

20^

/O % Z:^ %

30% -/^ •%

Fig. 2. Scale for estimating the intensity of the spot type of scab
attack on apple leaves

T,oo K°^ ^^a\
^^ *^^ sample leaves in this scale, the total area of the spots

.IL ^^^ determined and expressed as a percentage of its respective leaf
area. By comparing infected leaves with these measured examples, theavei^age leaf area occupied by scab lesions may be computed. See the text



Diseases of Fruit Trees, 1922-1928 433

^%

30 Z^

/o%

eo %

20 %

90 %

Fig. 3. Scale for estimating the intensity of the diffuse type of scab
attack on apple leaves

The shaded area shown on each of the leaf diagrams represents scab
lesions of the diffuse type illustrated in Fig. 4. This scale was prepared in
the same way as the spot scale shown in Fig. 2 and is used in the manner
described in the text.



434 Ir.i.ixois NATrRAL History Survey Bulletin

In using: these scales, the observer, walkinf]: from tree to tree in

the orchard, is required to select a number of leaves at random from

each tree, without refjard to the presence or absence of infection, and

to compare them, either then or immediately afterward, one by one

with the standards of the scale. The results of one such comparison

are shown on tlie sample field-record (page 418), but in practice the

appearance of the record differs in that each leaf is recorded under its

proper class by a penciled dot. In classifying the leaves, the rule is

that a leaf is to be considered as belonging to the class of infection

exemplified by the standard which it resembles most.

As an illustration, the data shown on the record sheet have been

brought forward in the following

:

Scale classes



Diseases of FRtriT Trees, 1922-1928 435

sive commercial orchards with thousands of trees were visited and 494
usable records were taken. The total number of trees represented by all

the data collected, from the first year to the last, is 132,943; and the
number of counties appearing annually in the records averages 23,

though the actual number has been as low as 4 (in 1925) and as high
as 50 (in 1924). The average number of usable records secured per
year stands at 70 and the average number of trees represented annually
by these records is 18,991.

As appears from Table IV, in which the yearly indexes for preva-

lence and intensity have been added and averaged for the seven year

term, the average prevalence of scab during the period covered by our
observations has been 76.68 per cent., while the average proportion of

fruit infected by the disease has been 16.56 per cent, and the index of

average intensity for the leaf attack has been 4.92.

Table IV

Prevalence and Intensity of Apple Scab



436 Illinois Natural History Survey Bulletin

Upon the infected trees, the scab lesions were, as the records show,

mainly on the fruit and the leaves. The actual extent of these infections

on the infected trees is not shown 'in Table TV, since the data have been
evaluated in the manner illustrated in Table I so as to show the extent

of those infections as averages for all trees, whether infected or not.

Hence, the field records taken in 1922 show that 37.45 per cent, of all

the fruit borne on all trees re])resented by the records were infected by
scab. In the same way. the leaf attack is given the average index, for all

leaves on all the recorded trees, of 8.13, which is a concrete statement

Fig. 4. Diffuse lesions and spot lesions produced by scab attack on
APPLE leaves

The leaf at the left bears lesions of the diffuse type, that at the right has
typical spot lesions. Diffuse scab is usually found along the veins of leaves,

while spot lesions are scattered irregularly over the leaf surface. These two
types of lesions are usually found on separate leaves but occasionally occur
on opposite sides of the same leaf. Their dissimilarity of appearance requires
the use of the two measuring scales shown in Figs. 2 and 3.

of the percentage of leaf surface occupied by scab lesions. Since the

number of records, the number of trees, and the counties in which these

intensity records were secured are the same as for the prevalence record,

the intensity indexes also indicate the intensity of the scab epidemic

throughout the State.

The second year of the period, 1923, when 43 records representing

14,805 trees were secured in orchards located in 30 widely distributed

counties, the prevalence index for scab stood at 74.85, the fruit intensity

index at 37.23, and the leaf intensity index at 14.45. For the calculation



Diseases of Fruit Trees, 1922-1928 437

of these indexes, there were available only about one-sixth as many
records as in 1923, but the average number of apple trees represented in
each record in 1923 was more than four times as great as in 1923. The
reason for this difference is found in the fact that in the later year more
orchards of commercial size were seen, and there is a reflection of this
in the prevalence indexes shown for the two years, that of the second
year, less by 18.15 than that of the first, being attributable in some

80



438 Illinois Natural History Survey Bulletin

occurred in the two preceding years. Based upon 83 records representing

48,865 apple trees and typifying the scab conditions prevailing in 50

of the 102 counties in the State, this season's observations, averaging

588 trees per record, are the most reliable of any thus far obtained. The
decreasing trend exhibited by the several indexes consequently may be

taken as an indication that in 1924 the growing season was distinctly

less favorable to the scab disease than was the case in either of the two
preceding years.

The indexes given for 1925 are the least reliable in Table IV. Cir-

cumstances over which we had no control so influenced the season's

work as to reduce the number of records to 5 and the number of counties

represented to 4, while the entire ninnber of trees entering into the

computation of the indexes was only a few over a thousand. The preva-

lence of scab, computed to be 98.58 per cent., may be unreasonably high

in view of the exceptionally low intensity indexes, which are given as a

"trace" (meaning that the actual amount was too small to be deter-

mined) for the fruit and 0.22 for the leaves. Yet, these indexes, when
compared with a report to the United States Department of Agriculture

made co-operatively by two observing agencies in Illinois, in which the

prevalence is placed at 100 per cent., the reduction in apple yield at

0.5 per cent., and the injury to the foliage as nil, are seen to be in rather

close agreement with the generally observed condition for the year.

Consequently, the fact that 1925 stands out in Table IV as marking a

very abrupt fall in the intensity of scab from the high levels of the

preceding years should be regarded as typifying conditions, though the

absolute index values are open to doubt. What appears to be a marked
increase in prevalence may, in fact, have been a fault due to too scant

data; but the high prevalence mentioned above as given by other

observers suggests that both the prevalence and the intensity phases of

the disease need not have followed the same trend.

In 1926, 16 records representing 11 counties and comprising a

total of about 9,500 trees, most of which were in commercial plantations,

gave a prevalence index for scab of 16.32. This is a great decrease from
the index of the previous year and may be regarded as reflecting in a

measure the exceptionally low intensity indexes of 1925. Of the 1926
intensity indexes, that for fruit disease increased to a measurable
amount, stated in the table as 0.01 per cent., while the leaf disease

index remained at the same level as in the preceding year. Though the

greatest difference between 1925 and 1926 is the decrease in prevalence,

the slight increase in fruit intensity in the latter year, coupled with the
maintenance of the leaf intensity index, is indicative of the beginning
of an increase in scab which ought, preventing circumstances being
absent, to become more and more apparent in succeeding years.

In 1927, the prevalence of scab rose to 93.77 per cent., and this

increase was accompanied by a distinct increase in the intensity of the

attack, the fruit index reaching 6.36 and the leaf index 1.95. The 42
records entering into the computation of the indexes were taken in 10
counties located mainly in the central and southern fruit sections of the

State, and a total of 12,755 trees, mostly in well-cared-for orchards,

were represented in the data. A general increase over the previous year,

both in prevalence and intensity, was decidedly apparent.



Diseases of Fruit Trees, 1922-1928 439

For the final year. 1928. the characters of the scab epidemic were
shown by 62 records draAvn from 8 central and southern counties, and
in the computation of the indexes there was entered a total of 26,984

apple trees. The prevalence of scab, determined as 96.32 per cent., in-

creased somewhat over 1927 and there was an accompanying but rela-

tively greater increase in the intensity indexes, that for the fruit attack

being 10.36 and that for the leaf attack being 2.84.

The trends of prevalence and intensity of the scab epidemics from
year to year for the entire period of observation are shown graphically

in Figure 5. The vertical scale in this graph is arranged in a manner
such that the slant of the trend lines expresses the relative rate of

increase or decrease of the prevalence and the intensity indexes, regard-

less of the actual numerical values they may have.

An examination of this diagram suggests that during the first three

years scab was maintaining itself at a rather high level, possibly the

highest it could attain under the climatic and cultural conditions pre-

vailing in Illinois, and was not being subjected to circumstances result-

ing in great variation in the prevalence or intensity of its annual
epidemics. How long this high level of disease had been maintained
can not be determined from the data at hand; but, as the graph shows,

it came to an abrupt end in 1925; Though the prevalence of scab in-

creased in that year niore than it had declined in the two preceding

years, the intensity of the attack upon both fruit and leaves fell to

exceedingly low levels. Their drop is so sharp as to outweigh by many
times the relatively small increase in prevalence.

The effect of the great scab decline of 1925 remains apparent in

1926. Perhaps because the amount of infective materials produced in

1925 was very small, the prevalence of scab dropped in 1926 to the

lowest point of which we have record. But the fact that the low levels

of the fruit and leaf intensities reached in 1925 were at least maintained
in 1926 probably is a rather definite indication that the conditions pre-

vailing in 1926 tended to favor, rather than inhibit, the disease: other-

wise, a further decline should have occurred. This view is also justified

by the large increases in prevalence and intensity in the last two years

shown on the graph*. In 1927 all phases of the disease increased very

greatly, but, except for prevalence, the indexes did not reach the high

levels common before 1925. Though a much smaller increase occurred

in 1928, it served to bring the scab indexes still nearer the level of the

first years. The trend of scab since 1925 has been upward, and it is rea-

sonable to suppose that it will continue upward until the usual levels

are reached, unless another unfavorable year such as 1925 halts its

advance.

Apple Blotch

Caused by Phyllosticta solitaria E. & E.

Blotch of apples, in contrast with scab, is limited in its distribution

practically to North America, and in Illinois it is confined, at least as a

destructive disease, to the southern two-thirds of the State. Within

this region it is often very serious. Fruit is attacked and defaced to

such an extent as to be unmarketable; leaves are spotted more or less



440 Ilonois Natural History Survey Bulletin

severely and the yielding abilities of the attacked trees lessened thereby

year after year; and the young wood, particularly the twigs, is subject

to infection, the resulting cankers remaining for three or more years to

provide new infective material and to contribute to the decreasing ability

of the trees to yield. There are, of course, other frequent types of infec-

tion, such as that on the bud scales, on the petioles of leaves, and on
the stems of the fruit, but these are intrinsically of little significance,

though it is true that the lesions on the lowest parts of the petioles often

extend into the twigs and cause a large proportion of tlie cnnkors found
at the nodes of twigs.

In the measurement of the annual blotch epidemics in Illinois, the

prevalence of the disease and the intensity of its attack have, as with

apple scab, been recognized as distinct phases. Prevalence has been

determined by counting, in each plantation examined, the number of

infected trees, regardless of the types or kinds of infection present on
the trees, and expressing this number as a percentage of the entire

number of trees in the plantation. Such a percentage, computed in the

manner previously described and applying to all the blotch-infested

orchards examined during a year, is regarded as an index of the general

prevalence of blotch for the je&r. As such, it may be compared with
similar indexes for other years.

The intensity of blotch attack cannot be measured as a single unit,

because the variations in the epidemic from one year to another tend to

emphasize sometimes one, sometimes another, of the various modes of

attack ; and there are also remarkable varietal differences among apples

with respect to their susceptibility to one or more forms of the disease.

Blotch injury on diseased trees is. however, most serious on the fruit, the

leaves, and the twigs. In each of these types of attack a distinct form
of intensity is represented, and each type must be measured separately.

Thus far, we have gauged fruit infection only in terms of infected

fruits ; that is, the percentage of infected apples has been determined by
careful counts, but no attempt has been made to record the number of

blotch lesions per apple. Similarly, our measurement of the intensity

of twig blotch has been limited to a determination of the percentage of

infected twigs per tree, without attempting to specify either the number,
the age, or the extent of the cankers.

For blotch on the leaves, however, a more accurate means of

measurement has been employed. The diagram shown in Figure 6 con-

stitutes a scale by which the intensity of leaf attack can be determined
as the percentage of leaf area occupied by blotch lesions. In the con-

struction of this scale, just as in the making of the scab scale, a large

number of leaves showing various degrees of infection Avere placed in

series and from among them representative samples were selected, which
graded from light to heavy by readily distinguisha])le intervals. Careful

tracings were made of these leaves, and of the spots borne by them, and
the area of each of these standard leaves was determined carefully with
a planimeter. To determine the area occupied by the blotch infections, it

was necessary first to find the average area of a spot, since the individual

spot is too small to be measured accurately. This average was obtained
by tracing contiguously the magnified outlines of a large number of



Diseases of Fruit Trees, 1922-1928 441

spots, measuring the area covered by them all, and dividing this area
equally among them after reducing it to its actual, or unmagnified, size.

The percentage of the area of each of the leaves serving as standards
in the scale occupied by the blotch spots could then be very closely

approximated by first counting the number of infections on each leaf,

multiplying the average spot area by these numbers, and dividing the

Fig. 6. Scale for estimating the intensity of the attack of blotch on
APPLE leaves

The figures given below each leaf diagram express the percentage of the

area of each leaf occupied by blotch spots. The scale is used in the same way
as the scab scales.

total spot areas so obtained for each leaf by the respective leaf areas.

This process is, of course, subject to an error arising from a random
sampling of spots in selecting the standard leaves, but even in the lesser

grades of infection the number of spots is s6 large (it is exactly 100 in

the first standard leaf) as to reduce this error to an insignificant point.



442 Illinois Natural History Survey Bulletin

For two of "the grades used in the scale, two standard leaves have

been supplied. The reason for this is that, with such small spots as

those made by blotch and Avith the small percentage intervals that must

be used to gauge blotch infection on leaves, the different appearance of

large and small leaves with the same percentage of occupied area is

often misleading to the eye, since the tendency in visual appraising is

to emphasize the number, rather than the combined areas, of the spots.

With light infections this danger is minimized by the fact that the

intervals between the standards of the scale are smaller, while with

heavy infections the greater intervals between standards, which is nec-

essary for accurate classification, tend to lessen its occurrence.

The manner in which the leaf blotch scale is used in measuring
the success of the blotch disease is similar in every respect to the method
previously described in detail for the scab scale.

Data on Blotch

In determining the prevalence and intensity of the blotch disease

of apples in Illinois, a total of 258 orchard records have been collected,

and in securing them a total of 219,574 trees have been considered.

Though the number of records, and the number of trees represented by

them, has varied considerably from year to year, as may be seen in

Table V, the average number of records per year was 36 and the average

number of trees considered per year was 31,^67. The extent of the ter-

ritory in which the records were taken has varied also, the maximum
number of counties represented in any one year being 38 in 1924 and

the minimum number being 2 in 1925; but the average number of

counties per year has been 18.

Table V shows the results of the survey of blotch. Over the 7

years of the survey term, the average prevalence of blotch is shown to

be 72.33 per cent., while the average intensity of its attack is 18.09 per

cent, for the fruit phase, 27.65 for the twig phase, and 1.19 for the leaf

phase. These figures may be taken to mean that, year after year, the

blotch disease attacks an average of 72 per cent, of the apple trees in

the blotch infested orchards of Illinois, that in these same orchards it

infects to an undetermined extent 18 per cent, of all the apples pro-

duced and 27 per cent, of all the young apple wood, and that the area

occupied by its leaf lesions amounts to 1.2 per cent, of all the leaf area

of all these trees.

The variations in all phases of the blotch epidemics, as they

occurred from year to year, are also shown in Table V by the compara-
tive sizes of the annual indexes given for each of the measured modes
of attack. The indexes for 1922, computed from 68 records representing

17,781 trees distributed among 33 counties, give blotch a prevalence of

53.03 per cent., while the percentage of diseased fruit amounted to

41.36, the percentage of diseased twigs to 10.49, and the diseased leaf

area 2.56 per cent.

In 1923, data computed from 54 records representing more than
75,000 trees located in 28 counties, showed a rise in blotch prevalence

to 89.14 per cent., but this increase in prevalence was accompanied by a



Diseases of Fruit Trees, 1922-1928 443

fall in the intensity of the fruit attack to 33.15 per cent., a decrease of

9.21 from the index of the preceding year. The rise in prevalence was
accompanied, however, by a rise in the intensity of twig attack to 81.29

per cent, and in the intensity of leaf attack to 3.89 per cent.

In 1924, blotch prevalence remained very high, the index of 87.68,

which was computed from 54 records derived from 38 counties and
representing 93,329 trees, being only 1.46 less than that for 1923. The
intensity of the disease was, however, consistently low, the attack on
fruit having an index of only 3.89, that on the twigs an index of 8.41,

and that on the leaves an index too small to be recorded other than
as a trace.

Blotch conditions in 1925 are represented by a very small number
of records, which exemplify the attack on only 238 trees parceled be-

Table V

Prevalence and Intensity of Apple Blotch



444 Illinois Nati'ral History Survey Bulletin

In 1926 tlie indexes were ooniputed from 21 records drawn from 14

counties and representing, in all, 11,622 trees. The prevalence indicated

by these records is 46.67 per cent., which is probably somewhat of a

decrease from 1925. The larger index for the fruit attack, 12.48 per

cent., is undoubtedly an indication of blotch recrudescence following the

two previous unfavorable years, though tliis is borne out only to a rela-

tively slight extent by the leaf disease index of 0.01 per cent. Data on
the twig attack are lacking for this year.

For 1927, 31 records made in 10 counties and representing 11,207

apple trees gave a prevalence of 71.87 per cent., which serves to indicate

that blotch had regained much of the ground lost in 1924 and 1925;
and an increase of the fruit intensity index to 18.34 per cent, bears out

this indication. The value of the index for twig disease, 7.59 per cent.,

is, however, not greatly different from that of 1924.

In 1928 the prevalence of blotch, as indicated by 27 records obtained

from 7 counties and representing 9,772 trees, fell to 57.92 per cent. In
keeping with this are the noticeable decreases shown by the fruit and
twig disease indexes, the former being 9.37 per cent, and the latter 3.59

per cent. The very small amount of leaf blotch is expressible, as in the

previous year, only as a trace.

Subject to the limitations in data pointed out above, the trends of

both the prevalence and the intensity of the apple blotch epidemics from
year to year through the period covered by our survey are shown graphi-

cally in Figure 7. The general trend of the disease has been downward,
and the changes from one year to another have, on the whole, been rather

large. From a fairly high point in 1922, blotch increased considerably

in 1923, and fell to a low point in 1924. The following year, 1925, it

regained a considerable part of what it lost in 1924, and immediately
thereafter, 1926, began a downward movement, interrupted by a slight

rise in 1927, that furnished the low indexes of 1928. The course which
the disease may follow in 1929 and later cannot be anticipated from the

diagram, though the suggestion of the trends shown there is that it will

continue downward.
By examining Figure 7 more closely, one may see that a rather

striking parallel exists in the year to year trends exhibited by the various

blotch indexes. The prevalence index stands at a relatively high point

throughout, but its general level during the first four years was dis-

tinctly higher than during the last three years. The indexes for leaf

and twig blotch appear to conform to this generalization ; and it is worthy

of note, in particular, that the upward trend in prevalence from 1922 to

1923 is reflected by similar though unequal trends in leaf and twig in-

fection. The very slight decrease in prevalence from 1923 to 1924
appears also, in a greatly magnified degree, in the sharp decrease exhib-

ited by the fruit, leaf, and twig infections, while the prevalence increase

in 1925 is reflected also in the increased indexes of disease intensity.

The fall in prevalence shown for 1926 was accompanied by a fall in the

intensity of leaf disease and probably also of twig disease, though there

is no adequate data to support the contention. For the slight rising and



Diseases of Fruit Trees, 1922-1928 445

falling of blotch prevalence in 1927 and 1938, respectively, the fruit

intensity index is seen to rise and fall, the twig intensity index to fall

in 1928 to a point lower than it held in 1927, and the leaf intensity

index to remain so low in both years as to be unmeasurable.
In general, it appears that the trend of prevalence exhibited by

apple blotch from year to year is reflected in a distinctly magnified
degree by the trends of each of the modes of blotch attack, but there is

no lagging behind of any phase of the disease. It should be noted, how-

100
80



446 Illinois Natural History Survey Bulletin

Apple Black-rot

Caused by Physalospora malorum (Pk.) Shear

The black-rot disease, known to occur widely in Europe, in South

Africa, and in Canada, as well as throughout the United States, is one

of the most common, though not usually the most serious, of apple

diseases in Illinois. It ranges throughout the State and is found, in

one or another of its forms, in practically every orchard. The most

important injury resulting from it is the fruit-rot from which it is

named. Generally, this rot is not of great importance, unless some pre-

disposing factor such as codling moth injury provides unusually good

conditions for its development. The canker form of black-rot does per-

manent damage to apple trees, but the fungus which causes the disease

is known to be a weak parasite, able to gain access to the woody partss

of the tree only through injuries already accomplished by more active

diseases such as fire-blight.

Upon apple leaves, the black-rot disease appears as a spot which,

because of its characteristically zonal markings, is commonly kno\\Ti as

"frog-eye." After being killed by the black-rot fungus, the leaf tissues

of these spots often are invaded by other fungi, and it has been our

experience in Illinois that these secondary invaders are found in the

lesions more often than the black-rot fungus. Hence, in measuring the

extent of black-rot on leaves we have counted all "frog-eye" spotting as

black-rot, regardless of the identity of the fungus actually found on the

spots by microscopic examination.

In measuring black-rot epidemics, the statistical phases taken into

consideration have been prevalence, intensity of fruit attack, the quantity

of twigs and woody parts infected, and the percentage of leaf area occu-

pied by the frog-eye spots. Prevalence, as with other diseases, is ex-

pressed as the percentage obtained by comparing the number of infected

trees with the total number ; and the intensity of both the fruit and tmg
infections is expressed as the percentage of these individual parts in-

fected by the disease. No attempt has been made to measure further

the severity of the infection on the individual apple or twig. With the

fruit, however, it should be stated that only the apples on the trees have

been counted, as the infection of fallen fruit, though one of the most
common causes of decay, is nearly always secondary to injury by codling

moth larvae.

In measuring the intensity of black-rot on leaves, we have used the

scale shown in Figure 8. The units of the scale were selected and meas-

ured, and the scale itself has been used, in the manner already described

in detail in' the discussions of scab and blotch. The very small leaf-area

values shown in Table VI and discussed in succeeding paragraphs seem
inconsistent with the relatively large values shown by the standards in

the scale, but it must be remembered in this connection that the scale

itself serves only to measure the infected leaves, which often constitute

only a small portion of a leaf sample, and that the total area occupied

by lesions, which is obtained by using the scale, is parceled evenly

among all the leaves of the sample.



Diseases of Fruit Trees, 1922-1928 447

Fig. 8. Scale for estimating the intensity of "frog-eye" spots, the
manifestation of black-rot attack, on apple leaves

This scale is used, in the manner described in detail in the discussion

of apple scab.



448 Illinois Natural History Survey Bulletin

Data on Black-rot

A total of 311 records have been taken on black-rot during the

course of our survey, and the data contained in these records present

the state of the disease on 115,729 apple trees. As may be seen in Table

VI, which gives the annual indexes of prevalence and intensity for

black-rot, the number of records taken in any one year has ranged from
a maximum of 99 in 1922 to a minimum of 2 in 1925, while the average

number per year has been 44. The number of trees represented by the

sets of annual records has varied also, ranging from a maximum of

26,452 in 1927 to a minimum of 198 in 1925, and the average yearly

number has been a little over 16,500. The average number of counties

represented in the yearly records is 15, but the actual number per year

lias ranged from a minimum of 1 in 1925 to a maximum of 32 in 1922.

Table VI

Prevalence and Intensity of Apple Black-rot



Diseases of Fruit Trees, 1922-1928 449

by frog-eye spots only to the extent of 0.36 per cent. In 1923, when
the prevalence of black-rot, computed as 55.54 per cent., was at the
lowest point observed by us, all forms of black-rot attack were low also,

only 0.45 per cent, of both the fruit and twigs being diseased and 0.18

per cent, of the leaf area occupied by lesions.

The sudden rise of prevalence in 1924 to 82.65 per cent, was
accompanied by increases in the indexes of all three types of attack, the
computations made with the data for the year giving an average of

1.05 per cent, of black-rot infected fruit, 2.26 per cent, of cankered
twigs, and 0.52 per cent, of leaf area occupied by frog-eye spots.

Fig. 9. Trends of prevalence and intensity shown by the black-rot
epidemics

Prior to 1925, prevalence and the three measured forms of intensity were
high, and their changes from year to year were uniform in direction. In

1925, howevei', an upward movement in prevalence and leaf intensity was
accompanied by sharp falls in both twig and fruit intensities. Subsequent
to 1925, prevalence continued to increase, but there was a general falling off

in all intensities.

This is in decided contrast with the conditions indicated by our

relatively small amount of data for 1925. Though in this year the

prevalence index rose to 91.41 per cent., the rise was accompanied by

rather steep falls in two types of black-rot attack, the percentage of

rot-infected fruit and cankered twigs sinking to amounts too small to



450 Ir.Lixois Natural History Survey Bulletin

estimate. There was, however, a rise in the leaf infection index to 1.09,

the highest point in our record. Respecting the value of these indexes, it

may be stated that though the number of observation, and of trees, upon
which the indexes are based are very small, the relative state of the

disease expressed by them is in accord with the conditions generally

observed to have prevailed that year, although it is possible that

the actual increase in black-rot leaf-spot is magnified somewhat in the

index.

In 1926, the prevalence index rose very slightly to 91.57, and there

were corresponding slight increases in the intensity indexes representing

the black-rot attack on fruit and on twigs, the former being 0.17 and the

latter 0.004. The index of leaf-attack, however, fell to the lowest point

recorded in our observations up to that year.

Continuing the tendency to increase shown during three preceding

years, the prevalence index rose in 1927 to 100 per cent. ; but all the

intensity indexes moved downward, that for fruit-rot to 0.01 per cent.,

that for twig cankers essentially to 0, and that for leaf spot to 0.15.

In 1928, the high prevalence of 1927 was maintained, and the

downward movement of the intensity indexes was continued to 0.01

per cent, for fruit-rot, an inestimable trace for twig canker, and 0.05

per cent, for leaf spot.

The trends of prevalence and of the intensity of the three important

forms of black-rot attack on apple are shown graphically in Figure 9. In

this figure, the portion of the graph which would ordinarily be seen

between the wavy lines has been omitted in order to bring the high

prevalence line into closer comparison with the low intensity lines.

From this graph, it may readily be seen that only from 1922 to 1923

and from 1923 to 1924 is there a complete parallelism among the preva-

lence and the three intensity curves. In 1923 all swing downward from
the previous year, and in 1924 they swing upward. The rate of change

is not, however, equal in any two cases. The upward trend of prevalence

and leaf infection from 1924 to 1925 contrasts sharply with the steep

fall of the twig and fruit infections; but the continued rise of preva-

lence from that year on is coupled with falling leaf intensity and low
twig and fruit intensities.

Apple Fire-blight

Caused by Bacillus aviylovorus (Burr.) Trev.

Fire-blight, like apple scab, occurs throughout Illinois; and just as

scab is the most feared of apple diseases because of its ability to ruin a

season's crop, so fire-blight furnishes the greatest threat to the continued

existence and productivity of apple orchards. There is no part of an

apple tree that cannot become blighted. With the appearance of the

blossoms in early spring, fire-blight begins its destructive work, and as

the season progresses new forms of attack may constantly be observed to

appear. The year-round nature of the disease is well illustrated by the

very generally recognized types of attack, commonly known as blossom-

blight, twig-blight, leaf-blight, fruit-blight, cankerous blight of the

limbs and trunk, and collar-blight at the base of the tree. Seasonal

phases of fire-blight, such as blossom-, twig-, and fruit-blight, constitute



Diseases of Fruit Trees, 1922-1928 451

the epidemic aspects of the disease, and it is these which cause the
annual reductions in yield, while the cankerous phases, represented by-

limb-, trunk-, and collar-blight, are permanent forms which constitute
continual hazards in orchard maintenance.

In the measurement of epidemic fire-blight we have thus far lim-
ited ourselves to acquiring data on two phases only, prevalence and
twig-blight. As a measure of the intensity of the annual blight epidemics
an index of twig-blight serves as the best total statement obtainable,

unless more exacting observations are made on individ\ial phases than
we have been able to undertake. Our twig-blight cotmt includes fruit

spurs which show evidence of blossom-blight, leaf-blight, or spur-blight,

as well as the real twig-blight which commonly occurs a little later in
the season. It is, therefore, a comprehensive measure of all epidemic
phases of the disease; and the data presented below should be accepted
in that sense.

Data on Fire-blight

In the course of onr survey, 373 records have been made of the

occurrence of fire-blight in apple plantations. These records typify the

condition of the disease with respect to a total of 169,346 trees, or an
average of 24,192 trees for each year's observations; and for the average

year, the records show conditions prevailing in 23 counties. As may be

seen in Table A^II, the largest number of records secured in any one

year was 105, in 1922, while the smallest number was 10, secured in

1927. The territory represented by these records has varied also, 41

counties furnishing the 1924 records and 7 counties the records for

1927 ; and the number of apple trees from which the recorded data was
collected has varied from a maximum of 43,567 in 1924 to a minimum
of 9,583 in 1927.

Table VII

Prevalence and Intensity of Apple Blight

*



452 Ilunois Natural History Survey Bulletin

Variable as the quantity of the yearly records is, they are in the

main representative of fire-blight conditions during the past seven

years. The average prevalence of the disease, secured by giving each

year's observations equal weight and dividing the sum of prevalence in-

dexes for the period equally among the years, is shown in Table VII to

be 57.38 per cent., and the average intensity of the fire-blight attack,

expressed in terms of blighted twigs and spurs, is 4.93 per cent.

The individual years, however, often show considerable variations

from the average. The prevalence of fire-blight in 1922, when 105

records were secured in 38 counties, was 42.64 per cent, in a total of

13,202 apple trees, less by 14.74 per cent, than the average, and the

too
do



Diseases of Fruit Trees, 1922-1928 453

blight, this intensity being higher by 4.68 per cent, than the average. In
1924, when 77 records taken in 41 counties showed a prevalence of

65.41 per cent, among 42.567 trees and an intensity of 10.69 per cent.,

the disease exceeded its average conditions by <S.03 and 5.76 per cent.,

respectively. This excessively high fire-blight condition was maintained
to a lesser extent in 1925 when, on the basis of 19 records, the preva-
lence of the disease was shown to be 59.85 per cent, among 25,132
trees, or 2.47 points above the average; but in this year the intensity

of fire-blight dropped to 4.76 per cent., or 0.17 point IdcIow the average.

On the basis of 60 observations taken in 28 counties, the prevalence of

blight in 1926 was shown to be 1.51 points above the average by the
index for the year, which was computed to be 58.89 per cent, among
25,919 trees. The intensity of the attack, however, was 0.44 of a point
less than the average, or 4.49 per cent.

The tendency of fire-blight to decrease, which was more or less

apparent in the years from 1923 to 1926, appeared in its full extent in

1927. In that year 10 observations drawn from 7 counties indicated a
prevalence of 21.04 per cent, among 9,583 trees and an accompanying
intensity of 0.09 per cent, for twig-blight, these indexes being respec-

tively 36.34 and 4.84 per cent, below the average conditions.

In 1928, there was a great recrudescence of blight, its prevalence,

as indicated by 44 records made in 9 counties, being 81.01 per cent,

among 16,824 trees; but the intensity of twig-blight, though it rose to

3.05 per cent., still remained at 1.88 points below the average.

The' trends of prevalence and intensity of fire-blight are shown
graphically in Figure 10 by the solid lines. The arrangement of the
vertical scale of the graph is such as to make the changes from one year
to another comparable in terms of the slope of the lines. It is readily

seen that the intensity of blight attack has varied miich more, propor-
tionately, than its prevalence. With the single exception of the change
from 1923 to 1924, when the prevalence trend fell and the intensity

trend rose, the direction of both trend lines is the same for every year.

And it is notable also that in every case the movement shown in the
prevalence curve is magnified, in the intensity curve, to an extent accu-
rately indicated by the relatively greater steepness of the year-to-year

sections of the intensity curve.

Apple Eust

Caused by Gymnosporangiiim jimiperi-virginianae (Schw.) Sw.

Among the apple tree diseases occurring in Illinois, apple rust is

distinctive because it is purely annual in occurrence on apple trees, and
for this reason it is the only one of the apple diseases that can be

classed as strictly epidemic. The destruction caused by it is limited to

the fruit and leaves, and with the falling of the leaves and harvesting

of the fruit it is entirely removed from the trees. The rust epidemic
of any one year is, therefore, entirely separate from the epidemics of the

preceding and following years. Since the fungus which causes the dis-

ease produces no permanent lesions upon the apple tree, it depends for

its annual propagation upon its ability to accomplish a portion of its

development upon another host, the cedar. So specialized are the



454 Illinois Natiral History Sirvey Bulletin

habits of the fungus that the spores developed by it upon the apple

tree are wholly unable to reinfect apple trees. They are, however, viru-

lent on cedars, and the rust epidemics consequently develop anew each

year from the infective material supplied by the "cedar-rust" phase of

the disease.

Since it is annually d^3endent entirely upon cedar trees for its

propagation apple rust is not so widely or generally distributed in Illi-

nois as the other epidemic diseases of apples but is limited in its occur-

rence to regions in which cedars are relatively common and grow in

close proximity with the orchards. Nevertheless, the extent of its range
includes the entire state.

In measuring the annual rust epidemics, we have limited the

accumulation of data to two phases of the disease. These are prevalence

and the intensity of rust attack on the leaves, prevalence being, as with
other diseases, expressed as the percentage of infected trees found in rust-

infested orchards and leaf intensity as the approximate percentage of

leaf area occupied by the rust spots as measured by the area-scale given
previously for the spot type of scab, as this scale approximates with
reasonable closeness the rust-spot condition.

Data on Apple Rust

Satisfactory data on apple rust were secured for each of the years

except 1925 included in the period of our survey. The less extensive

distribution of the disease is seen in the fact that the total number of

records of the disease is just 116, whereas 494 scab records were

secured in the same period. The total number of trees from which these

rust records were derived is 53,377. The average number of records per

year was 19, and the average number of trees per year was 8,896, the

data for each year representing, on an average, the rust conditions in

11 counties.

As is shown in Table YIII, the computations made with the accu-

mulated apple rust data indicate that in rust infested orchards the

average yearly prevalence has been 61.96 per cent., while the severity

of the attack has been equivalent to a rust occupation of 3.38 per cent,

of the leaf area produced by all trees in the orchards.

The exact condition of each year with respect to the rust attack is

also shown in Table VIII. For 1922, 16 records derived from 14

counties indicate a rust prevalence of 26.29 per cent, among 3,503 apple

trees, with an accompanying intensity of foliage disease amounting to

1.57 per cent, of leaf area. In 1923, when 14 records were obtained in

8 counties, the prevalence of rust was 57.29 per cent, in 6,210 trees, and

the intensity of the leaf attack amounted to 3.7 per cent, of the leaf

areas of these trees. In 1924, the 29 records secured in 23 counties indi-

cated the prevalence of rust to be 38.77 per cent, in 23,056 trees, with

a leaf-area occupation of 0.25 per cent. Data were not secured for

1925, but in 1926 the prevalence of rust, as shown by 12 records taken

in 9 counties, amounted to 72.51 per cent, among 2,445 trees, while the

intensity of the leaf attack is given the astonishingly high index of

14.63. In 1927, the prevalence of rust rose still further, the 23 records

taken that year giving a prevalence index of 81.22 per cent, among



Diseases of Fruit Trees, 1922-1928 455

6^998 trees; but the intensity index dropped to 0.06 per cent., the

lowest among our records. Again in 1928, prevalence increased, the

index derived from 22 records taken in 7 counties being 95.72 per cent.,

for 11,165 trees; but the intensity of the rust attack was, nevertheless,

quite light, the index of leaf-area occupation by rust spots being only

0.1 per cent.

The trends of rust prevalence and intensity from year to year are

shown by dotted lines in Figure 10. From 1922 to 1923 the trend of

apple rust, both in prevalence and in intensity, was upward, and the
relative increase was nearly equal for the two aspects of the disease.

From 1923 to 1924 both trends were downward, the intensity of the

attack decreasing nnich more rapidly than prevalence. Between 1924
and 1926 no record is available; and the trends from 1924 to 1925 and

Table VIII

Prevalence and Intensity of Apple Rust



456 Ilunois N.vtur.vl Hi.story Survey Bulletin

Summary of Apple Disease Data

The observations given in the preceding pages have dealt with the

individual diseases, and no comparisons have been made between dis-

eases, as regards either their prevalence or their intensities. It will be

of some interest, therefore, to compare changes in prevalence from year

to year, and changes in the intensity of various types of attack of all

diseases, and to determine the total effects of all over a period of years

as exemplified in the years covered by our survey.

Prevalence of Apple Diseases

In Table IX, the prevalence indexes for all apple diseases are

brought together so that the prevalence of each disease each year may
be compared with the prevalence of every other disease.

Table IX

Summary of Prevalence of Apple Diseases

Disease



Diseases of Fruit Trees, 1922-1928 457

If the annual prevalence indexes for these five epidemic diseases

were added, each yearly total would be greater than 100. Although it is

self-evident that the percentage of diseased apple trees can not be

actually in excess of 100 per cent., these totals of the prevalence indexes

would serve to express, for the purpose of comparison, the relative suc-

cess these apple . tree diseases attain annually. The total of the pre-

valence indexes for 1922 is 281, while for 1923 it is 350. As numerical
averages, these figures indicate that' in the first year the incidence of

apple diseases, as is shown in Table IX, was 2.81 per tree and in the

second year 3.5 per tree. Banking the years included in our survey ac-

cording to the incidence of diseases per tree, they stand in the order

shown by Table X.

Table X
Years Ranked According to the Average Incidence of

Diseases Per Apple Tree

Rank



458 Illinois Natural History Survey Bulletin

course, be determined. The result of the movement of the known pre-

valence indexes was, nevertheless, to bring about a rise in the disease

incidence curve to approximately the height of 1923.

In lf)2(). three diseases fell olf in prevalence, scab very sharply,

blotch quite sharply also, and fire-blight only slightly, while of the

other two diseases, black-rot increased somewhat in prevalence and rust

rose to the highest point recorded up to that time. 'The net result of

these changes was to drop the disease incidence curve to a point only
slightly higher than it occupied in 1922.

^o

SCOA
B/of-e/i
Black-rof
Firt B''i^t^

/o
/32Z 23 '24 '25 '26 '27 'Z8

Fig. 11. Comparison of the prevalence trends of apple diseases and of
THE average incidence OF DISEASE PER TREE. SeE TEXT FOR DISCUSSION

The general retrogression of disease prevalence in 1926 was com-
pensated during the 1927 season by a rise in prevalence of all diseases

except fire-blight, which fell to its lowest point in our series of records,

and the result was to raise the disease incidence curve to the highest

point recorded up to that time.

This general increase was continued into 1928. Fire-blight rose

very sharply to a high degree of prevalence, black-rot, rust, and scab

maintained or increased their former high indexes, and blotch alone de-

creased, with the net result that the average incidence of diseases per
tree rose, as the upper curve shows, to the high point of 4.31.



Diseases of Fruit Trees, 1922-1928 459

AVERAGE ANNUAL PREVALENCE

Our records, covering as they .do the changes in disease prevalence
exe;iiplified by seven growing seasons, during which each disease en-

countered at one time or another conditions that were favorable or un-
favorable to its propagation, may be held to represent to a rather exact
degree the average prevalence of each of the epidemic diseases which
the orchardist may expect to have to meet over a period of years under
Illinois conditions. This expectation, which may be expressed as the
average annual prevalence computed for each disease from the indexes
given in Table IX, is shown graphically in Figure 12. Fire-blight may
be expected to be least prevalent, with rust, blotch, scab, and black-rot
increasingly prevalent in the order named. And the individual tree,

as indicated by the uppermost block in Figure 13, may be expected to

encounter an average incidence of 3.4 diseases per year.

|3.43
1 1 1 1 1

10 zo
Av. Incidence



460 Illinois N.vrrR.vL History Survky Bulletin

FRUIT INTENSITY

Data are at hand to show the intensity of fruit attack in the case of

only, three diseases, scab, blotch, and black-rot. These are shown, in

Table XI. In the first year, 1922, blotch attack was the most intense,

occurring on 41.36 per cent, of the fruit in the plantation found to be

diseased and scab was only slightly less intense, with an index of 37.45

per cent.

Throughout the entire period black-rot occupied a very minor place

only once, in 1924, attacking in excess of 1 per cent, of the fruit. The

place of first importance as a fruit disease was held 4 times by blotch

(1922, 1925, 1926, and 1927), while scab was most important in the

three seasons 1923, 1924, and 1928.

The totals given for the annual indexes show the total incidence of

disease on the fruit each year. As these figures indicate, 1922 with its

total index of 79.61 is outstanding for the amount of fruit attacked.

Next in order stands 1923 with an index of 69.83. The remaining years

1924 to 1928 inclusive, have total indexes of a much lower grade; bul

1925 with its very low index of 9.09 is very certainly the year of least

fruit injury.

Table XI

SuMM.vRY OF Intensity of Apple Diseases on Fruit

Disease



Diseases of Fruit Trees, 1922-1928 461

however, even though the scab index continued to rise, for the blotch

index fell off so sharply as to cause a distinct lowering in the total

amount of diseased fruit.

The outstanding fact illustrated in Figure 13 is that in spite of the

annual fluctuations of individual disease indexes the total fruit-disease

index moved steadily from the high point of 1932 downward to the low
point of 1925, after which it began again to rise; and though its upwarci
progress has not been uniform, the upward trend is evident through 1928.

/oo

I9ZZ

Pig. 13. Comparison of the trends of the total annual fruit intensity
indexes with those of the annual fruit intensity indexes of each
apple disease. See text for discussion.

INTENSITY OF ATTACK ON TWIGS

Data have been taken, as shown in the special discussion of diseases,

for twig disease produced by blotch, black-rot, and fire-blight. The in-

dexes of intensity derived from these data are brought together in Table
XII, where they are shown comparatively by years; and the total effect

of these diseases is estimated for each year by summing the intensity

indexes.



462 ILUNOIS N.VTIR.VL HlSTOKY SlRVKY Bui.I.KTI.N

With the single exception of 1924, blotch was more intense in its

twig attack than either fire-blight or black-rot; and black-rot was the

least intense in all years. Fire-blight occupied the middle place in all

years except 1924, when it exceeded blotch by the small amount of 2.28

per cent. The year 1923 is outstanding for its exceptionally heavy

manifestation of twig disease ; and it is also worthy of special notice that

1925, a year when fruit attack was at a minimum and leaf disease at a

very, low ebb, has the second place for intensity of twig disease.

The trends of the individual diseases, as manifested in terms of

diseased twigs, is shown in Figure 14 in comparison with the total

percentage of diseased twigs. Starting at the moderate total of 13.45

in 1922, the total intensity of twig disease rose very sharply in 1923

to the high total of 91.35, following very closely in this the upward
trends of blotch and fire-blight, which were opposed by a sharp fall in

the slight intensity of Ijlack-rot. From this high point, the total in-

tensity fell in 1924 to 21.36, chiefly as a result of the sharp decline in

the blotch intensity index; but the rate of fall of the total intensity

line was lessened by an appreciable rise in the fire-blight index and a

Table XII

Summary of Twig Intensity of Apple Diseases

Diseases



Diseases of Fruit Trees, 1922-1928 463

It is of interest to note that, of the three diseases causing twig
injury, fire-blight appears to fluctuate less from year to year than the
other two. The changes exhibited by both blotch and black-rot, as
exemplified in the years 1922-1925, are rather extensive and cause their
lines on the graph to move sharply up and down. It is striking also
that in the four comparisons which the data for year-to-year trends make
possible, the indexes for these two diseases travel consistently in opposite
directions. In the four possible comparisons between blotch and fire-

blight, there are three instances of opposed trends, and one of similar

/oo
ao



464 Illinois N.xriK.vr, Histoky Survey Bulletin

in Illinois, an epidoniic leaf disease. Scab, blotch, black-rot. and rust,

however, have a distinctive type of leaf attack, which results in the kill-

ing or injurying of a particular portion of a leaf while the remainder

of the leaf maintains a healthy, functioning condition. The extent of

the leaf injury which these diseases cause by direct att<ack has been

shown for each disease in the individual discussions, where an index

was given representing the average leaf area occupied by lesions each

vear. These indexes are gathered together in Table XIII for comparison,

and their annual totals, given at the bottom of the table, furnish within

the limits under which they were gathered, a statement of their total

effect in terms of the percentage of leaf area occupied or destroyed.

Taule XIII

Summary of Leaf Intensity of Apple Diseases

Diseases



Diseases of Fruit Trees, 1922-1928 465

broken, the downward trend of the total index line resulting almost
entirely from the steep fall of the scab index, since blotch and rust unite

in the opposite trend. In 1926, again, rust alone is responsible for the

upward turn in the total index line, the trend of all other diseases being
downward, except scab which moved neither up nor down.

The rapid fall in 1927 of the rust index, coupled with only very

slight changes in all other indexes except that of scab, brought the total

index trend down practically to the -Icn-el of the increasing scab index;

/oo
3oY ~^—— 7o/-a/ Index.

Scab
3/o/ch
jBus^
0/ock-ro^

/922 '23 '2&

Fig. 15. Comparisox of the trends of the total ax.xual leaf i>'te>"Sity

indexes with those of the annual leaf intensity indexes of each
APPLE DISE.\SE. SeE TEXT FOR DISCUSSION.

and in 1928 the continued rise of the scab index, though unaccom])anied

by marked changes in the insignificantly low indexes of the other leaf

diseases, started the total index of leaf disease again in an upward
direction.

The general trend of leaf disease was upward from 1922 to 1923,

but downward from 1923 through 1925, while in 192() the constant

level of scab and the high rust index predict the beginning of a later

general rise which becomes somewhat more apparent in 1927 and 1928.



466 Ii.i.ixoTs Natiral History Survey Bulletin

Total Effect of Apple Diseases

The diseases of apple trees combine, in the intensity of their

various types of attack, to injure the trees permanently and to interfere

with the annual production of crops both by the permanent injury they

do and by the season's infection on the fruit and the damage done to

leaves, upon which the tree depends for a supply of material with which

to form its fruit.

The effects of these diseases, as we have observed them in Illinois,

are shown graphically in Fi'gurc 16. The tabulation presented as part

of the graph shows the totals of the annual indexes for each type of

attack, and the cross-hatched blocks of the diagram present a visual

comparison of the yearly sums. Thus, it is evident that 1923 held first

place as a year of destructive disease, while 1926 was the year of least

1922

192 J

1924

1925

15)20

\92~/

192 a

Total Intetibity of

Apple Diseasea

192279.61

1925 69.8:0

1924 29.46

1925 9.09

1^26 12.60

1927 24.71

1928' 19.73

TwLg Leaf_

13.45 12.62

91.35 2222

2U6 7.42

59.30 i24
4.49 I5D2

7.6a 2J6

6.64 2.99

10 aO 50 40 50 60 70 SO 90 100 160 I8t) 190

Fig. 16. Comparison of the total intensity of the annual epidemics of
APPLE diseases

The total indexe.s of fruit, twig and leaf intensity shown in the table

are combined graphically in the block diagrams to show the relative intensity

of all epidemics in each of the years covered by the survey. The tendency of

fruit, twig, and leaf disease to fluctu&.te independently from one year to

another is also shown.

destructive disease. It appears also that the two first years, 1922 and

1923, were characterized by a very great severity in disease, but that in

1924, and in the succeeding years, the intensity of disease attack de-

creased to moderate or small amounts.
The graph also illustrates the proportional effects of each type of

attack in each year and shows how an increase in the intensity of one

type of attack, even when accompanied by decreases in the intensity of



Diseases of Fruit Trees, 1922-1928 467

the other types, may serve to heighten the total effect of disease. Thus
in 1923, the intensity of attack on frnit constitutes a little over 75 per

cent, of the entire manifestation of intensity, while the remaining 25

per cent, is divided quite evenly between twig and leaf intensities. In
the following year, however, the intensity of fruit attack was actually

less by one-eighth and the leaf intensity nearly double that of 1922, yet

the length of the column is greater by nearly three-fourths, because of

the excessive increase in twig disease, which amounts to nearly half the

total manifestation for the year. The very great decrease in the total

manifest disease in 1924 is due to marked shrinkage in all types of

attack, the proportional reduction being greatest in twig disease, which
was less than one-fo\irth as intense as in 1923, while the reduction in

both leaf and fruit intensity was approximately two-thirds.

In 1925, a year known among commercial growers for the very

small amount of apple tree disease, there appears actually to have been
a total manifestation of disease large enough to rank third among the

years covered by our observations ; ])ut it will be seen that this is due
entirely to an increase in twig disease to nearly three times its amount
in the preceding year, and that the intensities of leaf and fruit attack,

the latter especially noticed by the orchardist, decreased to approxi-

mately one-half and one-third, respectively, of their preceding amounts.
The still further decrease in total manifestation of disease evident in

1926 did not materially impress the grower, for the reason that the

decrease resulted very largely from the heavy falling off of twig disease

from an index of 59.30 in the previous year to 4.49 in that year, while

the fruit and leaf attacks, both of which he was more apt to observe,

showed definite increases over the previous year and thereby gave,

falsely, the impression of an increased amount of disease.

In 1927, there was only a very slight increase in the total mani-
festation of disease, but the change from 1926 consisted in doubling of

the fruit intensity index, thus characterizing the season's disease attack

as being commercially destructive to almost the same extent as that of

1924, with an increase in twig disease to but twice its former small

amount and a decrease in leaf intensity to one-seventh. In 1928 there

was not much change in the amounts of leaf and twig disease, but the

total manifestation of disease appeared somewhat less through a decrease

in the fruit infection to about four-fifths of the amount present in 1927.

In general. Figure 16 serves to illustrate and emphasize, from the

point of view of total disease manifestation, the tendency of each type

of attack to fluctuate from year to year entirely independently of other

types of attack. It is also evident that commercial phases of disease, or

those readily observable by growers, are apt by themselves to lead to

erroneous estimates of the total effects of disease attack. It is only by
compiling data on all phases that the real characteristics of any year

with respect to disease severity can be determined.

It is of interest, also, to see what our data indicate in the way of a

general condition which the orchardist must face in the intensity of

disease attack through a period of years. As Avas illustrated in Figure

16, the total intensity of disease and also the intensity of each type of

attack may vary Avidely from one year to another. The average condi-



468 Ilunois Natural History Sirvky Bui.i.ktin

tion derived from these variations may, however, be indicative of the

relative importance of each disease and, in general, also of the relative

importance of each type of disease manifestation over a period of years.

Figure IT conijjares the average annual intensity in fruit, twig, and
leaf attack of the diseases listed in tabh's on previous pages, as shown
by our data. The tabulation which appears as part of the diagram gives

the average annual intensities of each type of attack as exemplified by
our data for each disease and also the total average intensity of each
type of attack brought about by all diseases. The figures in the table are
shown graphically by the series of blocks.

Blach-roti

Blotch. I

Total

Average laterisity of

/Apple Diseases



Diseases of Fruit Trees, 1922-1928 469

justify giving it the first rank in importance for the State as a whole.

In its destructive effect upon the trees, however, it is plainly less im-
portant than blotch, as its leaf infection index of 4.92, though more
than four times as great as that of blotch, is more than compensated
for by the high intensity of blotch twig attack. Hence, it appears that of

all the epidemic diseases, blotch holds first place in importance, at least

in the commercial regions of the state.

The average total intensity of the epidemic diseases, taken over a

period of years, is shown also in Figure 17. Of the three manifestations

of intensity, the fruit attack is the largest, though it exceeds the twig
attack by only a small amount. Leaf disease, often the most conspicuous
phase, appears on the whole to be the least significant, though the more
particular pains taken to measure it may have reduced its expression in

a somewhat misleading way. It seems, however, from the data at hand,
that over a period of years, a fruit grower may expect fruit disease and
twig disease to dominate leaf attack, the first averaging annually about
35 per cent, of the crop, the second, year after year involving an average
of about 33 per cent, of the new tree growth, and the leaf disease

amounting on an average to slightly less than 10 per cent, of the total

leaf area.

In these totals, black rot will play but a small part ; rust will con-

tribute chiefly to leaf-injury; fire-blight will be important in twig
injury; scab will seriously effect both the fruit and leaves; and blotch

Avill be the most serious of the diseases on fruit and twigs, but of minor
importance on leaves.

PEAR DISEASES

In Illinois, pears suffer as a rule from the attack of only one serious

disease, namely blight. There are, of course, other diseases, particularly

two that appear as leaf spots; but the damage they do is very slight

indeed, and it has been our experience that they occur but rarely in

destructive amounts and then only in very restricted areas. The 'leaf

blight" due to the fungus Fahraea macvlata (Lev.) Atk. is perhaps the

more serious of these, as it also appears at times upon the fruit; and
the more common Septoria leaf-spot (caused by Mycosphaerella sentina

(Fr.) Schr.) does very little injury. Con.sequently, our data on pear

diseases is concerned only with pear blight.

Pear Blight

Caused by Bacillus amylovorits (Burr.) Trev.

The commercial production of pears in Illinois is very largely con-

ditioned by the annual epidemics of pear blight. The disease, caused by

the same bacterium as fire-blight on apples, manifests itself as ])lossom

and twig blight, as spur blight, and as a very severe canker of the limbs

and trunk. Its destructiveness, which may be seen to be very great, has

been observed by pear growers to vary to such an extent from year to

year that they conmionly designate years as of mild or of heavy blight.



470 Il-UNOIS N.\TURAL HISTORY SlRVEY BULLETIN

In measuring the annual epidemics of pear blight in Illinois, we
have taken into account only the phase of its attack known as twg-
bliglit. including in this general type, however, the spur-blight phase

and leaving out for the present any separate consideration of blossom-

blight or cankers. Our data consist of percentage statements, obtained

by representative counts, of the prevalence of blight infection among
trees and of the intensity of the attack, expressed as the percentage of

blighted twigs and fruit spurs.

Data on Pear Blight

Because of the scattered distribution of pear ])laii1iitions, as well as

because of the relatively small number of pear trees grown in the State,

we have not been able, in the course of our survey, to secure as abundant
records for pear diseases as for the diseases of the more important
fruits.

Table XIV

Prevalence and Intensity of Pear Blight



Diseases of Fruit Trees, 1922-1928 471

prevalence, the index being 96.28, and an even greater relative increase

in twig-blight intensity, Avhich has an index of 18.44.

For 1924, the prevalence of pear blight was determined to be 71.64
per cent, and the twig-blight intensity 20.53 per cent. This shows a
decided fall in prevalence from the previous year but a slight increase

in intensity.

The epidemic of 1925, as shown by our 13 observations, was milder
than those of either 1923 or 1924, but more severe than that of 1922.
Its prevalence was 45.33 per cent, and its intensity 5.61 per cent. This

Fig. 18. Trends of prevalence and intensity of peae-blight.
discussion

See text for

tendency to decrease Avas not maintained in 1926, however, for in that

year 21 records representing 4,806 trees gave the high prevalence index

of 81.19 and a twig intensity index of 14.21.

In 1927 the epidemic was again mild, the prevalence of the dis-

ease being only 30.72 per cent, and its twig intensity 2.31 per cent.

This very sharp reduction in the severity of blight was continued into

1928, when the prevalence index reached the very low point of 5.53 and
the twig intensity index fell to 0.16.

The year-to-year trends of the prevalence and twig intensity phases

of the pear blight epidemics are shown graphically in Figure 18. In



472 Illinois Natural Histoky Siuvky Bulletin

general, the movement of both phases follows the same trend, though

between 1923 and 1924, when prevalence moved downward and twig

intensity upward, tlie opposite appears to liave been true. The upward
or downward trend is usually more pronounced in the intensity phase

than in the prevalence phase.

The graph illustrates, also, tiie great prevalence and destructive-

ness of the blight epidemics in the years 1923, 1924, 1925, and 1926,

which are in marked contrast with the rather mild destructiveness of

1922, 1927, and 1928.

Through the period covered by our data, as may be seen in Table

XIV, pear-blight has been present on an average of 50.16 per cent, of

the trees we have examined, and 8.98 per cent, of all twigs on all these

trees have been attacked by the disease.

PEACH DISEASES

Though they have always been important limiting factors in pro-

duction, peach diseases have recently taken on a still greater economic

significance in Illinois because of a great increase within the last 10

years in the number of commercial peach plantations set out within the

State. The Bureau of Agricultural Economics of the U. S. Department
of Agriculture made a survey of the peach trees in Illinois during the

fall and winter of 1925-1926, the results of which, made available in

May, 1926, indicate that there were in the State at that time 2,293

peach orchards of 100 or more trees. And it was estimated that there

were in these orchards 1,923,012 peach trees, of which a little over

320,000—approximately one-sixth of the entire number—were less than
two years old and 772,000, or 40 per cent., were two and three years old.

Since the completion of this survey, these trees have come into bearing

and more orchards have been planted, and both together are yielding

crops estimated at 1,122,000 bushels in 1927 and 1,638,000 bushels in

1928. The farm value of the 1927 crop was estimated to be $2,300,000
and that of the 1928 crop $2,293,000.

All the important or destructive diseases that occur on peaches in

Illinois are epidemic in their behavior, that is, they attack one or more
parts of the tree anew each year; and the success of each year's attack

is determined in large measure by the prevailing weather. In the

measurement of these annual epidemics, the methods that have been
used are the same as those described for apple diseases.

Peach Brown-rot

Caused by Sclerotinia cinerea (Bon.) Schroeter

The attack of the brown-rot disease on peach trees is by no means
limited to the fruit, from which it takes its name, but is destructive also

to blossoms and twigs. In measuring its annual epidemics, however, we
have had to limit the collection of data to the fruit-rot phase of the

disease, except for the years 1926 and after, when data were taken on
twig-blight also. At present, however, we intend to present the accumu-
lated data on prevalence and fruit-rot only.



Diseases of Fruit Trees, 1922-1928 473

Data on Brown-rot

During the period of our survey, we have observed brown-rot in

135 peach plantations, an average of 19 per year. The average number
of trees furnishing the annual records is 15,973 and the average number
of counties represented yearly is 8. As may be seen in Table XV, the

average annual prevalence of brown-rot indicated by these records is

42.67 per cent., while the average intensity of its attack on fruit is

5.73 per cent.

Table XV

Prevalence and Intensity of Peach Brown-rot



474 Illinois Natural Histoby Slrvky Bli.i.etin

rose to 55.88 per cent, and that for fruit intensity to 8.47 per cent.,

marking the epidemic of that year as second in intensity and third in

prevalence among the years covered by the survey. With the arrival of

1928. however, brown-rot once more encountered unfavorable conditions

and dropped in prevalence to 22.68 per cent, and in intensity to 1.52

per cent.

The trend of these annual brown-rot epidemics is sho^vn graphically

in Figure 19. It may be seen that with the exception of the change

from 1925 to 1926, where the intensity of the fruit attack appears to

have lagged behind the rising prevalence, the trends of both the pre-

valence and the intensity indexes follow the same courses throughout.

'27 '26

Fig. 19. Trends of the prevalence and intensity of the brown-rot and

bacterial shot-hole diseases of peach

The trend lines for both diseases show sharp changes from year to year,

indicating their sensitivity to weather change. The year 1925 was marked

by a sudden drop in the prevalence and intensity of both diseases, from which

the bacterial shot-hole recovered in 1926. Brown-rot, however, though recover-

ing its prevalence within a year, exhibits a lag of a year in recovering its

former intensity.

The sharp movements both upward and downward of the prevalence and

intensity lines indicate also that the disease is very greatly influenced

bv annual fluctuations in weather.



Diseases of Fruit Trees, 1922-1928 475

Bacterial Shot-hole

Caused by Pseudovionas Priini E. F. Smith

This disease, named from the shot-hole effect which its spots pro-

duce on peach leaves, also attacks the fruit and twigs. Of these three

modes of manifestation, leaf attacks and fruit attacks are the most seri-

ous. Our data, however, will he presented to show only the prevalence of

the disease and the intensity of its leaf attack.

In order to determine the annual intensity of shot-hole on [jeaeh

leaves, the scale shown in Figure 20 has been devised. It is used in the

same manner as the other leaf-spot scales which we have already de-

scribed in connection with apple scab and other diseases. It should be

emphasized especially that in this case as in all others the purpose of

the scale is to measure the intensity of the disease attack in terms of

Fig. 20. Scale for measuring the intensity of attack of bacterial shot-

hole ON PEACH leaves

diseased leaf area and that it does not take into account the effect of the

disease, which is to produce a distressingly high percentage of defolia-

tion whenever the trees it attacks are growing in poor soil or when \\'arm,

moist atmospheric conditions prevail.

Data on Shot-hole

A total of 259 records have been secured for the bacterial shot-hole

of peach during the course of our survey. As summarized in Table XVI,
from an annual average of 37 records of the disease on 39,221 trees dis-

tributed among 17 counties, they indicate that shot-hole prevalence has
—

5



476 Illinois N.\tubal History Survey Bulletin

had. the high average annual index of 77.11 per cent, and that the in-

tensity of its attack on leaves has resulted, as an average, in occupation

by lesions of 10.33 per cent, of the leaf area of all trees observed.

Table XXl also shows the variation, as indexes of prevalence and
intensity, of the annual epidemics. Data taken in 1922 from 90 plan-

tations containing 12,931 trees indicate a prevalence for that year of

79.63 per cent, and a corresponding leaf intensity of 3.31 per cent.

In 1933, according to records taken in 21 plantations containing a total

of over 76,000 trees, the prevalence of shot-hole dropped to 59.59 per

cent, and its leaf intensity fell to 1.37 per cent. The following year,

1934, the data contained in 40 records representing 44,640 trees show a

considerable increase in prevalence, the index rising to 95.53 per cent,

and an even shar])er rise in leaf intensity, the index for which stood

at the very high level of 53.53 per cent.

Table XVI

Prevalence and Intensity of Peach Shot-hole



Diseases of Fkuit Trees, 1922-1928 477

vailed among 73,505 trees, prevalence was shown to have fallen slightly

to 97.47 per cent, and leaf intensity somewhat more sharply to 2.87

per cent.

The trends from year to year of the shot-hole epidemics, as ex-

hibited by their prevalence and leaf intensity indexes, are shown in

Figure 19. "With the exception of the change from 1926 to 1927, the

trends of prevalence and intensity follow the same course; but it is

evident that the changes in intensity are much sharper than those in pre-

valence. Following the very unfavorable year of 1925; both trends rise

sharply and there is no apparent tendency for one to lag behind the

other, as appeared to be the case with brown-rot. However, the changes

from one year to another are rather sharp, as is observable also for

brown-rot, and indicate that the disease is remarkably sensitive to an-

nual changes in weather.

Peach Leaf-cdel

Caused by Exoascus de-formans (Berk.) Fuckel.

Leaf-curl is a common and often destructive disease in every peach

region in the world and it is, naturally, prevalent throughout Illinois.

It attacks the tree in a number of ways, causing a severe type of twig

blight, attacking the blossoms and fruit and causing them to fall from
the tree, and producing especially a leaf disease, which is termed curl

from its characteristic appearance.

We have thus far found it impractical to measure accurately the

intensity of leaf-curl attack. As the disease manifests itself in Illinois,

it seldom produces any noticeable damage on either the flowers or the

fruit, but it occasionally appears in limited regions as a severe twig blight

and is everywhere prevalent as a leaf disease. Upon the leaves, it may
appear in very small or in large amounts ; but it always results in such

a curling and malforming of the leaves that we have not been able to

devise a workable series of measuring standards with Avhich to gauge its

leaf intensity. Consequently, we furnish here only our data on its pre-

valence.

Data on Leaf-curl

We have obtained an average of 25 records per year on the pre-

valence of leaf-curl during the years covered by our survey. As shown

in Table XVII, these records represent annually an average of 8,879

trees, and the disease is indicated to have an average annual prevalence

of 52.02 per cent.

In 1922, when the prevalence of leaf-curl among 5,873 peach trees

was shown by the data in 64 records obtained in 27 counties, the index

stood at 36.23 per cent. In the following year prevalence increased to

the highest point in our record, 95.64 per cent.; but in 1924 it de-

creased, reaching 19.6 per cent., the lowest point in our records. For

1925 we have no adequate data; but an increase in prevalence to 51.17

was shown by the 15 records taken in 1926; and this tendency to in-

crease was continued into 1927, when on the basis of data contained in

26 records drawn from 21,548 trees the prevalence index was placed at



478 Illinois Natural History Survey Bulletix

Taule XVII

Prevalence of Peach Leaf-curl



Diseases of Fruit Trees, 1922-1928 479

84.97 per cent. This high level was not maintained, however, for in

1928, when 7 records showed the state of the disease among 13,400 trees

located mainly in the worst leaf-curl region of the State, the prevalence

index stood only at 24.51 per cent.

The trends of these annual epidemics are shown graphically hy the

solid line in Figure 21. The tendency to fluctuate sharply from one year

to another, Avhich has been apparent with other peach diseases, is also

marked in the case of leaf-curl. Though data are lacking for 1925, it is

reasonable to suppose that the disease behaved as did most other diseases

that year, falling to a very low point of prevalence ; and this would give

the leaf-curl graph an even more irregular aspect.

Peach Scab

Caused by Cladosporium carpophiluni Thuemen.

AVith its general range including Europe and the entire eastern

half of the United States, peach scab is, of course, found commonly
throughout Illinois. Like the other peach diseases, it manifests its

presence by attacking the twigs, the leaves, and the fruit; but the two
phases confined to twigs and leaves are relatively rare and do but little

injury to the trees. It is the fruit attack that gives this disease its im-

portance; and as this is its most common manifestation in Illinois, we
have chosen to measure the intensity of the annual scab epidemics in

terms of infected fruit.

Data on Peach Scab

In all, 122 records of peach scab have been gathered during our

survey, an average of 20 per year, representing as an average 14,098

trees per year. As is indicated by the summary of our data given in

Table XVIII, the average annual prevalence of the disease in Illinois

stands at 66.53 per cent., while its average annual intensity, expressed

as the relative proportion of infected peaches, stands at 41.34 per cent.

This latter figure does not, of course, indicate the economic loss which

results from the disease, since in taking our counts we include all in-

stances of infection, no matter how light they may be.

The annual variations exhibited by the peach scab epidemics are

also stated in Table XVIII. In 1923, when a total of 4.097 trees dis-

tributed among 16 counties furnished 3.6 usable records, the prevalence

of the disease amounted to 62.68 per cent, and the intensity of the fruit

attack to 24.31 per cent. The following year these indexes were found,

on the basis of .25 records including 9.673 trees distributed among 18

counties, to have increased quite decidedly, that for prevalence having

reached 88.83 per cent, and that for fruit intensity 71.99 per cent.

And though in 1924 there were only 9 records representing only 1.526

trees, prevalence continued to rise, reaching the very high index of 100

per cent., which was attained also by the fruit intensity index.

No records are available for 1925; but since the disease entirely

escaped the notice of horticultural experts that year, it was probably

neither very prevalent nor very destructive. In this, it would have fol-

lowed the same course that was taken by other peach diseases that year.



480 Ir.r.iNois N.\TiK.\L History Survtsy Bulletin

In 192G. however, there appears to have been abundant scab in-

fection. Thoiifjli only 11 records were secured and only 113 trees were

represented by them, they were distributed among 6 important peach-

growing counties; and the high prevalence index of 100 per cent, and
the moderately high fruit intensity index of 43.7fi per cent. ])robably

represent the relative state of the disease rather accurately.

In contrast with these years of great prevalence and intensity, 1927
and 1928 stand out as years of mild scab. The six records taken in

1927 apply to orchards containing a total of 4,100 trees and furnish a

prevalence index of 24.48 per cent., while in 1928 the 35 records ac-

cumulated represent a total of 65,080 trees and give a prevalence index
for scab of 23.22 per cent. The indexes indicating the intensity of the

scab attack on fruit for these two years were 4.27 per cent, and 4.75 per
cent, respectively.

Table XVIII

Prevalence and Intensity of Peach Scab



Diseases of Fruit Trees, 1922-1928 481

SUMMAKY OF PeACH DISEASE DaTA

In the preceding discussion of peach diseases the data gathered

each year for each disease have been given in tal)ular summaries, and
comparisons have been made of the prevalence and intensity of each

disease.

Prevalence of Peach Diseases

In Table XIX. the annual indexes of prevalence for the several

peach diseases have been brought together so that the diseases may be

compared with one another.

In the first year of our survey, brown-rot stands out as the most
prevalent of the peach diseases, but the bacterial shot-hole disease and
scab also had high indexes, while leaf-curl occupied the lowest position.

For 1933, the average incidence of diseases per tree was 3.67. In 1933,
the diseases formerly occupying first and last placecs changed positions

Table XIX

Summary of Prevalence of Peach Diseases

Diseases



482 Ii.uNois Natikal Hi.stouy SruvKY Bulletin

to a very low point. The average incidence of diseases per tree attained

the high point, 2.64,

In 1928 bacterial shot-hole was the only disease exhibiting a high

degree of prevalence. Brown-rot fell off markedly, and so also did leaf-

curl, while seal) remained at essentially the level of the year before.

Consequeatly, the average incidence of disease per tree fell to 1.68, the

lowest point in our record with the exception of the incomplete 1925.

The trends of prevalence for each of the peach diseases are shown
in the lower ])art of Figure 22, and the trend from year to year of the

average incid(Mice of diseases is shown in the upper part. From 1922 to

Fig. 22. Comparison of the prevalence trends of peach diseases

The lower part of the figure gives the trends for each disease and the

upper part the trend of average annual incidence of disease per tree. The
break in the lower part is necessary to show the great drop in brown-rot
prevalence in 1925. Data for 1925 are lacking for leaf-curl and scab.

1923 the trends of leaf-curl and scab moved upward, but this was
counterbalanced by a downward movement by brown-rot and shot-hole,

with the result that the level of average incidence was scarcely changed.

The upward movement of scab was continued into 1924, and
brown-rot and shot-hole both reversed their downward trends, as did



Diseases of Fruit Trees, 1922-1928 483

leaf-curl its up^-ard trend, with the result that, in spite of the steep fall

of leaf-curl, the trend of average incidence was turned distinctly-

upward.
Partly through lack of data for two diseases, but more evidently

because the year was very unfavorable to peach diseases, as is indicated
by the steep falls of the brown-rot and shot-hole trend lines from 1924
to 1926, the average incidence curve was bent down very sharply in
1925. With all four diseases tending to increase their prevalence in
1926, the average incidence line returned again to nearly a normal
level; and with the continued rise in the trends of brown-rot, leaf-ciirl,

and shot-hole, the incidence curve rose a trifle higher in 1927 and was
held to a level slightly below that of the 1922 only by the sharp decline

of scab. Between 1927 and 1928 all trends were downward, those of

brown-rot and leaf-curl very distinctly so, those of scab and shot-hole
perceptibly so, with the result that the trend of average incidence moved
downward also.

I 2.21

J I L
O /O 20
/f//e. /r7c/c/<z/7ce

ScaS 66!53\

^^^^^^^^^^^^^^^^

ZO 30 4iO so so 70 8o so /oo

Fig. 23. Comparison of the average annual prevalence of peach diseases

Brown-rot is prevalent to the least extent and bacterial shot-hole to the
greatest, while leaf-curl and scab are intermediate. These four diseases, by
their combined attack, give an average annual incidence of disease of 2.21

per tree.

There apj)ears to be but little agreement among peach di.seases as

to their prevalence reaction in ordinary years. While certain ones may
move together in one year they are apt to move in opposite directions

in another year. Each disease appears to be influenced to a marked
degree, either favorably or unfavorably, by changes in some one or more
factors in the environment provided from year to year. It is notable,

hoAvever, that in spite of the great fluctuations exhibited by the in-

dividual diseases, their various reactions result, on the whole, in main-
taining the curve for the average incidence of disease at a surprisingly

uniform level.



484 Ilunois Natitral History Survey Bulletin

By summing the annual indexes of ])reYalence for each disease and

apportioning this sum equally among all the years for ^vhich we have

records, a statement may be obtained of the average prevalence of each

disease over a period of years. This is illustrated in Figure 23. Brown-

rot is shown to be, on an average, the least prevalent of the peach diseases,

having an average annual prevalence index of 42.67 per cent. Leaf-

curl, with its average annual prevalence index of 52.02 per cent., is

approximately one and one-fifth times as prevalent as brown-rot; scab,

with its average index of 66.53 per cent, is one and a half times as

prevalent as brown-rot ; and bacterial shot-hole, with an average index

of 77.11 per cent., is one and four-fifths times as prevalent as brown-rot.

The average incidence of peach diseases per tree, as indicated in

Figure 23, is 2.21.

Intensity of Peach Diseases

Owing to the fact that in our survey of peach diseases we have not,

as a rule, measured the intensity of any one disease by more than one

of its various manifestations, we shall not attempt to make the same

variety of comparisons that were made with the apple diseases. But in

order to make as complete a statement as possible, we shall introduce

some data on types of intensity not given in the previous discussion.

These data, as will be seen by examining Table XX. show the results

of our observations on the intensity of the fruit attack of the bacterial

shot-hole disease in 1927 and 1928, the intensity of the twig attack of

brown-rot in 1926, 1927, and 1928, and the intensity of the twig attack

of leaf-curl for all years except 1925 when no data were taken. We do

not believe, however, that the twig intensity indexes for leaf-curl for

the years prior to 1926 are sufficiently accurate to indicate in more than

a relative way the intensity of the disease.

Table XX

Summary of Intensity of Peach Diseases

Diseases



Diseases of Fruit Trees, 1922-1928 485

Always bearing in mind the limitations of accuracy that affect these

data, the material given in Table XX can be rearranged graphically, as

is done in Figure 2-i, to show the variations in total annual intensity

of the peach diseases. The table included in this figure gives by years
the totals of the annual intensity indexes of all diseases, as listed in

Table XX, for each type of disease manifestation, and these totals are
represented graphically by the blocks in the diagram. Hence, each year's

block shows in black the percentage of fruit infected by all diseases, by
cross-hatching the percentage of twigs infected by all diseases, and by
vertical shading the total percentage of leaf-area occupied by disease

lesions.

The year 1924 stands out as the year of most intense disease epi-

demics; but 1925, though following immediately, had the least intense

1922

/923

/924

/9Z5

/926

/927

/92S

C Toto/Inf'ensity
ofPeachO/'seases

Fruif 7iv/g /.ea^

1922 26.60343 ~22/

/923 72^22273 /.27

/924I2Z36 4.70^353
/925TnKe — Oj?l

/92642.76 4.03 8.4/

/92726^2/S3e 4j37

/928 37/t7S97 2JSr7

Hey

/O 20 30 jiO 50 SO TO SO 90 /OO //O /20 /30 AX? /50 /60 /70 /SO /90

Fig. 24. Comparison of the total intensity of peach diseases by years

The table in the chart gives the sums of the indexes of fruit, twig, and
leaf intensity of all diseases, and these sums are shown graphically by bars
for each year. See text for further explanation.

epidemics. All phases of intensity vary from year to year; but that on
fruit, though varying to a greater extent than any other, has always'

been the most important. Excepting in 1924 and 1926, twig intensity

was of more importance than leaf intensity, but it is readily seen that

in the 1924 block fruit intensity and leaf intensity make up by far the

greater part, as is true also of the 1926 block.

Through 1924, peach diseases appear to have been increasing very

rapidly in the intensity of their epidemics; but with the advent of the

universally unfavorable 1925 all epidemics appeared in mild form. Sub-



486 Ilunois Natural History Sirvky Bui.lktix

sequently. however, peach diseases increased the intensity of their at-

tacks and attained a \e\e\ in 1926, 1927, and 192.S somewhat higher than

they liad occupied in 1922.

It is worthy of note that the tendency of peach disease to attack

fruit to a greater extent than any other part of the tree gives them an

especial importance, inasmuch as they are therefore an annual threat

against the harvesting of an ahundant and profitable crop.

The average comparative intensities of each of the peach diseases,

as they have manifest themselves during the period covered by our sur-

vey and as they probably may be expected to manifest themselves over a

similar period of years, are shown by the table and diagrams in Figure

25. In the table, the figures show the averages of the annual intensity

indexes of each disease for each of its measured manifestations, and

these values are shown diagrannnatically for easy comparison by the

horizontal shaded blocks.

Leafcur/

Bacfizna/

5hof/7o/z

Scab

Average //fferrsifyof
Peac/;D/sec7ses

rrui'f Twig Leaf
Bro^/7-rof- J5.73 /34~^^
^eafcur/ — ao7 —
5hof-Me Z2/i9 — /Q33
5cab 4/.34 — —
Aey ^^ ^

I I I I'l I I I I I I I M I I i
I 1 I I M I I I I I I I h I I I I H I I I I I I I I

O 5 /O /^ ZO 25 JO 35 ^O 45

Fig. 25. Comparison of the average annual intensity of peach diseases

The table gives the average annual intensity index for each measured

manifestation of each peach disease and the length of the bars expresses

these indexes graphically. For further explanation see text.

Brown-rot, though it has been measured in terms of fruit intensity

and twig intensity, does not equal any other diseases in either of these

respects; and its epidemics may therefore be considered the mildest

from the disease standpoint, though not from the commercial, of any of

the peach diseases. ' Leaf-curl, the intensity of which we have measured

only in terms of diseased young growth, exceeds in this respect brown-

rot, and the two diseases combine to furnish almost the total amount of

twig destruction. Bacterial shot-hole, for which indexes have' been com-

puted to show the intensity of attack on fruit and leaves, exceeds brown-

rot by nearly four times in its fruit intensity and contributes almost

the entire amount of leaf area reduction not attributable to the attaciv

of leaf-curl. Scab, which we have measured only as a fruit disease,

exhibits the greatest average intensity of attack of any of the peacn

diseases.



Diseases of Fruit Trees, 1922-1928 487

The comparisons made in Figure 24 ought not, however, to be un-

derstood as in any way representing the relative commercial importance
of the peach diseases. Brown-rot, ranked on the basis of our data as

least important in the intensity of its epidemics, is the most important
commercially of the four ; and bacterial shot-hole, shown by our diagram
to be milder in its epidemics than scab, usually out-ranks scab very

greatly in its injurious effects upon the peach crop. This, however, only

serves to emphasize the necessity of considering the disease separately

from its effects on yield, if data are to be useful in explaining the annual
fluctuations in prevalence of disease and in intensity of disease attack.

CHEERY AND PLUM DISEASES

As was pointed out in our discussion of the distribution of fruit

trees in Illinois, neither cherries nor plums are grown as extensively as

apples and peaches. For the most part, they are to be found in small

groups in farm orchards, though there are of course larger plantations

of a hundred trees or more scattered here and there through the State.

As a consequence, these fruit trees do not in general receive the same
care, either in cultivation or in spraying, as the more important kinds

of fruit trees; and this naturally results in a greater prevalence of

disease and usually in a greater intensity of disease attack.

The epidemic diseases of cherries and of plums are similar. Brown-
rot, previously discussed as an important peach disease, also attacks both

cherries and plums. The bacterial shot-hole disease of peaches is very

common on ]jlums and is occasionally encountered on cherries. Both
cherries and plums suffer also from leaf-spot injury resulting from the

attack of fungi classed as species of Coccomyces.

In meavsuring the annual epidemics of cherry and plum disea.ses, we
have taken into account both prevalence and intensity; but owing to the

abundance of disease upon the leaves, we have distinguished in our

records only brown-rot and leaf spotting, including in the latter cate-

gory all types of leaf spotting, without regard to cause. Figure 26 shows

the scale we have used in estimating the leaf-area occupied or destroyed

by leaf-spot diseases of cherries, and Figure 27 shows the scale we have

used for plum leaf spotting.

Data on Cherry and Plum Diseases

During the period covered by our survey, we have secured data on

the diseases of plums and cherries more or less incidentally as we carried

on our work with the diseases of more important commercial crops.

Nevertheless in 1922, 1923, and 192-1 data were obtained on brown-rot

attack from a total of 2,127 cherry trees; in the years 1922-1924 and

1926 and 1927, data on brown-rot were secured from 1,069 plum trees;

and through the period of the survey data on cherry leaf spotting was

taken from a total of 10,796 trees and on plum leaf spotting from a total

of 408 trees. The results of these examinations are summarized in Table

XXI.
In 1922 the brown-rot epidemic on cherries was very light. The

prevalence of the disease had the very low index of 12.17 per cent..



488 Illinois Natural History Survey Bulletin

while the intensity of its attack on fruit was only 1.12 per cent. But

in 1923 the epidemic was severe, prevalence then rising to 100 per cent,

and intensity to 17.21 per cent. In the following year, 192-1. there was

no appreciable reduction in brown-rot prevalence. The index for that

year stood at 99.94 per cent., a reduction of only .06 per cent, from 1923;

but the intensity of brown-rot attack on the fruit increased to 29.05 per

cent.

Fig. 26. Scale for estimating the intensity of spotting on cherry leaves

In each example the area of the spots is expressed as a percentage of the

area of the leaf. No effort is made in using this scale to distinguish between

leaf-spots resulting from different causes.

Though these three are the only years of the period covered by our

survey for which we have representative data, they serve to indicate

the importance of brown-rot as a cherry disease. With an average an-



Diseases of Fruit Trees, 1922-1928 489

nual prevalence of 70.7 per cent, and an average annual index of fruit

intensity amounting to 15.79 per cent., this disease is annually a menace
to cherry production.

Leaf spotting of cherry trees, as may be seen in Table XXI, al-

ways has a high prevalence. In 1932 its prevalence index was 93.66

per cent., in 1931 it was 99.35 per cent., and in all other years except

Fig. 27. Scale for measuring the intensity of spotting on plum leaves

In each example the area of the spots is expressed as a percentage of the
area of the leaf. This scale measures roughly the combined intensity of the
bacterial shot-hole and coccomyces leaf-spot lesions.

1935, for which we have no adequate data, it was computed to be 100

per cent. Its prevalence in Illinois, as exemplified by our data for six

years, averages annually 98.66 per cent.



490 Illinoi.s N.\TiR.\i, History Subvky Bulletiist

There is, however, a very considerable annual variation in the in-

tensity of cherry leaf spotting. In 1922 the index of intensity stood at

.55 per cent.; in 1923 it rose to 1.97 ])er cent., nearly four times the

previous amount : and in 1924 it ro.^c still further, reachinu- 4.G5 per cent.

No data are available for 1925, but the index for 192G, 2.28 per cent.,

suggests that leaf spotting in common with several diseases of other

fruit trees, may have decreased very considerably. The year 1927 is

remarkable for the exceptionally intense attack of leaf spotting diseases

on cherries. The index for that year, 15.69 per cent., the highest we
have recorded, is merely three and a half times as great as the previous

high index of 4.65 per cent, recorded in 1924. It is rather surprising,

after so intense an attack, to find the index of intensity falling in the

next year, 1928, to so small an amount as .69 per cent., but this serves

to emphasize once more the tendency of diseases to fluctuate greatly

from one year to another, under the influence of varying weather con-

ditions.

Table XXI

Prevalence and Intensity of Cherry and Plum Diseases

Diseases



Diseases of Fruit Trees, 1922-1928 491

disease increased in severity in 1923 to the point of attaining an index
of 49 per cent. Though this high percentage of fruit disease fell off to

the not very moderate amount of 29.42 per cent, in 1924, the attack

again attained such serious proportions in 1926 that the intensity index
stood at 80 per cent. This was reduced in 1927 to 25 per cent.; but
the seriousness of plum brown-rot as an annual menace to the plum
crop is well expressed by its average annual fruit intensity index of

38.52 per cent.

Leaf spotting of plums appears in our records only for the years

1922, 1923, and 1924. In the first of these years its prevalence among
the plum trees examined was 95.58 per cent.; in 1923 it was 100 per
cent. ; and in 1924 it fell slightly to 96.49 per cent. Its average annual
prevalence, as indicated by these three indexes, is 97.36 per cent.

Cherry

P/um

3rouvn-rof 70.70\

Srotvn-rof 96.45\

Lzaf-spof 9736 \

/O 20 30 40 £0 60 70 SO
Preya/e.rjce /r7c/e.x

90 /OO

C/rerry

P/(//?7

\ /5.7S\ 6roy<fn-rof

I

Leafspof

38.S2\Brown- rof

/O /5 20 25 30 35 40 45 50
/nferrsify /r?c/ex

Fig. 28. Comparison of the average annual prevalence and intensity of
cherry and plum diseases

Brown-1'ot is less prevalent on cherries than on plums but leaf spotting
is nearly equally prevalent on both. In intensity brown-rot is more severe
on both cherries and plums than is leaf spotting. The intensity of leaf spot-

ting is nearly equal on both fruits. (Brown-rot prevalence should read 97.04

instead of 96.45. See Table XXI.)

The intensity of the leaf-spot diseases for these vears is shown in

Table XXI as 1.53 per cent, in 1922, 6.71 per cent.^in 1923, and 4.8

per cent, in 1924. These three indexes indicate an average annual in-

tensity of 4.35 per cent.

The average annual prevalence and intensity of the cherry and
plum diseases are compared graphically in Figure 28. Brown-rot may
be seen to be less prevalent on cherries than on plums and much less

prevalent on cherries than is the leaf spot either on cherries or plums.

On plums, brown-rot and leaf spotting are nearly equally prevalent.

—6



492 Illinois Natural History Survey Bulletin

In the average intensity of its attack on cherries, as illustrated in

Figure 28, brown-rot exceeds leaf spotting, but is less than one-half as

severe as it is on plums. There is little difference between the average

intensities of leaf spotting on cherries and on plums.

SUMMAEY OF DATA

In our detailed discussion, the methods employed in taking data on
fruit-tree diseases have been described; the simple statistical methods
used in evaluating these data have been explained; and the results of

our annual surveys of fruit-tree diseases, determined by this statistical

treatment, have been presented in a manner designed to show with

exactness the year-by-year variations in the prevalence and severity

phases, as well as the average condition, for each important epidemic

disease of each kind of fruit tree. These results, beihg based, without

exception, upon separate percentage calculations for every phase of dis-

ease attack, are entirely comparable, whether comparisons be made be-

tween the indexes of one disease for different years or between the in-

dexes of different diseases of the same or of different kinds of trees.

The only limitation that can be "placed upon these comparisons arises

from natural and unavoidable variations in the quantity and depend-

ability of field data; but these matters can readily be tested and evalu-

ated by several well-known statistical indicators, such as the standard

deviation or the probable error.

It now remains to present, from these same data, a general picture

of fruit-tree disease, in Avhich the total effect of disease can be clearly

seen, in its year-to-year variations and in its average condition, upon
each kind of fruit tree. In so doing, the mass of detail given earlier

must be combined as much as possible; but it is still desirable to main-
tain, both for clearness and because of the impossibility of uniting the

several modes of disease attack in one inclusive measurement, the dis-

tinctions we have made not only between prevalence and intensity but
also between fruit disease, twig disease, and leaf disease.

We have enumerated for apples five separate epidemic diseases; for

pears, one disease; for peaches, four; and for cherries and plums, two
each. Fruit trees, like other living things, are not rendered immune to

further maladies by becoming infected with one disease. As every

orchardist knows, an apple tree may suffer from scab alone, from blotch

alone, or from both, a:nd even from blight, black rot, and rust also. So
it may readily be understood that if the several sets of indexes of all the
diseases of one kind of fruit-tree for any one year are added together
their totals will show the amount of disease on the average tree.

Annual Prevalence of Feuit-teee Diseases

For apple diseases in 1922, the scab index for prevalence of 93, the

black rot index of 65.64, the blotch index of 53.03, the fire blight index

of 42.64, and the rust index of 26.29, when added, give a total of 280.60.

This at first suggests what is apparently unreasonable, that in every 100

trees the percentage of diseased trees was 280.6; but actually the mean-
ing is that, on an average, in every 100 trees there were about 281 cases



Diseases of Fruit Trees, 1922-1928 493

of disease, when the separate kinds of disease are recognized. An equally
concrete, perhaps a more easily understood statement of the same fact
is that the individual tree was attacked on an average by 3.8 diseases.

The sums of prevalence indexes, when regarded in this way, are an ex-
pression of the number of cases of disease per 100 trees, and by dividing
this by 100 we obtain a number which we may very properly term the
average incidence of disease per tree.

The average incidence of disease per tree, because of the manner in
which it is derived, expresses briefly and accurately the prevalence of
disease in any year or, by an average of years, for any desired longer
period of time.

In Table XXII the average incidence of disease per tree, derived
by totaling the prevalence indexes given previously for the several dis-

5 r

^4

^3

:«

r
w
^

Wk Peach
\P/um U Cherry

WtPear

1922 /SZ3 /S24 /925 /926 /927 /928

Fig. 29. Comparison of the annual incidence of disease per tree for each
of the five tree fruits

Apples are most subject to disease attack every year and peaches are
next. Plums and cherries are nearly equally subjected to attack, and pears
less than any. This ranking on the basis of disease prevalence would not,
however, correspond to a ranking on the basis of commercial injury.

Table XXII

Average Incidence of Diseases Per Tree

Kind of tree



494 Illinois Nati-ual History Survky Bulletin

eases and dividin;r the sum by 100, is fjivcn by years for each of the five

kinds of fruit trees dealt with.

Upon comparing: tlie items in tliis table it will be found that the

ap])le and the peach, our two most important horticultural crops, suc-

cumb quite consistently to the attack of more diseases than do the less

important pear, cherry, and plum. It may be seen also that the total of

disease prevalence fluctuates from year to year just as noticeably as the

prevalences of single diseases.

The data tabulated in Table XXII are shown <;ra])hically in Figure

29. The height of the columns represents the average incidence of disease

per tree. It is evident that of the five kinds of fruit trees important in

Illinois horticulture, the apple has in every year been most subject to

the attack of epidemic diseases. In comparison -with other kinds of fruit

trees, the prevalence of disease on the apple has varied the least from

year to year. The peach, next to the apple, has been most subject to

disease attack; but the prevalence of disease among peach trees has

varied to a far greater extent from year to year than it has among apple

trees. The plum and the cherry, being as a rule susceptible to the same

diseases, have ])een subjected almost equally to disease attack, though

the actual prevalence of disease may be greater on one in one year and

on the other in an other year. With pears, for w^hich in our state there

is but one generally severe epidemic disease, the columns in Fig. 29

serve only to represent its annual fluctuations in prevalence.

Annual Intensity of Disease Attack

Intensity of attack, as our discussion of the individual diseases has

suggested, is in many respects distinct from disease prevalence and, as it

has been measured separately throughout our surveys, it should be main-

tained separately in our summary.

In order to determine, for each kind of fruit, to what extent the

intensity of attack of all the epidemic diseases affects the average tree,

it is necessary to add together in separate categories the annual indexes

for fruit disease, for leaf disease, and for twig disease determined from

the field data. The results may then be regarded as showing the annual

intensity of disease attack on the average tree. This summation is given

in Table XXIII. The total intensity of disease attack on the apple is

shown for the fruit, twig, and leaf, on the pear for twig-disease only, on

the peach for fruit, twig, and leaf, and on the cherry and plum only

for the fruit and leaf disease.

For clearer presentation and more ready comparison, these data are

given graphically in Figure 30. The intensity of the various phases of

disease for the several kinds of fruit trees is shown year by year. The
first column of bars relates to intensity of attack on fruit, the second

to intensity on twigs, and the third to the intensity of leaf disease.

Intensity of Disease on the Fruit

Fruit intensity data in Table XXIII apply only to apples, peaches,

cherries, and plums. The bars representing these data in Figure 30 are



Diseases of Fruit Trees, 1922-1928 495

arranged for each year in the order in which the fruit were named. In
the years 1922, 1923, and 1924, the intensity of fruit disease on apples,

shown by black bars, tended to decrease, while fruit diseases on peaches,

shown by doubly cross-hatched bars, tended to increase. This opposition

of trends is of interest, not only because it suggests a possible general

and opposite difference in the effect of weather conditions upon apple

and peach fruit-diseases but also because the general increase in fruit

disease 'intensity on cherries and plums in the same years indicates that

conditions increasingly unfavorable to diseases on pomaceous fruits may
be increasingly favorable to diseases attacking fruits of drupaceous trees.

The year 1925 is outstanding because of the very slight intensity of

disease attack on fruit, but in the succeeding years, 1926, 1927, and
1928, the attack on fruit is shown to have increased again in the case of

every kind of fruit tree for which data were obtained. And in these

Table XXIII

IXTEXSITY OF DISEASE ATTACK OX FRUIT TrEES

Kind of fruit



496 Illinois Natural History Survey Bulletin

Fig. 30. Comparison of the annual intensity of disease on each of the
five tree fruits

The bars in the column at the left compare fruit intensity by years, those
in the middle column twig intensity, and those in the column at the right
leaf intensity. For further explanation, see the text.



Diseases of Fruit Trees, 1922-1928 497

The causes of this oppositeness of reaction exhihited hy the dis-

eases of pomaceous and drupaceous fruits may be sought for in the

weather which prevailed during those periods when the distinctly dif-

ferent diseases of the two types of fruits were developing.

Intensity of Twig Disease

Data on intensity of twig-disease are given in Table XXIII for

each kind of tree for which they are available. These data are shown
graphically by the middle column of bars in Figure 30. Data have been

obtained for twig-disease of apples, pears, and peaches. The bars in

Figure 30 are arranged in this order. The blank spaces marked "no

data" indicate the absence of twig-disease data for the plum and cherry,

except for 1935 when data were also lacking for peaches.

The fluctuations from year to year in intensity of disease attack on
twigs are more generally in agreement for all kinds of fruit trees than

was the case with fruit-disease. In 1922, indexes for twig-disease in-

tensity were relatively low for apples, pears, and peaches, but in 1923

they increased, that for apples very greatly and those for pears and
peaches to a lesser extent. In 1924 the twig-disease intensity indexes for

the apple and the peach fell, that, for the peach nearly to the level of

1922, while the index for pears rose somewhat above its 1933 level. The
year 1935, conspicuous for its detrimental influence upon fruit and leaf

infection, allowed the twig-disease intensity index for apples to reach

a very high point, but at the same time the pear index dropped to about

one-fourth its former value. Lacking data, the trend of the peach index

cannot be determined. In the following years, 1926, 1927, and 1938, all

indexes for the intensity of twig disease are low, and it m9,y be assumed
from this that 1935 eventually proved as detrimental to this form of

disease as it was to other forms, though the effect was manifested later.

The small increases and decreases of the twig-disease indexes mani-

fested from year to year in this last period of our survey are followed

consistently by the diseases of apple and peach, and no distinction can

be made l^etween the twig-disease trends of drupaceous and pomaceous
trees. They appear to react uniformly to weather conditions.

Intensity of Leaf Disease

The totaled indexes of leaf-disease intensity are given in Table

XXIII and are shown graphically by the third column of bars in

Figure 30. Data are available for leaf-disease on apples, peaches,

cherries, and plums, and the bars in Figure 30 which represent these

data for each year are arranged in the order in which the fruits have

just been named. Intensity of leaf-disease attack was low for all fruits

in 1922. There is, however, no observable subsequent general tendency

toward increase. In 1933, apple leaf-disease increased in intensity,

peach leaf-disease decreased, and leaf-disease on both cherry and plum
increased. In 193-4, a marked falling off in the intensity of apple leaf-

disease by two-thirds of its former amount was accompanied by an
even more remarkable increase in peach leaf-disease intensity, while the

corresponding index for cherries increased and that for plums decreased.



498 Iluxois Natural History Survey Bulletin

The year 1935 was very severe in its effect on leaf-disease, reducing!: the

index for apple to a low point but being apparently more favorable to

apply diseases than to peach diseases, as the latter was too slight to

measure. With 1926, the leaf-disease intensity indexes rose again, those

for the peach and the cherry increasing to moderate points and that for

the apple rising above its 1923 level. In 1937. the rearrangement of

these indexes placed that for cherry highest, with the peach index in-

termediate and that for apple lowest. Tliere were no data u])()n which to

base an index, for plum leaf-disease. In 1928, the apple index rose

slightly above its previous level, the peach index was diminished to about

three-fourths of its former value, and the cherry index fell to the lowest

recorded point.

In the fluctuations of the yearly indexes of intensity for disease

attack there is no complete agreement, but it is worthy of note that the

tendency to move in opposite directions previously demonstrated by the

indexes of fruit-disease intensity for the apple and peach is again well

defined in the movements of the leaf-disease indexes of these trees.

From 1922 to 1933 the change in the leaf-disease intensity index for

apple was upward, the change in the peach index downward; from 1933
to 1924 the apple index moved downward, the peach index upward; the

indexes of both trees suffered great reductions in the disastrous season

of 1925 (though the apple index was affected less than the peach index)

;

the very considerable increase in the apple index in 1926 was accom-
panied by similar increase in the peach index; in 1927 a great decrease

in the apple index was accompanied by a decrease only half as great in

the peach index ; and in 1928 the slight increase in the apple index was
again contrary to the decrease in the peach index.

There is, not, in this case, the completeness of opposing trends that

was exhibited by the intensity indexes for fruit-disease, and the move-
ments of the leaf-disease indexes for the cherry and pliuTi do not up-
hold a consistent difference in trends between leaf-disease on drupaceous
and on pomaceous trees ; but it does appear that seasons favoring intense

leaf-disease attack on apple trees are adverse to the development of leaf-

disease on peaches.

Average Prevalence and Average Intensity of Fruit-tree

Diseases

In summarizing our data on the prevalence and intensity of attack

exhibited by the individual diseases of each kind of fruit-tree, we have

computed for each disease a set of indexes to show its average annual

prevalence and intensity through the period of our survey, and from

these averaged indexes we have also computed indexes which express

numerically the average annual incidence of disease per tree and the

average annual intensity of disease attack on fruit, twig, and leaf for

each kind of fruit tree. These final indexes are all brought together in

Table XXIV, and the values there shown are given graphical expression

in Figure 31.

As would naturally be expected from the fact that it is subject to

attack by the largest number of epidemic diseases, the apple has the

highest average disease incidence per tree. The peach ranks second in



Diseases of Frtjit Trees, 1922-1928 499

this respect, the plum third, the cherry fourth, and the pear, with its

one important disease, fifth. But it is interesting to find that in suscepti-
bility to disease attack the apple also ranks first, and this is true in spite
of the larger number of apple diseases. On an average, an apple dis-

ease succeeds in attacking 68 or 69 out of every 100 trees. Avhile a
peach disease succeeds in attacking only 55 trees in a hundred. The
average susceptibility of the plum and cherry to disease attack appears
rather high when the figures given in Table XXIV are interpreted
strictly, being respectively 67 and 77 trees per 100; but since the leaf

spot disease, as recognized in the accumulation of data, is in each case
really a composite of two diseases, their apparent susceptibility is re-

duced thereby to about 44 and 48 trees per 100, respectively, or less than
the susceptibility of the peach. From the graphical presentation of
data in Figure 31, it may be seen that the differences between kinds of
fruit-trees that have been shown to exist with relation to disease pre-
valence are neither constant nor parallel with relation to the intensitv of

APPLE

PEAR

FEACh

CMERRy

PLUM



500 Illinois Natiral History Sirvky Bulletin

disease attack on fruit, twig, and leaf. The second column of bars in

Figure 31 compares the average intensity of fruit-disease on our fruit

trees. Though attacked by fewer diseases, as a rule, than the apple, the

average ])each tree suffers more from fruit-disease, while the plum, with

nearlv the same incidence of disease as the cherry, suffers over twice as

much fruit infection.

The average annual intensity of twig attack, which is compared

in the third column of bars in Figure 31 for the apple, pear, and peach,

is greatest on the apple, least on the pear, and intermediate on the

peach. This order corresponds exactly to the order of disease preva-

lence.

The average intensity of leaf-disease, shown for all our fruit trees

except the pear by the fourth set of bars in Figure 31, is again greatest

on the peach, though on the apple it is hot much less. On cherry and

plum it is about eijual, but is, on each, less than half as severe as on

peach or apple. The average intensity of leaf-disease runs parallel, in

respect to the kind of fruit trees, with the average intensity of fruit-

disease on peach and apple, but parallel with the average incidence of

disease on plum and cherry trees.

RECAPITULATION

In Illinois, as elsewhere, the productiveness of orchards is curtailed

by the injury to trees and their fruit that results from the attack of

destructive diseases. Each kind of fruit tree is subject to a particular

group of diseases, of which some are serious and some mild. Certain

diseases, after obtaining a foot-hold, work continuously to accomplish the

death of the trees they attack. Others leave their host-trees during

some part of every year and, returning annually at some particular time

to renew their attack on special parts of their hosts. exhi])it striking

variations both in prevalence and in destructiveness, from one year to

another. In this respect they are similar to the recurring diseases of

mankind and equally deserve to be called epidemic.

Research and investigation have revealed the causes of these dis-

eases, the life histories of the parasites, and the poisons most effective

against them. Sprays are, however, seldom completely effective. Often

they fail.

To increase the efficiency of disease control, an understanding of

the effects of weather variations upon epidemics is necessary. This may
be gained by studying simultaneously, and in detail, both the weather

and the epidemics. The former has for many years been accurately

measured and recorded, but epidemics have not been subjected to a

similar scrutiny.

From '1921 through 1928, a survey was made in Illinois of the

epidemic diseases of fruit-trees. During these years, methods of measur-

ing the epidemics had to be devised, tested, and perfected ; in every year

the epidemics were measured with the means at hand, and the measure-

ments were recorded according to definite plans. The full results of this

survey are now reported in the text of this paper.

An epidemic is considered to have two primary aspects. These are.

first, the prevalence of the disease and, second, the intensity of its attack.



Diseases of Fruit Trees, 1922-1928 501

The former vre have measured and recorded simply as the percentage of

trees succumbing to infection; but the latter, being more complex, has
had to be considered from three points of view.

Diseases of an epidemic nature usually produce lesions on fruit, on
new and immaturely lignified twigs, and on leaves. The attack may he

on any one, on any two, or on all three of these tree parts, according to

the disease. To measure fully the intensity of an epidemic, each phase
of its attack must be taken into account and the measurements must be
maintained as a separate indicator of the success of the epidemic.

Prevalence and intensity of attack have been measured for 5 apple
diseases, 1 pear disease, 4 peach diseases, 2 plum diseases, and 3 cherry

diseases. The data so accumulated have been evaluated and reduced to

averages, which we have called indexes.

Each epidemic disease has an index of prevalence for each year of

the period of our survey, unless data were not obtainable, and, according

to the mode of attack characteristic of the particular -disease, its annual
intensity is represented by one or more indexes. The blotch disease of

apple, for example, is given the following indexes for 1922 : prevalence,

53.03; intensity of fruit attack, 41.36; intensity of twig attack, 10.49;

intensity of leaf attack, 2.56. These indexes, the exact significance of

which is fully explained in the section entitled Analysis of Data, present

a complete evaluation of apple blotch, in all its epidemic aspects, for the

summer of 1922. Corresponding sets of indexes for 1923 and each suc-

ceeding year state with exactness the increases and decreases of the

disease in prevalence and intensity through the period covered by the

survey. Sets of indexes for the other epidemic diseases of the apple and
for the diseases of other fruit trees make it possible to compare the

fluctuations of all the apple diseases with one another from year to year

and to compare at will the variations of any disease of any kind of fruit

tree with any disease of any other kind of tree. Many such comparisons

have been made in the text, both directly from the tabulated indexes or

by means of diagrams based on the indexes.

It seldom happens that all the diseases of one kind of fruit tree

follow, in one year, the same course of increase or decrease from the

previous year. Two diseases may increase in nearly the same proportion

from one year to the next, but in the third year their trends are apt to

become opposed, one disease continuing to increase, the other manifesting

a great diminution in prevalence and in one or more of its phases of

intensity.

The changes exhibited from year to year by fruit-tree diseases

appear to be governed by no general law. Fluctuations of individual

diseases are often so great, and occur in such diverse directions and

amounts, as to suggest only capriciousness. The interdependence that

might be expected between prevalence and intensity of attack does not

occur consistently. High prevalence and mild intensity are often com-

bined, as are also moderate prevalence and high intensity. Of the three

phases of disease intensity, one may be high in a given year and the

others low, and the changes that occur in succeeding years follow no

apparent rule. Evidently, each phase of each disease fluctuates from

year to year with considerable independence from the other phases.



502 Ilunois Natubal Histoey Survey Bulletin

To these general statements, there is one exception. The year 1925

proved disastrous to all diseases. Whatever the conditions of that year

may have been, whatever their differences from previous and succeeding

years, they were beyond doubt so unfavorable that, excepting only apple

blotch in its twig phase, no fruit-tree disease was able to achieve signifi-

cant proportions, either in prevalence or intensity. So fatal was this

year to the organisms that cause fruit-tree diseases, that in the three

subsequent years included in the survey few diseases have been able to

attain their former levels of prevalence and intensity.

The fruit trees of Illinois are of two classes. Apples and pears, with

their several-seeded cores, are called pomaceous; while peaches, plums,

and cherries, with their hard-pitted fruits, are drupaceous. This botani-

cal separation of the two groups is accompanied by an equally distinct

difference in diseases. And more remarkable still, our comparisons show
that the fluctuations of disease on pomaceous and on drupaceous trees

are nearly always 'in opposite directions. A year that favors apple and
pear disease is detrimental to disease on the drupaceous trees; and, con-

versely, a year favorable to disease on peaches, plums, and cherries in-

hibits the development of disease on apples and pears.

In the production of such a diversity of disease phenomena, many
elements are combined. As direct causes of disease, there are fungi and
bacteria. These have various means for reproducing themselves and vari-

ous abilities for infecting their hosts through chance openings, by exert-

ing mechanical force, or by accomplishing the chemical dissolution of

protective membranes. Success in reproduction, in accomplishment of

infection, and in the subsequent growth and extension of these parasites

is conditioned and governed by many external factors, among them being

light, heat, and moisture. In different amounts or combinations, these

factors impede or expedite the production of disease, determine times

of infection, and influence reproductive processes. Since the ability of

the parasites to maintain themselves vicariously in adverse conditions

and to flourish under favorable conditions results in the variations in

epidemics, the prevalence and intensity a disease attains is a definite

indication of the favorableness of the conditions under which it

developed.

Epidemics have now been analyzed and measured, and their

characteristics have been stated in terms sufficiently concrete to enable

the making of exact comparisons for similar and dissimilar environ-

mental conditions. Of the environment, temperature and precipitation

are measured regularly in Illinois and a large mass of data is available.

The next step is to determine what combinations of these elements, as

indicated by the characteristics of accompanying epidemics, favor or

inhibit the development of fruit-tree diseases.



INDEX

Acacia, 232.

Acer. 234.

Negundo, 246.

violaceum, 285.

nigrum, 238.

Palmeri, 285.

rubrum, 243.

Drummondii, 244.

tomentosum. 285.

tridens. 285.

saccliarinum, 240.

saccharum, 235.

Rugelii, 236.

Schneckii, 236.

Acroneuria, 366.

Aesculus, 248.

discolor, 285.

glabra, 250.

Amelanchier, 202.

canadensis, 203.

Allocapnia, 351-354, 357, 360, 364, 367,

368, 370, 371, 388.

forbesl, 347, 354, 358, 360, 397-399.

granulata, 353, 358, 359, 360, 361, 362,

363, 366, 369.

mystica, 347, 353. 358, 360, 361, 363,

364, 366, 399-403.

pygmaea, 353, 358.

recta, 353, 358, 359, 360, 361, 363, 364,

366, 368.

vivipara, 353, 358, 359, 360, 361, 363,

364, 366, 368, 392-403.

Apple, 198.

Black-rot, 446-450.

Blotch, 439-445.

Crab, 200.

Diseases, 429-469.

Fire-blight, 450-453.

Rust, 453-455.

Scab, 430-439.

Arbor Vitae, 41.

Ash, 268.

Biltmore. 271.

Black, 280.

Blue, 278.

Brown, 280.

Green, 275.

Ash—Continued.

Mountain, 200.

Pumpkin, 274.

Red, 275.

Swell-butted. 274.

White, 270.

Asimina, 185.

triloba, 186.

Asp, Quaking. 47.

Aspen. 47.

Large-toothed, 50.

B

Bacterial shot-hole, 475-477.

Bacillus amylovorus, 450, 469.

Balm of Gilead, 59.

Bass Wood, 252.

Beech, 103, 104.

Blue, 94.

Betula. 98.

lenta, 103.

lutea, 103.

nigra, 98.

papyrifera, 101.

Birch, 98.

Canoe, 101.

Cherry, 103.

Paper, 101.

Red, 98.

River, 98.

Yellow, 103.

Bitternut, 77.

Black Jack, 128.

Blue Beech, 94.

Bow Wood, 177.

Box Elder, 246.

Brachyptera, 347, 373.

Buckeye, 248.

Fetid, 250.

Ohio, 250.

Butternut, 65.

Buttonwood, 195.

Capnia, 355, 360, 388.

Capnia vernalis, 351, 407.

Carpinus, 93.

caroliniana, 94.

[503]



504 Illinois Natikal Histoiiy Sikvky Bi i.lkti.n

Caiya. 71.

alba, S6.

aquatica, SO.

Buckleyi. 93.

arkansana, 93.

villosa. 93.

cordiformis, 77.

glabra. SS.

megacarpa, 286.

lacinlosa, S3,

ovalis. 90.

obcordata, 286.

obovalis, 286.

odorata, 286.

ovata. SI.

Xuttallii. 286.

pubescens, 286.

pecan. 73.

Castanea. 107.

dentata. 107.

Catalpa, 282.

speciosa. 282.

Cedar. Red. 42.

White. 41.

Celtis, 16S.

laevigata. 171.

occidentalis, 169.

canina, 286.

crasslfolia, 286.

pumila, 286.

Cercis. 221.

canadensis, 222.

Cherry, Bird. 216.

Choke. 217.

Rum, 220.

Wild Black, 220.

Wild Red, 216.

Cherry diseases, 487-492.

Che.stnut, 107.

Cladastris lutea, 286.

Coccomyces, 487.

Coffee Bean. 222.

Cornus. 260.

alternifolia. 264.

florida. 260.

Cottonwood. 54.

Black. .52.

Swamp, 52.

Virginia, 56.

Crab Apple, 198.

Crataegus, 205.

apiomorpha, 206.

Crus-galli, 205, 206.

Hillii, 206.

illinoiensis, 206.

lucorum, 206.

macrancantha, 206.

mollis, 205, 206.

nitida. 206.

peoriensis, 206.

Phaenopyrum, 205-206.

pratensis, 206.

punctata, 206.

pruinosa, 206.

rotundifolia, 206.

tomentosa, 206.

viridis, 206.

Cucumber Tree, 180.

Cypress, Bald, 37.

Deciduous, 39.

Diospyros, 265.

virginiana, 265.

platycarpa, 286.

Diplodontus, 370.

Dogwood, 260, 264.

Flowering, 260.

Ebony. 265.

Elm, 156.

American, 156.

Cork, 159.

Red. 163.

Rock, 159.

Slippery, 163.

Water, 166.

White, 156.

Winged, 162.

Endostigme inaequalis, 430.

Exoascus deformans, 477.

Fagus, 103.

grandifolia, 104.

caroliniana, 286.

Fraxinus, 268.

americana, 270.

biltmoreana, 271.



I.MIF.X 50.=

Fraxinus—Continued.

nigra, 280.

pennyslvanica, 275.

lanceolata, 277.

profunda, 274.

quadrangulata, 278.

Fruit trees, diseases of, 415-502.

G

Gleditsia, 226.

aquatica, 229.

triacanthos, 228.

Gum, Cotton. 257.

Sour. 256.

Sweet, 192.

Tupelo, 257.

Gymnocladus, 222.

dioicus, 224.

Gymnosporangium
ae. 453.

juniperi-virginian-

H

Hackberry, 168, 169.

Mississippi, 171.

Hawthorn, 205.

Hickory. 71. 86.

Big Shellbark, 83.

Buckley's 93.

King Nut. 83.

Mockernut. 86.

Pignut, 77, 88.

Oval, 90.

Shagbark, 81.

Shellbark, 81.

Water, 80.

White, 86.

Hornbeam, 93, 94.

Hop, 96.

I

Infusoria, 370.

Insects, see Stoneflies.

Ironwood, 96.

Judas Tree, 221, 222.

Juglans, 64.

cinerea, 65.

nigra, 68.

Juniperus, 42.

virginiana, 43.

Kentucky Coffee Tree, 224.

King Nut, 83.

Larch, 33.

European, 36.

Larix, 33.

decidua, 36.

laricina, 33.

Leuctra claasseni, 347, 351, 355, 360,

404-407.

Linden, 251, 252.

Liquidambar, 191.

orientalis, 194.

Styraciflua, 192.

Liriodendron, 182.

Tulipifera, 183.

Locust, 226, 231.

Black, 232.

Honey, 228.

Water. 229.

Yellow, 232.

M

Madura. 177.

pomifera, 177.

Magnolia, 179, 330.

acuminata, 180.

Mountain, ISO.

Mahogany, 224.

Malus, 198.

angustifolia, 198.

coronaria, 200.

ioensis, 198.

plena, 200.

lancifolia, 198.

platycarpa Hoopesii, 198.

Maple, 234.

Ash-leaved, 246.

Black, 238.

Hard, 235.

Red, 243.

Scarlet, 243.

Silver, 240.

Soft, 240.

Sugar, 235.

Mites, 370.

Mockernut, 86.

Morus, 174.

rubra. 174.



506 ll.I.INOIS NaTIKAI. HisTOKY SlKVKY BlI.I.KTlN

Mountain Ash, 200. 202.

Mulberry. 174.

Red. 174.

N

Nemoura vallicularia. 251, 359, 365,

367. 36S.

Nephelopteryx, 347. 372.

Nyssa. 254.

aquatica, 257.

sylvatica. 250.

Oak, 110.

Basket. 149.

Black. 117. 122.

Black Jack, 128.

Burr. 135.

Chestnut. 152.

Chinquapin, 153.

Cow, 149.

Hill's. 117.

Jack. 117, 128.

Laurel, 134.

Mossy Cup, 135.

Overcup, 138.

Pin. 120.

Post. 140.

Red. 112. 116, 124, 126.

Rock chestnut, 152.

Scarlet. 155.

Schneck's, 116.

Scrub, 128.

Shingle. 134.

Spanish. 124.

Swamp Spanish, 120, 126.

Swamp White, 138, 146.

White. 144.

Willow, 131.

Yellow. 153.

Yellow-bark, 122.

Opercularia, 370.

Osage Orange, 177.

Ostrya. 94.

virginiana, 96.

P

Pawpaw. 185, 186.

Peach diseases, 472-487.

Brown-rot, 472-474.

Leaf-curl, 477-479.

Scab, 479-480.

Pear blight, 469-472.

Pecan, 73.

Niblack, 75.

Busseron, 75.

Posey, 75.

Perla, 366.

Persimmon. 265.

Phyllosticta solitaria, 439.

Physalospora malorum, 446.

Pignut, 77, 88, 90.

Pine, 23.

Gray, 28.

Jack, 28.

Short-leaf, 29.

White, 24.

Yellow, 29.

Pinus, 23.

Banksiana, 28.

echinata, 29.

laricio. 32.

mitis, 30.

pungens. 32.

rigida, 32.

Strobus, 24.

sylvestris, 32.

virginiana, 30.

Plant disease survey methods, 416-420.

Plane-tree, 195.

Planera, 164.

aquatica, 166.

Platanus, 195.

occidentalis, 195.

Plecoptera, 345-409.

Plum diseases, 487-492.

Plum, 210.

Wild, 212.

Wild Goose, 213.

Poplar, 46, 50.

Carolina, 59.

Lombardy, 59.

Yellow, 182, 183.

Populus, 46.

balsamlfera, 54, 59.

virginiana, 56.

canadensis, 59.

Eugenie, 59.

candicans, 59.

grandidentata, 50.

heterophylla, 52.



Indkx 507

Populus—Continued.

nigra, 59.

Italica, 59.

tacamaliacca, 59.

tremuloides, 47.

Prunus, 210.

americana, 212.

liortulana, 213.

pennsylvanica, 21G.

serotina. 220.

virginiana, 217.

Pseudomonas Pruni. 475.

Pteroiiarcvs, 366.

Quaking Asp, 47.

Quercus. 110.

alba, 144.

pinnatifida, 2S6.

alba X bicolor, 155.

alba X macrocarpa. 155.

alba X stellata, 155.

bicolor, 146.

bicolor X lyrata. 155.

borealis, 112.

maxima, 112, 155.

coccinea, 155.

ellipsoidalis, 117, 155.

X exacta, 155.

X Fernowii, 155.

imbricaria, 134, 155.

imbricaria X rubra. 155.

X Jackiana, 155.

X Leana, 155.

lyrata, 138.

macrocarpa, 135.

marilandica, 128, 155.

montana, 152.

Muehlenbergii, 153.

palustris, 120, 155.

palustris X Phellos. 155.

Phellos, 131.

Prinus, 149.

rubra, 124.

pagcdaefolia, 126.

X runcinata, 155.

X Schockiana, 155.

X Shuettei, 155.

Shumardii, 116.

Schneckii, 116.

stellata, 140.

Quercus—Continued.

X tridentata, 155.

velutina, 122, 155.

R

Red Cedar, 42.

Redbud, 222.

Rhabdiopteryx, 374.

Robinia, 231.

Pseudoacacia, 232.

Salix, 59.

amygdaloides, 62.

nigra, 60.

Sassafras, 188, 189.

officinale, 189.

albidum, 286.

Savin, 43.

Sclerotinia cinerea, 472.

Service Berry, 203.

Shad Bush, 203.

Sorbus, 200.

americana, 202.

Stoneflies, 345-409.

Strophopteryx, 347, 351, 356, 360, 367,

368, 370, 374.

fasciata, 355, 358, 360, 361, 362, 366,

369, 384-388.

Sugarberry, 169, 171.

Sycamore, 195.

Tacamahac, 59.

Taenionema, 347, 359, 373.

Taeniopteryx 347, 351, 356, 358, 360,

364, 367, 368, 370, 372, 373, 376.

maura, 379, 380.

nivalis, 354, 356, 358, 359, 360, 361,

362, 366, 368, 378, 379, 380, 381-383.

parvula, 354, 360, 383.

Tamarack, 33.

Taxodium, 37.

distichum, 39.

Thuja, 41.

occidentalis, 41.

Tilia, 251.

floridana, 251.

glabra, 252.

heterophylla Michauxii, 251.

reglecta, 251.



50S Il.l.INOlS XaTIKAI. HisTOKY SlItVKY Bri.I.KTIX

Tulip Tree. IS:?.

Tupelo. 254. 256.

Tupelo Gum. 257.

U
Ulmus. 156.

alata, 162.

aniericana. 156.

fulva. 16.3.

racemosa. 159. 162, 163.

W
Wahoo, 162.

Walnut. 64.

Black. 68.

Ohio, 71.

Stabler, 71.

Thomas, 71.

White. 65.

Water Elm, 166.

White Cedar, 41.

Willow. 59.

Almond, 62.

Black, 60.

Peach, 62.



EliliATA 509

ERRATA

Page 6, footnote, for il read IS. and omit .;/.

Page 112, first line, for The read This: tenth line from bottom, for The Red Oak
read Q. borealis.