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JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND
Maataloustieteellinen A ikakauskirja

Vol. 58: 33—42, 1986

Spring wheat mixtures in northern crop production: ability of mixtures
to buffer disease development and yield loss caused by Septoria nodorum

REIJO KARJALAINEN

Departments of Plant Pathology and Plant Breeding
University of Helsinki, SF-00710 HELSINKI, Finland

Abstract. Epidemic development of Seploria nodorum was studied in pure stands and mixtures of
two spring wheat cultivars Tähti (susceptible) and Kadett (moderately resistant) in 1983 —1985. Apparent
infection rates in the mixture were similar to that of the more resistant pure stand. In all three years,
disease levels in mixed stands were lower than the arithmetic mean of the pure stands.

The yield experiments indicated that under low or moderate disease stress mixtures can buffer yield
reduction effectively. However, when disease levels were high, mixtures appeared to be less effective in
that respect. The results are discussed in relation to possible mechanisms of intraspecific mixtures to retard
non-specialized pathogens. The use of wheat mixtures to prevent disease induced yield losses caused by
S. nodorum is also discussed.

Index words: spring wheat, cultivar mixtures, yield stability, disease resistance, Seploria nodorum

introduction

Considerable theoretical and practical inter-
est has been focused in recent years on the
evaluation of genotypic mixtures in cereal
crop production, and there is some evidence
to support the idea that mixtures have several
advantages over monocultures, such as greater
stability of performance across diverse en-
vironments (Jensen 1952, Simmonds 1962,
Trenbath 1974) and more diverse and stable
resistance to diseases (Jensen 1952, Borlaug

1958, Suneson 1960, Browning and Frey

1969). The reason why mixtures seem to be
advantageous over monocultures lies in the in-

teraction between genotype and environment
for each cultivar (Wolfe and Barrett 1980,
Rajeswara Rao and Prasad 1984). It is
known (Wolfe and Barrett 1980) that the
way each cultivar responds to the range of en-
vironments may be unique to that particular
cultivar, and in many cases such differences
are extremely difficult to measure and predict.
It has been suggested (Marshall and Brown
1973) that even in the absence of intergeno-
typic interactions, a mixture would be more
stable than its components, provided at least
one component line responds differentially to
at least one environment. Extensive data of
oat mixtures and multilines (Jensen 1952,

33

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Pfahler and Linskens 1979, Shorter and
Frey 1979) indicate that genotype environ-
ment interaction variances for grain yields of
mixtures are often smaller than those for oat
lines and cultivars grown in monoculture. The
better utilization of environmental resources
such as water, light, and nutrients under sub-
optimal conditions (Frey and Maldonado
1967, Rajeswara Rao and Prasad 1982)
might partly account for the yield advantage
of mixtures over monocultures. Consequently,
the better ability to compensate yield losses
in a stress environment is likely to provide
stable performance compared to pure stands.
However, Clay and Allard (1969) have
shown that in some cases barley mixtures can
be less stable than monocultures.

It is well demonstrated (Adams et al. 1971,
Day 1973, Marshall 1977) that the increased
uniformity of modern crop production has
sometimes caused unpredictably serious crop
losses due to disease epidemics. Particularly
in modern cereal production the widespread
use of one or a few similar and genetically
homogeneous cultivars provides ideal condi-
tions for the rapid spread of virulent isolates
of foliar pathogens. Therefore, the introduc-
tion of single major resistance genes into com-
mercial cultivars and the cultivation of the
varieties without exact knowledge of the viru-
lence data of the pathogen population has
often led to the breakdown of resistance to
biotrophic fungi of cereals. It was suggested
by Jensen (1952) and Borlaug (1958) that
difficult cereal pathogens can be controlled by
mixing cultivars of different resistances.

Extensive data of recent studies of bio-
trophic pathogens of oats (Frey et al. 1977),
wheat (Fried et al. 1979), and barley (Wolfe
and Barrett 1982) indicate that disease pro-
gress in mixtures is much less than the mean
of component pure stands. In addition, long
practical evidence of growing oat multilines
(Browning et al. 1979) also indicates that
multilines can effectively buffer against the
yield reduction caused by crown rust epi-
demics.

However, very little is known of how mix-

tures affect the disease progress of pathogens
not specialized to cultivars. Recently, Jeger
et al. (1981 a) proposed theoretical models
suggesting that in many circumstances the
amount of unspecialized pathogens in mix-
tures will be equal to or less than the arith-
metic mean of component pure stands. So far,
experimental evidence (Jeger et al. 1981 b) is
too limited to verify the predictive ability of
this model.

Septoria nodorum Berk, is an important
and widespread pathogen of wheat and bar-
ley (King et al. 1983), and it is capable of
causing yield reduction even at a moderate
level of infection (Obst 1977). In the last
years S. nodorum has caused significant yield
reductions to Finnish wheat crops (Karjalai-
nen et al. 1983). The cheapest and economi-
cally most feasible method of controlling the
disease caused by S. nodorum is to use re-
sistant cultivars. However, recent studies
(Karjalainen 1984, 1985) have indicated that
all Finnish cultivars are susceptible to the dis-
ease. Therefore, the question arises whether
cultivar mixtures can be used to reduce dis-
ease progress and the yield losses caused by
S. nodorum. The purpose of the present study
was to evaluate the idea proposed by Jeger

et al. (1981 a, b) that cultivar mixtures can
reduce the amount of unspecialized disease in
mixtures compared with the mean of the pure
components. The present study was carried
out in 1983—1985 using mixtures of suscep-
tible Finnish cv. Tähti and moderately resis-
tant Swedish cv. Kadett.

Materials and methods

The data presented in this study are based
on trials carried out at the experimental farm
of the University of Helsinki in the years
1983—1985. Two spring wheat cultivars dif-
fering in their resistance to Septoria nodorum
were selected for the study. Tähti, a late Finn-
ish variety has been cultivated in southernFin-
land for several years mainly because of its
good baking quality, stiff straw and resistance
to sprouting damage. Recent cultivar trials

34



(Karjalainen et ai. 1983, Karjalainen 1984)
have shown that Tähti is very susceptible to
S. nodorum, which has caused serious crop
losses in the last, rainy years (Karjalainen
1985), and the popularity of Tähti has been
dramatically reduced. Instead, a high-yielding
Swedish spring wheat cultivar Kadett has be-
come popular in southern Finland. Kadett is
moderately resistant to S. nodorum and
clearly outyields Tähti.

The experiment was laid out in a random-
ized block design with eight replications in
1983, six in 1984, and eight in 1985. Plot size
was 10m2

. Pure stands and their binary mix-
tures (1983 and 1984) as well as more compli-
cated mixtures, Tähti 30 %/Kadett 70 %,

Tähti 50 %/Kadett 50 %, and Tähti 70 %/

Kadett 30 % (1985), were used.
Half of the experimental plots were arti-

ficially inoculated with S. nodorum and sepa-
rated by oat plots in order to prevent inoculum
from spreading into controls. Inoculum was
applied onto the plants by spraying (106 co-
nidia/ml) twice, starting at the four leaf stage.
The preparation of inoculum has been previ-
ously described by Karjalainen et ai. (1983).

Disease assessment based on estimating the
percentage area covered by lesions on flag and
second leaves. Two assessments were made in
1983 and 1985, four in 1984. The data on dis-
ease severity are based on the measurement of

Table 1. Monthly mean temperatures and total rainfall
in May—August 1983—1985 at Helsinki-
Malmi airport.

Mean Rainfall
temperature mm

°C

1983 May 11.8 37.5
June 13.9 55.3
July 18.3 23.8
August 15.8 52.4

1984 May 12.9 43.5
June 14.4 55.2
July 15.6 155.2
August 15.5 29.9

1985 May 9.4 60.5
June 13.7 66.3
July 16.1 77.5
August 16.3 95.4

60 tillers per plot. Data comparisons between
yield and 1000-grain weight were made in rela-
tion to uninoculated control plots. The per-
centage of diseased leafarea was transformed
using the arc-sin transformation.

Results

Effects of mixtures on disease progress

Disease progress in experimental fields in
1983 was slow due to relatively long dry and
very warm periods (Table 1). The assessment
of disease severity (Fig. 1) indicates that the
amount of disease in the mixed stand was
16 % less than the arithmetic mean of the pure
stands. Under low level infection stress,
Kadett seems to avoid disease effectively, and
the disease level in the mixtures appears to be
closer to the more resistant component than
the susceptible one.

Disease progress in experimental fields in
1984 was very rapid due to favourable weather
conditions, frequent rains during the latter
part of the growing period. Following inocu-
lation, the development of disease on flag
leaves and two uppermost leaves (Figs 2,3)

Fig. I. Disease severity of S. nodorum (average of two
upper leaves) of spring wheat cultivars Tähti and
Kadett and their binary mixture based on the
average of two disease observations in 1983. The
percentage diseased leaf area values were trans-
formed using the arc-sin transformation.

35



was first slow compared with later phases.
Every assessment indicated that the amount
of disease was highest for Tähti and lowest for
Kadett. The disease level of the mixtures ap-
peared to fall between the two components,
but was closer to theresistant cultivar, Kadett

(Fig. 4). Apparent infection rates were calcu-
lated in order to compare pathogen reproduc-
tion rates between pure stands and mixtures.
The data show (Fig. 3) that the r-values for
the mixture and the resistant component are
identical (r = 0.18), while the r-value for Tähti

Fig. 2. Disease progress of S. nodorum on the flag leaves of spring wheat cultivars Tähti and Kadett and their
binary mixture in 1984.

Fig. 3. Disease progress of S. nodorum (average of two upper leaves) of spring wheat cultivars Tähti and Kadett
and theirbinary mixture in 1984. Apparent infection rates (r) were calculated on the basis of three last sampl-
ing dates and using the log e transformation according to Zadoks and Schein (1979).1 A

36



37

is somewhat higher (r = 0.23).
In 1985 the development of the disease was

moderate, and the data indicate (Fig. 5) that
the disease amount in mixed stands was closer
to theresistant component, although the mix-
ture benefit was small compared with the
arithmetic mean of the pure stands. In all
three years, S. nodorum was practically the
only leaf pathogen present in trials, only
mildew was occasionally seen.

The ability of mixtures to buffer yield loss

Disease induced yield reduction in 1983 was
very small and statistically insignificant,
4.3 °7o for Tähti, 4.4 % for Kadett, and only
0.5 % for the mixture (Fig. 6). The data
clearly show that under low infection condi-
tions the mixture appears to buffer against
yield reduction caused by S. nodorum.

In 1984, the infection by S. nodorum was
heavy and caused statistically significant yield
reductions, 35 % for Tähti, 27 ®/o for Kadett,
and 31 *7o for the mixture (Fig. 7). The data
show that the mixture can only slightly buffer
high yield reduction. In 1984 the infection
caused high and statistically significant reduc-
tions in grain weight, 21 °lo for Tähti, 20 %

for Kadett, and 18 °7o for the mixture (Fig. 8).
In this case, the grain weight of the mixture

Fig. 4. Disease severity of S. nodorum (average of two
upper leaves) of spring wheat cultivars Tähti and
Kadett and their binary mixture in 1984.

Fig. 5. Disease severity of S. nodorum (average of two
upper leaves) of spring wheat cultivars Tähti and
Kadett and their binary mixture in 1985.

Fig. 6. The data of grain yield of spring wheat cultivars
Tähti and Kadett and their mixture under dis-
ease induced stress compared with the control
in 1983.

Fig. 7. The data ofgrain yield of spring wheat cultivars
Tähti and Kadett and their mixtureunder heavy
disease induced stress compared with the con-
trol in 1984.



appears to be reduced even less than that of
the resistant component of the mixture.

The disease induced moderate yield reduc-
tions in 1985, 16.7 °7o for Tähti, 13.7 °7o for
Kadett, and 12.1—13.9 % for the mixture
(Fig. 9). Again the data suggest that the mix-
ture and the more resistant component of the
mixture are rather similar in their ability to
buffer against yield reductions. The grain
weight data (Fig. 10) show that disease in-
duced a reduction of 10.5 % for Tähti, while
the grain weight of Kadett was reduced only
by 4.5 % and that of the mixture by 3.7
6.3 °7o. This supports the idea that mixtures
can buffer disease induced yield and yield
component reductions.

Discussion
The data reported in this study suggest that

spring wheat mixtures involving susceptible
and fairly resistant cultivars can retard the dis-
ease progress of S. nodorum in relation to the
mean of the component pure stands. This is
in agreement with the model prediction pre-
sented by Jeger et al. (1981 a), since their
model suggests that the unspecialized disease
progress in a mixture is less than the arithmetic
mean of the pure components. Jeger et al.
(1981 b) also provided experimental evidence
for their model, when they found that mix-

tures of two cultivars differing in their par-
tial resistance to S. nodorum can retard dis-
ease progress to such an extent that the pres-
ence of only 25 % of the more resistant cul-
tivar reduces disease to approximately the
amount in the more resistant pure stand.

Disease progress in this experiment varied
in three years. In 1983, the infection level was
low, and S. nodorum progressed slowly from
lower to upper leaves due to unfavourable dry
weather. Under low infection conditions,
however, the mixture benefit was clear, and
the amount of disease in mixtures was sub-
stantially less than the expected mean of the
component pure stands.

Fig. 8. Effects of S. nodorum on the grain weight of
spring wheat cultivars Tähti and Kadett and their
mixture in relation to uninoculated controls in
1984.

Fig. 9. The data of grain yield of spring wheat cultivars
Tähti and Kadett and their mixtures under dis-
ease induced stress compared with the control
in 1985.

Fig. 10. Effects of S. nodorum on the grain weight of
spring wheat cultivars Tähti and Kadett and
theirmixtures in relation to uninoculated con-
trols in 1985.

38



In 1984, the latter part of the growing sea-
son was rainy, and following inoculation sub-
sequent disease progress was rapid. The over-
all disease level was high, and even moderately
resistant Kadett was rather badly diseased
(Karjalainen et al. 1983). However, under
heavy infection conditions the disease progress
in mixtures seemed to be less than the expected
mean of component pure stands. The data
from experiments in 1985 contribute to the
previous observations, although in this case
the mixture benefit was not as high as in 1984.

Extensive experiments on variety mixtures
and the way they affect the disease progress
of race-specialized pathogens have been car-
ried out (Wolfe and Barrett 1980, Fried et
al. 1981, Munk 1983). However, only limited
data are available on the effects of mixtures
against pathogens not specialized to cultivars.
Brönnimann and Fossati (1976) studied mu-
tant lines of cultivar Zenith in mixtures and
found that mixtures reduced the disease de-
velopment and consequent yield reduction
caused by S. nodorum. Significant disease
protection was also found by Ayanru and
Browning (1977) studying the disease epi-
demics of Victoria blight in heterogeneous oat
populations, as well as by Grummer and Roy

(1966) examining the brown spot disease
caused by Helminthospohum oryzae on inter-
varietal mixtures of rice. Limited data also
suggest (Clark 1980) that such interspecific
mixtures as oats and barley can retard the
progress of the disease caused by unspecial-
ized Bipolaris sorokiniana. Although the data
sofar are not extensive, it seems evident that
cultivar mixtures can provide a practical
means for the control of non-specialized pa-
thogens.

The reason why mixtures can retard disease
development is not well understood. It has
been proposed (Burdon 1978) that in a pure
stand of plants having uniform susceptibility
to a particular pathogen, the replacement of
a proportion of these plants by resistant ones
reduces the amount of inoculum available for
subsequent dispersal within the stand. In ad-
dition, the replacement of susceptible plants

by resistant ones results in a decline in the
density of the remaining susceptible plants
thus increasing the average distance that in-
oculum has to travel between one susceptible
plant and another. Consequently, the in-
creased distance often means a reduced prob-
ability of the spread of inoculum. Other fac-
tors, such as induced resistance (Johnson and
Allen 1975) have been suggested, and recent
experiments on barley mixtures and powdery
mildew (Chin et al. 1984, Wolfe 1985) con-
firm therole of induced resistance as a factor
influencing the disease development in mix-
tures.

In the case of S. nodorum and wheat mix-
tures, it is probable that the reduction in the
proportion of susceptible tissue and the sub-
sequent reduction in the probability of in-
oculum spread may play an importantrole in
retarding epidemic development. In addition,
it is apparent that crop morphological traits
such as leaf angle, leaf form, distance between
different leaves, and some other factors af-
fecting microclimatic conditions may play
some role in regulating the force of escape
mechanisms. It is widely known (e.g. Eyal

1981) that escape mechanisms are among the
important basic factors concerning the disease
resistance of wheat to S. nodorum. S. nodo-
rum spreads from lower parts of the canopy
by step-wise movement to the upper leaves,
and the speed of the disease progress is great-
ly dependent on humid conditions inside the
canopy (Scharen 1964). Consequently, any
crop traits that reduce the humidity in the
canopy might have some important conse-
quences for disease development.

The yield data of the present study indicate
that disease induced yield reduction was quite
different in three years. In 1983, the yield re-
duction caused by S. nodorum was small and
in 1985 moderate, while in 1984 the crop losses
were very serious as a consequence of heavy
disease pressure. The grain yield reduction was
obviously mainly due to the overall reduction
in photosynthetic leaf area. It was observed
in 1984 and 1985 that S. nodorum clearly
shortened the duration of green leaf area by

39



destroying plants leaf by leaf. Consequently,
infection greatly reduced grain weight, but
grain number per ear was also affected.

The data from 1983 and 1985 experiments
provide evidence that mixtures can prevent
disease induced yield loss to a large extent
under low or moderate infection conditions,
since both grain yield and grain weight of the
mixture were less affected by the disease than
the pure components. However, in 1984 the
infection was severe and caused high losses,
and the mixtures did not seem to prevent yield
losses effectively. Hence, although mixtures
appear to buffer disease development substan-
tially, it is not always obvious that comparable
yield advantages will be achieved. This con-
troversy between disease data and yield mea-
surements has been a well-known problem for
a long time in mixture trials (Suneson 1949,
Parlevliet 1979). Jeger et al. (1981 b) have
pointed out that great care must be taken be-
fore ascribing any yield benefits observed in
mixtures to any disease reduction also ob-
served.

The apparently low yield benefits for mix-
tures in 1984 compared with the observed dis-
ease reduction may be the outcome of several
reasons. First, it may partly be due to the
small plot size used in this experiment, as it
is known that mixture benefit is best obtained
in large plot yield trials. Another explanation
for the low yield benefit in relation to the sig-
nificant disease reduction may be the observa-
tion (Scharen and Taylor 1968, Obst 1977)
that the ability of S. nodorum to reduce grain

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41



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SELOSTUS

Lajikeseosten viljelyarvo kevätvehnän
tuotannossa; seosten kyky vähentää
Sepfor/a-taudin aiheuttamia satotappioita

Reijo Karjalainen
Helsingin yliopiston kasvipatologian laitos
ja kasvinjalostustieteen taitos
00710 Helsinki

Helsingin yliopiston Viikin koetilalla selvitettiin vuo-
sina 1983—1985 kevätvehnän lajikeseosten vaikutusta
Septoria-taudin leviämiseen ja sen aiheuttamiin satotap-
pioihin. Koeruudut infektoitiin keinotekoisesti Septoria
nodorum -sienellä, ja taudin aiheuttamia satotappioita
verrattiin käsittelemättömiin koeruutuihin. Koelajikkei-
na olivat taudinaltis Tähti sekä tautia kohtalaisesti kes-
tävä Kadett.

Tulokset osoittivat, että taudin lisääntymisnopeus la-

mixtures. Proc. 4th Barley Genet. Symp., Univ. of
Edinburgh, Edinburgh, p. 435—440.

Zadoks, J. C. & Schein, R. D. 1979. Epidemiology and
plant disease management. 427 p. Oxford University
Press, Oxford.

Ms received

jikeseoksessa väheni kestävämmän Kadett-lajikkeen ta-
solle. Taudin määrä seoksessa oli lähempänä kestäväm-
pää lajiketta ja aina vähäisempikuin seoksen komponent-
tien aritmeettinen keskiarvo. Tehokkaimmin lajikeseok-
set pystyivät korvaamaan kohtalaisen voimakkaan
Septoria-laudm aiheuttamia satotappioita mutta voimak-
kaan saastunnan aiheuttamaa satotappiota ne eivät sa-
nottavasti estäneet. Parhaan sadon tuotti Kadett-lajike
voimakkaankin tauti-infektion jälkeen.

42