JOURNAL OF AGRICULTURAL SCIENCE IN FINLAND
Maataloustieteellinen A ikakauskirja

Vol. 56: 143—154

Rhizoclonia solani damping-off on spring turnip rape and spring
rape (Brassica spp.) in Finland

RISTO TAHVONEN*, JUKKA HOLLO*, ASKO HANNUKKALA
and AARNE KURPPA*
Institute of Plant Pathology, University of Helsinki
SF-00710 HELSINKI 71, Finland

Abstract. Damping-off occurred in 90 % of the spring oilseed rape fields surveyed in
1981—1982. On the average, 10 % of the plants were infected with damping-off. 6 % of the
fields were severely infected (> 30 % of the plants affected) by damping-off and 38 % were
uninfected or only slightly infected ( < 5% of the plants affected). The incidence of damping-
off was higher in those fields which had been under oilseed rape in earlier years. In a crop
rotation experiment, the amount of damping-off increased from 2 % to 20 %, and finally to
38 %, depending upon whether turnip rape had been grown on the same part of the field
once, twice or three times. Crops other than Cruciferous ones were grown for 1 or 3 years
between the turnip rape crops. Rhizoclonia solani Kiitin was isolated from 76 % of the
affected plants. The R. solani isolates produced severe damping-off on rape and turnip rape
in pathogenicity tests. R. sotani isolates from barley, potato and lettuce brought about only
mild cases of damping-off, or else only a decrease in the growth of the plants. Fusarium
avenaceum Sacc. was the only other isolated fungus which was pathogenic.

Introduction

The cultivation of spring turnip rape
(Brassica campestris L. ssp. oleifera) and
spring rape (Brassica napus L. var. oleifera)

increased sharply in Finland during the
1970’5. The area under these crops at the
beginning of the 1980’s has been around

* New adress: Agricultural Research Centre, Depart-
ment of Plant Pathology, SF-31600 JOKIOINEN,
Finland

55 000—60 000 ha, spring turnip rape ac-
counting for over 50 000 ha of this area. The
yields of spring oilseed rape have remained
stable under the varying growing conditions
in Finland and oilseed rape has been con-
sidered to be extremely suitable as a break
crop in cereal monoculture (Köylijärvi
1982).

The pathogens affecting spring turnip rape
and spring rape under Finnish conditions
have been largely neglected. In Sweden and

Index words; spring oilseed rape, damping-off, Rhizoclonia solani

143

https://www.c-info.fi/en/info/?token=vRC6zoYvzpcCfIvC._QoTR98qHAywU1vGYMVhLw.ke3AKrQT5Z4FfVWlhbXECpO3elNlwl8P0KaxD_cuPndzNJ58aRWAQLzsHhYEzVAitz0IC9nF1P2uR1rsFbb6NaMJbuVxpy-dr55CbZusQexlPkssR-y-FHAxsOMQ6CPxnaHO_u2MWbtcE6d0nQ1JdOc1L8QNfFN5kqeXldtW_q_1dpzeDh_fyB1kAyMiXFE6EuF6vzUiR0TcXSZFAm_9PTqbCyYnZn4JsML1


elsewhere in Europe, Leptosphaeria macu-
lans (Desm.) Ces. & de Not, Sclerotina scle-
rotiorum de Bary and Alternaria spp. are the
most serious pathogens affecting oilseed
rapes. In addition to these, damping-off and
foot rot are also serious pathogens in Cana-
da (Berkenkamp and Vaartnou 1972, Raw-
linson and Muthyalu 1979, Kruger 1979,
Svensson 1982). In a long-term crop rotation
experiment carried out by the Department of
Plant Pathology, the University of Helsinki,
damping-off started to increase strongly on
turnip rape after the crop had been grown on
the same area twice or three times. This led
to the decision to determine whether the
damping-off situation was the same with
commercial crops of turnip rape and rape.

Material and methods

1. The crop rotation experiment

Spring turnip rape is included in two
rotations in the crop rotation experiment
being carried out by the Department of Plant
Pathology at viikki.

In one of the rotations, turnip rape is
grown on the same block of the experiment
every second year. Either barley or wheat are
used as the preceding crop. In the other rota-
tion, spring turnip rape is grown on the same
site every fourth year, the order of the crops
in the cycle being turnip rape barley/
wheat oats broad beans.

The experiment was established in 1977.
During the period 1979—1982 the turnip
rape variety was »Torch», and in 1979 the
variety »Span» was used.

In 1979 a sample of 300 plant individual
was analysed. Owing to the increase in the
amount of damping-off, the number of anal-
ysed plants was increased during 1980—1982
to 1200 plants/year, i.e. 25 plants/sample
plot. The analysis was carried out at the
growth stage 2 (Harper and Berkenkamp

1975).

2. Collection of seedling samples from fields
and their analysis

Samples of spring turnip rape and spring
rape seedlings were collected from 62 fields
in southern Finland in 1981 and 50 fields in
1982. There were usually 2—4 true leaves on
the seedlings at the time when they were col-
lected. A single sample (about 400 seedlings)
consisted of 4 subsamples which were taken
from different points in the field. Each sub-
sample consisted of about 100 seedlings
which were collected from 2—4 adjacent
rows. Soil samples (10 litres/field) were also
taken from the topsoil in 1981. In addition,
information about the cultivation history of
each field during the preceding four years
was collected in 1981.

The subsamples from each field were anal-
ysed separately. The growth stage of the
seedlings was determined and the seedlings
classified into three groups: healthy = 0, in-
fected = 1 and dead = 2. The damping-off
°7o and disease index were calculated for each
field as the mean of the subsamples.

Seedlings were grown in the soil samples at
a high temperature ( + 25°C) and a low tem-
perature (+ 11°C). Spring turnip rape of the
»Torch» variety were used as the test plants.
Plastic pots (1.5 1) were filled with the sample
soil and 30 seeds sown in each at a depth of
2 cm. A pot containing sterilised sand in-
stead of soil was used as a control. There
were three replications. After the plants had
grown for three weeks they were classified in
the same way as the seedling samples taken
from the fields. Only 30 soil samples could
be studied at the low temperature.

3. Fungal isolations

Fungal isolations were made on plants suf-
fering from damping-off. A small piece of
tissue was cut from the borderline between
healthy and infected tissue. The specimen
was surface sterilised for 30 s in 1 % NaOCl
and then rinsed in 94 % ethanol. When dry
the specimens were transferred onto cornmeal

144



agar (Difco) containing 100 ppm strepto- pension (1 Petri dish/100 ml sterile water)
mycin (Streptomycin sulphate). The petri
dishes were incubated at room temperature
and the fungi which developed from the
plant tissue were identified under a stereo
and a light microscope after 7 days and after
21 days.

4. Pathogenicity tests

The pathogenicity of 16 fungal isolates to
spring turnip rape and spring rape was tested
in 1981. Seven out of the ten Rhizoctonia
solani Kuhn isolates were from spring turnip
rape or spring rape. The other 3 Rhizoctonia
solani isolates were obtained from potato,
barley and lettuce.

The Fusarium species included in the test
were Fusarium avenaceum Sacc., Fusarium
culmorum Sacc. Fusarium oxysporum
Schlecht. and Fusarium sambucinum Fuc-
kel. In addition, the pathogenicity of Ulocla-
dium consortiale (Tiihm.) Simmons and
Preussia aemulans (Rehm) v. Arx was tested.
The latter fungus was isolated from each of
the spring turnip rape seedlings suffering
from damping-off which were growing on
one field in Halikko.

The pathogenicity test was carried out
both in a Jacobsen germinator and as a pot
experiment in the greenhouse. The fungal
isolates for the test were grown for one week
on PDA medium (Difco) in 9-cm Petri
dishes.

The test plants used in the Jacobsen germi-
nator were spring turnip rape »Span». The
seeds were immersed in an aqueous suspen-
sion (1 petri dish/100 ml sterile water) of the
test fungus. When the infected seeds were
dry they were sown in the Jacobsen germina-
tor. The control seeds were immersed in an
aqueous suspension of the growth medium.
There were four replications of each fungus
(25 seeds/replication). The germination of
the seeds and condition of the seedlings was
checked 7 and 10 days after sowing.

Fresh peat was used as the growing
medium in the greenhouse. An aqueous sus-

was mixed into plastic boxes (50 X 25 X
9,5 cm), each containing 7 litres of peat.

In the control treatment a mixture of
water and the growth medium was added to
the peat. After being innoculated, the boxes
were kept in the greenhouse for one week
and the test seeds then sown in the boxes.
The spring turnip rape varieties »Span»
(0-variety), »WW-1644» (00-variety) and
»Candle» (000-variety), and the spring rape
variety »Oro» were used as the test plants.
One row of seeds of each variety were sown
in each box (30 seeds/row). The seedlings
were grown until they reached the 3—4 leaf
stage. The number of seedlings which deve-
loped was counted weekly. The mean tempe-
rature throughout the experiment was
+ 20°C. At the end of the experiment the
height of the seedlings from the root collar to
the tip of the uppermost leaf was measured
and the degree of infection determined ac-
cording to the previously described criteria.

5. Statistical treatment

The number of seedlings suffering from
pre-emergence damping-off was calculated
in the pot experiment as the differences in
emergence between t(ie control and test treat-
ments. Seedlings killed by pre-emergence
damping-off were placed in group 2 of the
disease scale.

The fields were classified into four groups
on the basis of the incidence of damping-off:
o—s0—5 %, 6—15 %, 16—30 % and over 30 °7o.

A value weighted on the basis of the culti-
vation history of the field was given for the
cultivation of oil seed crops. The more fre-
quent and the closer to 1981 that oil seed
crops had been grown in the field, the larger
was the weighted value (Table 3).

The material was tested using the Stu-
dent’s t-test, analysis of variance and regres-
sion analysis. The differences between the
different treatments were analysed by means
of LSD in cases where the F value obtained
in analysis of variance was statistically signif-

145



146

icant. The degree of statistical significance is
denoted as follows: P < 0.05 = +, P <

0.01 = + +
, and P < 0.001 = + + +.

Results

1. The symptoms of damping-off

The initial symptoms of post-emergence
damping-off are a partial darkening of the
hypocotyl and radicle of the turnip rape or

rape seedlings. As the disease progresses the
whole of the hypocotyl becomes dark and
shrinks to form a thread-like strand. (Fig.
1). The plants are either killed off by the
post-emergence damping-off or recover to
some extent as a result of secondary growth
of the hypocotyl, but remain poorly devel-
oped. Pre-emergence damping-off is only
evident as a poor plant stand.

2. Incidence of damping-off

In the crop rotation experiment, spring
turnip rape was grown as the first and second
crop on the same experimental block in the
longer crop rotation, and as the second and
third crop in the shorter crop rotation during
1979—1982. The number of plants suffering
from damping-off increased in both crop
rotations as the number of times spring tur-
nip rape was grown on the same site in-
creased (Table 1). When turnip rape was
grown for the second time on the same block
the number of plants suffering from damp-
ing-off increased to about 20 °7o, and when it
was cultivated for the third time the number
of damping-off seedlings was already about
38 %.

Damping-off occurred on 56 of the 62
fields examined in 1981 and on 46 of the 50
fields in 1982. The mean number of seedlings
affected by damping-off on the fields in each
year was about 10 %. About 20 % of the
fields in both years had a damping-off rate
of 16—30 % of the seedlings, and 6 % of the
fields had a damping-off rate of over 30 %

(Fig. 2a). There were no regional differences
as regards the incidence of damping-off. The
mean number of seedlings suffering from
damping-off in spring turnip rape fields was
9.5 %, and in spring rape fields 12 %. There
were considerable differences in the inciden-
ce of damping-off in the subsamples from
the same fields.

It was also possible to estimate the amount
of pre-emergence damping-off in the experi-
ments in which seedlings were grown in the
soil samples collected in 1981. The incidence

Fig. I. Damping-off caused by Rhizoctonia solani on
turnip rape, a) on germlings, b) large damaged
plants.



Table 1. The occurrence of damping-off on spring turnip rape in the crop rotation experiment during 1979—1982.

Crop rotation Year Number of times turnip rape
grown on the same area

1. 2. 3.
Damping-off %

Turnip rape cereal turnip rape etc. 1979 16,0
1980 22,0
1981 54,8
1982 20,5

19,0 37,7
Turnip rape cereal cereal 1979 3,0
broad bean turnip rape etc. 1980 1,5

1981 35,3
1982 11,5

2,3 23,4

of damping-off was 18 % at the higher tem-
perature, and 26 % at the lower tempera-
ture. (Fig. 2b). The proportion of pre-
emergence damping-off was considerably
greater at the lower temperature than at the
higher one (Table 2).

Information was collected in 1981 about
the cultivation history of 38 fields. Spring

Table 2. Effect of a high and a low growing tempera-
ture on the amount of damping-off in spring
turnip rape seedlings. Mean of 30 soil sam-
ples.

Post-emergence Pre-emergence
damping-off damping-off

°7o %

High temperature
+ 25°C 14,9 4,8
Low temperature
+ 11°C 11,5 14,9
Difference 3,4 —lO,l
t-value 0,8 —4,l +* +

turnip rape or spring rape had been grown at
least once on most of the fields during the
previous four years, which indicates that
spring oil seed crops are very frequently
grown on the same area (Table 3). On the ba-
sis of the experiments involving the growing
of seedlings at higher temperatures, the
amount of damping-off increased as the
weighted value for the cultivation of oil seed
crops increases (Fig. 3). The incidence of
damping-off varied between wide limits in
the case where oilseed rape had been grown
in the fields during the preceding four years.
If oilseed rape had not been grown in the
fields during the preceding four years, the in-
cidence of damping-off was low.

Fig. 2. The abundance of damping-off on spring tur-
nip rape and spring rape in 1981 and 1982. A)
on the basis of seedling samples taken from the
fields, B) in seedling growing experiments car-
ried out at temperatures of 25°C and 11°C in
soil samples collected in 1981.

147



Table 3. Distribution of the number of times oil seed crops were grown and the weighted value for the cultivation
of oilseed rape in the sample fields in 1981.

Cropl Number of fields Weighted value for,
1977 1978 1979 1980 % oilseed rape 2

0 0 0 0 10 26,3 0
0 1 0 0 8 21,1 2
1100 1 2,6 3
0 0 1 0 7 18,4 3
1010 1 2,6 4
0110 3 7,9 5
0001 2 5,3 4
0101 1 2,6 6
1101 1 2,6 7
0011 2 5,3 7
0111 2 5,3 9

Total 38 100

1 1 = spring turnip rape or spring rape, 0) = other crop (mainly cereals).
2 The weighted value for the cultivation of oil seed rape was obtained by multiplying the value for the 1977 crop

by 1, for the 1978crop by 2, for the 1979crop by 3 and the 1980 crop by 4, and then adding the results together.

4. Fungal isolations from plants suffering
from damping-off

The fungus most frequently isolated from
spring turnip rape and spring rape plants suf-
fering from damping-off was Rhizoctonia

solaniKiitin (Table 4). The fungus was iden-
tified on the basis of the colour and morpho-
logical characteristics of the mycelia and
sclerotia. There were no differences in the
occurrence of Rhizoctonia solani in different
years.

Fusarium fungi which were most frequent-
ly isolated were Fusarium avenaceum Sacc.,
Fusarium culmorum Sacc., Fusarium oxys-
porum Schlecht. and Fusarium sambucinum
Fucel. None of these Fusarium species were
clearly dominant. The species from the fol-
lowing genera were also isolated from the
seedlings: Gliocladium spp., Ulocladium
spp., Penicillium spp., Mucor spp. and
Chaetomium spp. and more rarely species
belonging to the genera of Absidia, Alter-
naria, Aspergillus, Botrytis, Cephalospo-
rium, Chrysosporium, Cladosporium, Co-
niothyrium, Cylindrocarpon, Dendryphion,
Graphium, Humicola, Monodictys, Mortie-
riella, Papulaspora, Phoma, Preussia, Rhi-
zopus, Thamnidium, Trichoderma and
Torula.

5. Pathogenicity tests

All the Rhizoctonia solani isolates obtain-

Fig. 3. Dependence of the incidence of damping-off
on the cultivation of oil seed crops. The
damping-off observations are from seedling
growing experiments using soil samples at
25°C. The figures are for both pre-emergence
and post-emergence damping-off.
y = 5.15 e 0 24 », r = 0.52 + + +

148



4

Table 4. The fungi most frequently isolated from turnip rape and rape suffering from damping-off.

% of the isolations

Crop rotation Field samples Seedling culti-
experiment 1981—1982 vation 1981
1979—1982 25 „c uoc

Rhizoclonia solani 89,7 62,3 80,1 71,7
Fusarium spp. 5,0 11,4 19,2 7,4
GHocladium spp. 7,9 1,6 8,1 5,2
Ulodadium spp. 1,5 1,2 3,3 2,1
Penicillium spp. 6,2 2,7 4,2 5,7
Mucor spp. 7,3 12,2 2,3 2,3
Chaetomium spp. 2,2 1,4 2,9 4,7
Other fungi 11,3 5,9 14,0 7,0
Number of isolations 406 1330 307 191

ed from the turnip rape and rape plants were
pathogenic to the test plants (Fig. 4). They
almost completely prevented the devel-
opment of the seedlings on the peat substrate
in the greenhouse (Table 5). In the case of
the experiment with the Jacobsen germinator,

three of the Rhizoctonia solani isolates were
weaker pathogens than the other four which
were isolated from turnip rape and rape. The
Rhizoctonia solani isolate obtained from
potato was not pathogenic in either of the
tests. The Rhizoctonia solani isolate from

Fig. 4. The pathogenicity of the fungal isolations to spring turnip rape and spring rape determined in a Jacobsen
germinator and in peat substrate in the greenhouse. F values: Jacobsen germinator, fungi = 25,1***,
Greenhouse, fungi = 100***, varieties = 3,5, fungi x varieties = 1,2. If there is the same letter at the
top of two columns, then there is no significant difference (P = 0,05) as regards the degree of infection.

149



lettuce was pathogenic in the Jacobsen ger-
minator but not in the peat substrate. On the
other hand, the Rhizoctonia solani isolate
from barley was pathogenic in the peat
substrate but not in the Jacobsen germi-
nator.

As far as the Fusahum species were con-
cerned, Fusahum avenaceum was pathogen-
ic in both tests. Fusahum oxysporum was
pathogenic in the Jacobsen gemination but
not in the peat substrate. Fusahum culmo-
rum, Fusahum sambucinum, Ulocladium
consortiale (Tiihm) Simmons and Preussia
aemulans (Rhem) v. Arx did not exhibit any
pathogenicity.

The interaction for pre-emergence damp-
ing-off between the fungi and the plant
varieties in the pathogenicity test carried out
in the peat substrate was statistically sig-
nificant. Pre-emergence damping-off oc-
curred most in Oro spring rape and least
in the number variety WW-1644 (Table 5).
Fusahum avenaceum caused the most pre-
emergence damping-off to the Oro variety,
and less damage to WW-1644 than to the oth-
er varieties. The Rhizoctonia solani isolates
obtained from potato and barley caused less
pre-emergence damping-off to the WW-1244
variety than to the other varieties. However,
there were no significant differences at the
end of the test between the varieties of turnip
rape and rape as regards the degree of infec-
tion.

The seedlings grown in peat substrate in-
fected with Rhizoctonia solani isolates were
shorter (Table 6). All the fungi, apart from
Fusahum sambucinum, clearly decreased the
height growth of the seedlings in comparison
to the controls. Although the Rhizoctonia
solani isolates from potato and lettuce did
not cause very much damping-off, they sig-
nificantly reduced the height growth of the
seedlings.

Discussion

Rhizoctonia solaniKiitin was found to be
the most common and most pathogenic fungal

pathogen causing damping-off on spring tur-
nip rape and spring rape. The fungus is
polyphagic and a number of strains with
varying properties are known (Mordue
1974). In Finland, R. solani causes diseases
on cereal and grass crops and on potato
(Ylimäki 1967, Seppänen 1979, Mäkelä &

Parikka 1980). It has earlier been found to
be a causal agent of damping-off in green-
house cultivations (Linnasalmi 1952). R.
solani has also been found to be pathogenic
to turnip rape and rape in Europe and in
Canada (Raabe 1939, Berkenkamp& Vaart-
nou 1974). After the seedling stage has been
reached, R. solani has been frequently iso-
lated from turnip rape and rape suffering
from foot rot (Petrie 1974, Kruger 1979).

All the tested R. solani isolates from
spring turnip rape and spring rape were ex-
ceedingly pathogenic. The difference
between the pathogenicity of these isolates in
the Jacobsen germinator may be due to the
nutrient requirements of the isolates. An ex-
ternal source of nitrogen is an important fac-
tor in infection by R. solani (Weinhold et
al. 1972). In the case of the peat substrate,
which is rich in nitrogen, there were no dif-
ferences between the isolates obtained from
spring turnip rape and spring rape, and the
isolate obtained from barley was also patho-
genic. Behaviour of the isolate obtained
from lettuce was, on the other hand, quite
the opposite as it was pathogenic only in the
Jacobsen germination. The isolate from
potato was not pathogenic to turnip rape or
rape. This supports the generally held belief
concerning the specialised nature of R. sola-
ni isolates from potato (Parmeter et al.
1969).

As far as the other tested fungi were con-
cerned, only Fusahum avenaceum Sacc. was
clearly pathogenic to turnip rape and rape.
However, owing to the fact that it oc-
curred in very small numbers only, its im-
portance as a causal agent of damping-off
was small. As it appears, according to Tah-
vonen (1979) and the results of this study
concerning the isolations made on plants af-

150



Table 5. Pre-emergence damping-off on turnip rape and rape in the pathogenicity tests carried out in the peat
substrate.

Fungus Pre-emergence daraping-off Vo

Turnip rape Rape

Span WW-1644 Candle Oro Mean
0-variety 00-variety 000-variety

Rhizoctonia solani »turnip rape»* 98,8 98,6 99,6 99,7 99,2
Rhizoclonia solani nbarley» 31,9 10,0 31,7 28,8 25,6
Rhizoctonia solani »potato» 12,9 1,0 17,3 11,1 10,6
Rhizoctonia solani »lettuce» 8,6 0,0 3,8 5,6 4,5
Fusarium avenaceum 13,0 4,0 18,3 33,3 17,1
Fusarium culmorum 12,0 2,0 0,0 5,6 4,9
Fusarium oxysporum 5,2 0,0 0,0 7,4 3,1
Fusarium sambucinum 5,2 0,0 1,0 5,6 1,4
Ulocladium consorliale 9,5 1,0 5,8 14,8 7,8
Preussia aemulans 7,7 0,0 0,0 7,4 3,8
Control 0,0 0,0 0,0 0,0 0,0
Mean 46,9 41,6 45,6 48,1
* mean of 7 isolates
F values: fungi > 100+ ++

, varieties = 15,8+ + + and fungi x varieties = 2,2 + ++ .

LSD (P = 0,05) fungi = 3,4 %, varieties = 2,0 % and variety with the same fungus = 8,2 %.

Table 6. Effect of the fungal isolates on the height growth of spring turnip rape and spring rape in the pathogeni-
city tests carried out on the peat substrate.

Fungus Seedling height (cm)

Turnip rape Rape

Span WW-1644 Candle Oro Mean

Control 11,9 11,8 12,0 11,7 11,9 a 1
Fusarium sambucinum 11,9 12,2 11,4 11,6 11,8 a
Fusarium oxysporum 10,8 10,7 10,4 10,9 10,7 b
Fusarium culmorum 10,2 10,7 10,5 10,6 10,5 b
Ulocladium consorliale 10,8 9,9 9,6 10,2 10,1 b
Preussia aemulans 9,6 10,5 9,6 10,0 10,0 b
Fusarium avenaceum 10,7 9,1 7,3 8,9 9,0 c
Rhizoclonia solani »lettuce» 8,7 8,6 7,6 7,9 8,2 cd
Rhizoctonia solani »potato» 7,7 8,0 6,6 7,5 7,4 d
Rhizoclonia solani »barley» 4,8 4,3 3,4 4,0 4,1 e
Rhizoctonia solani »turnip rape
and rape» 0 0 0 F 0 0

F values: fungi = 63,04+ ++
, varieties = 6,4 ++ and fungi x varieties = 1,05.

1 If two mean values are followed by the same letter, then the means do not differ significantly (P = 0,05) from
each other.

fected by damping-off, that the importance
of seed-borne fungi is of only slight im-
portance in Finland, thiram dusting which is
widely used here is of dubious usefulness.

No information was obtained about pre-
emergence damping-off when collecting the
plant samples from the experimental plots
and the fields used in commercial farming.

151



In Linnasalmi’s (1952) experiments, for tion experiment and the field observations,
instance, R. solani caused a considerable
amount of pre-emergence damping-off. The
proportion of pre-emergence damping-off
was considerably large in the case of seed-
lings grown in soil taken from fields, espe-
cially at a low temperature. The low tempe-
rature used in the experiments is quite equi-
valent to the mean soil temperature after
sowing dates in Finland. This indicates that a
plant stand of spring oilseed rape may be re-
duced as a result of pre-emergence damping-
off. The large variance in the subsamples
also indicates that the pathogen is present in
the soil in patches. The patchy occurrence of
the disease caused by R. solani is due to the
growth pattern of the fungus in the soil
(Baker et al. 1967). In addition to reducing
the plant stand, the initial development of
the seedlings can also be slowed down. This
was reflected in the pathogenicity tests as
slower growth of the seedlings. R. solani has
been found to slow down the initial develop-
ment of soy beans, although there were no
clear symptoms of the disease (Grau & Mar-
tinson 1979). The sparse stands of turnip
rape and rape, and their slow initial develop-
ment, weakens their ability to compete with
weed plants (Köylijärvi and Tulisalo 1982).

According to the results of the crop rota-

the incidence of damping-off clearly in-
creased as the number of times spring turnip
rape and spring rape were grown on the same
site increased. It was possible to explain
27 % of the variation in the incidence of
damping-off in the fields by means of the
weighted value for oil seed crops. In the
crop rotation experiment, the incidence of
damping-off increased in both rotations,
although spring turnip rape was grown on
the same area only every fourth year in the
longer rotation cycle. Furthermore, the R.
solani isolate obtained from barley was
pathogenic to spring turnip rape and to
spring rape. These observations indicate a
need for further studies into the behaviour of
R. solani in cereal oilseed rape rotations. In
addition, the role of R. solani as a causal
agent of foot rot on spring oilseed rape in
Finland should also be studied, as well as the
effect of Rhizoclonia diseases on the yield
from spring oilseed rape.

Acknowledgements. We extend our sincere thanks to
Raision tehtaat, and especially to Tuula Laukkanen,
M.Sc., and Mrs Pirkko Koivula who assisted in collec-
ting the soil and seedling samples and who enabled addi-
tional information to be obtained from the farms in
question.

References

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Ms received April 24, 1984

SELOSTUS

Rhizoctonia solani kevätrypsin ja
-rapsin taimipoltteen aiheuttajana

Risto Tahvonen*, Jukka Hollo*,
Asko Hannukkala ja Aarne Kurppa*
Helsingin yliopiston kasvipatologian laitos,
00710 Helsinki 71

Kevätrypsin ja -rapsin viljely lisääntyi voimakkaasti
1970-luvulla Suomessa, mutta näitä kasveja vaivaavat
taudit ovat jääneet tutkimuksessa vähälle huomiolle.
Kasvipatologian laitoksen vuoroviljelykokeessa Viikissä
on taimipolte ollut kevätrypsin vuosittainen tauti. Tä-
män vuoksi päätettiin selvittää, onko tilanne sama käy-
tännön viljelmillä.

Kevätrypsi ja -rapsi viljelmiltä kerättiin vuosina 1981
ja 1982 taiminäytteitä kasvien ollessa 2—4 lehtiasteella.
Taimien tautisuus määritettiin ja sairaista kasveista teh-
tiin sienieristyksiä. Lisäksi vuonna 1981 kerättiin pelloil-
ta maanäytteet, joissa tehtiin kahdessa lämpötilassa
(+ 11°C ja +25°C) taimikasvatuksia.

Rhizoctonia solani Kiihn oli yleisin taimipoltteisista
kevätrypseistä ja -rapseista eristetty sieni sekä vuorovil-
jelykokeesta että pelloilta kerätyistä näytteistä. R. sola-
ni oli myös voimakkain taimipoltteen aiheuttaja pato-
geenisuustesteissä. Patogeenisuustesteissä perunalta, sa-
laatilta ja ohralta eristetyt R. solani isolaatit eivät ol-
leet niin voimakkaita taimipoltteen aiheuttajia kuin ke-

* Uusi osoite: Kasvilauliosasto, MTTK, 31600 Jokioinen

vätrypsiltä ja -rapsilta eristetyt isolaatit. Fusarium -sie-
niä eristettiin myös yleisesti, mutta vain F. avenaceum
Sacc. oli selvästi kevätrypsin ja -rapsin patogeeni. Kos-
ka siemenlevintäisten taimipoltteen aiheuttajien merki-
tys näyttää vähäiseltä aikaisemmin Suomessa tehtyjen
havaintojen ja tämän tutkimuksen sienieristysten perus-
teella, on kevätrypsin ja -rapsin siementen tiraamipeit-
taus kyseenalainen toimenpide.

Taimipoltetta esiintyi tutkimusjaksona 1979—82 vuo-
sittain kasvipatologian laitoksen vuoroviljelykokeessa.
Myös käytännön viljelmiltä tehtyjen havaintojen mu-
kaan on taimipolte yleinen tauti. Vuonna 1981 tutkituis-
ta 62 pellosta 56 pellolla esiintyi taimipoltetta ja vuonna
1982 tutkituista 50 pellosta 46 pellolla esiintyi tätä tau-
tia. Taimipoltteisia taimia oli keskimäärin 10 % pelloil-
la. Peltoja, joilla oli sairaita kasveja 16—30 %, oli
kumpanakin vuotena noin 30 % ja peltoja, joilla oli tai-
mipoltteisia taimia yli 30 %, oli vastaavasti noin 6 %.

Maanalaisen taimipoltteen määrää ei voitu määrittää
kerättäessä kasvinäytteitä pelloilta ja vuoroviljelyko-
keesta. Peltomaissa tehdyissä taimikasvatuksissa pystyt-
tiin arvioimaan verrannemaiden avulla maanalaisen tai-

153



mipoltteen suuruus. Matalassa lämpötilassa kuoli
maanalaiseen taimipoltteeseen keskimäärin 14.9 % tai-
mista, mikä oli merkittävästi enemmän kuin korkeassa
lämpötilassa maanalaiseen taimipoltteeseen kuolleiden
taimien luku (4.8 %).

Vuoroviljelykokeessa taimipoltteisten kevätrypsien
määrä lisääntyi, kun kevätrypsin viljelykerrat samalla

paikalla lisääntyivät. Sama ilmiö oli havaittavissa myös
käytännön viljelmillä.

Saatujen tulosten perusteella taimipolte harventaa
kevätrypsi- ja -rapsikasvustojasekä hidastaa taimien al-
kukehitystä. R. solani -sienen käyttäytyminen rypsi-
viljakierrossa vaatii kuitenkin lisätutkimuksia.

154