STUDY OF RAPESEED SUSCEPTIBILITY TO PRIMARY CONTAMINATION OF LEPTOSPHAERIA MACULANS IN RELATION TO THE PLANT VEGETATIVE STAGE

Blandine Poisson and André Pérès

CETIOM - Centre de Grignon - B.P.4 - 78850 Thiverval Grignon - France

e-mail : poisson@cetiom.fr

ABSTRACT

 

The purpose of this study was to check the hypothesis considering higher phoma susceptibility of young vegetative stages (2 - 4 and 6 leaves) at autumn with cultivar (susceptible and tolerant). The results showed that the longer the delay of leaf lesions, the earlier the contamination stage was (mainly noticed on tolerant cultivar). Furthermore, the stem canker severity observed at maturity revealed a differential behaviour according to the susceptibility or tolerance of the cultivar. So, this study involved that susceptibility to primary contamination of known vegetative stages seemed different according to cultivars. These results should be taken in the global understanding of agronomic, fungicidal and genetic control.

 

KEYWORDS : winter oilseed rape, plastic tunnel, cultivars, stem canker.

 

INTRODUCTION

 

Previous works have indicated that rapeseed was most susceptible to infection by Leptosphaeria maculans (anamorph Phoma lingam) prior to the 6-leaf stage of growth (Alabouvette and Brunin, 1974; MacGee and Petrie, 1979). This present study aims at checking the hypothesis (or the report which is generally established) of the higher susceptibility to phoma of the young vegetative stages in autumn, taking into account the varietal aspect (approach limited to three cultivars).

 

MATERIALS AND METHODS

 

The study was carried out in the field under a plastic tunnel which was fitted out with a self-regulated fog-system. It consisted in only one contamination realized on three vegetative stages of rapeseed obtained by sowing which were delayed in time. Three cultivars with a different behaviour to phoma were compared: Bristol (susceptible), Goeland (slightly susceptible) and Vivol (tolerant). A supply of 18 stubbles naturally infected with the previous year's phoma (two stubbles per block) was realized on the 16/10/96. Some observations through a microscope have shown the presence of numerous mature pseudothecia on these stubbles which can produce contaminating ascospores. On the 16/10/96 (the day of inoculation), the vegetative stages of the rapeseed were as follow: 6 leaves for the first seedling (S1), 4 leaves for the second seedling (S2), and 2 leaves for the third seedling (S3).

Two plants a cultivar and a day of seedling on each block were taken every four or five days. The observations on these plants consisted in writing down the presence or the absence of leaf spots and also in counting them. They consisted in observing the presence or not of a stem canker as well. On each taken plant, in vitro analysises were realized on different organs (cotyledon, leaf and collar) in order to detect the presence of the fungus.

 

RESULTS AND DISCUSSION

 

Analysis of the climatic and contaminating context

The fog-system was put into a plastic tunnel in order to keep a high humidity to favour the ascospores releases and for that reason has allowed the contamination. From the day of the the stubbles supply which contained mature pseudothecia, the climatic conditions into the tunnel were relatively favourable to a contamination (generally 10 hours a day of relative humidity> 90 per cent - the  average temperature between 8 and 14°C).

 

Dates and stages of appearance of the first leaf spots in autumn

If one observes the most distant stages of contamination (2 and 6 leaves), it appears that the time period of appearance of the first leaf spots is longer for the earliest stage of contamination (for 35 days in average for the 2-leaf stage against 19 days for the 6-leaf stage) - table I. This phenomenon was  particularly marked on Vivol (63 days for the 2-leaf stage against 29 days for the 6-leaf stage). Consequently, this report goes against the hypothesis of the higher susceptibility of the young stages (generally established in relation the attack on collar). Concerning the varietal behaviour, the usual observations of the previous studies were found again (Pérès and Poisson ``unpublished data''). Vivol, tolerant cultivar, expressed more lately the disease on its leaves than the more susceptible cultivars such as Bristol or Goeland (15 to 42 days later according to the stage of contamination). The observation of the 4-leaf stage seemed to reveal a fairely high susceptibility of this stage to the contamination with a very fast expression of the symptoms, particularly on Vivol and Goeland cultivars (six days). On the contrary, Bristol was an exception, expressing the disease more lately than for a contamination at the 6-leaf stage. (21 days against 14 days). 

 

Table I : Date of the first leaf spots and time period of the supply of contaminating stubbles (16/10) -  First leaf spots

 

Cultivars

Seedlings/Stage of contamination

Date of the first leaf spots

Number of days between the supply of the stubbles infected by phoma and the first leaf spots

VIVOL

S1/6-leaf stage

14/11 

29

 

S2/4-leaf stage

22/10

6

 

S3/2-leaf stage

18/12 

63

GOELAND

S1/6- leaf stage

30/10 

14

 

S2/4- leaf stage

22/10 

6

 

S3/2-leaf stage

06/11 

21

BRISTOL

S1/6-leaf stage

30/10 

14

 

S2/4-leaf stage

06/11 

21

 

S3/2-leaf stage

06/11 

21

 

Evolution of the leaf infection (leaf spots)

 

¨ Overall evolution of the leaf infection (for all cultivars)

The evolution of the number of leaf spots for all cultivars showed an increasing rate of the infected plants at the three stages from the 30/10 to the 14/11  (Figure 1). Then, either a stagnation (for the infected plants at the 6-leaf stage) or a decreasing (for the infected plants at the 4-leaf and 2-leaf stages) were observed. It was probably due to the loss of older leaves, which have fallen or withered.

The number of leaf spots varied according to the stage of contamination: the infected plants at 2-leaf stage were less infected than the infected ones at the 4-leaf and 6-leaf stages and certainly because the number of leaves and the leaf surface of the plants at the 2-leaf stage were lower. Focusing the plants contaminated at 4-leaf stage, it seemed that in this case they had got as many leaf spots as the infected ones at 6-leaf stage; this observation seemed to confirm the hypothesis of the highest susceptibility of this stage which was previously evoked.

                                                                                      

Figure 1 : Evolution of the number of leaf spots per plant for all cultivars at the three stages of contamination

 

 

¨ Individual evolution of the leaf infection (per cultivar and per stage of contamination)

The evolution of leaf infection showed a very good level of tolerance of Vivol (low level of infection and very slow evolution) and also a more important susceptibility of Goéland in particular on the 14/11, and this whatever the vegetative stage at the date of contamination.

 

 In vitro analysises

 

¨  In vitro analysis on leaves

The results  showed that the fungus was faster isolated on the infected plants at the earliest stages ( 2 to 9 days for the 2-leaf and 4-leaf stages against 14 days for the 6-leaf stage)- table II. These analysises revealed consequently the highest susceptibility to the leaf infection at early stages (and more particularly at 2-leaf stage) and this, whatever the cultivar. In particular, concerning the 2-leaf stage, if the contamination was the fastest among the three modalities, it was the slowest concerning the expression of the leaf spots. This tends to demonstrate that the incubation time was longer in the case of a very young plant.

 

 

Table II: Date of the first isolation on leaf and time period of the supply of infected stubbles - first isolation on leaf

 

Cultivars

Seedling/Stage of contamination

Date of the first isolation on leaf

Number of days between the supply of infected stubbles and first isolation on leaf

VIVOL

S1/6-leaf stage

30/10 

14

 

S2/4-leaf stage

22/10 

6

 

S3/2-leaf stage

18/10 

2

GOELAND

S1/6-leaf stage

30/10 

14

 

S2/4-leaf stage

22/10 

6

 

S3/2-leaf stage

22/10

6

BRISTOL

S1/6-leaf stage

30/10 

14

 

S2/4-leaf stage

25/10 

9

 

S3/2-leaf stage

18/10

2

 

¨ In vitro analysis on collar

In the case of the 4-leaf and 6-leaf stages of contamination, the fungus was present in the collar and in the leaf the same day (table III). It revealed a very fast progression of the mycelium of the leaf to the collar, if one admits  the hypothesis of an internal mycelium progression from the leaf to the collar (hypothesis to check). This was observed on Vivol but also on Goeland and Bristol. On the contrary, in the case of the contamination at the 2-leaf stage, the time period was longer (11 days in average), particularly on Vivol.

 

Table III: Time period of the first isolation on leaf - first leaf spots and first isolation on leaf - first isolation on collar

Cultivars

Seedling/Stage of contamination

Time period of first isolation leaf/ first leaf spots (day)

Time period of the first isolation on leaf/ first isolation on collar (day)

VIVOL

S1/6-leaf stage

15

0

 

S2/4-leaf stage

0

0

 

S3/2-leaf stage

61

19

GOELAND

S1/6-leaf stage

0

0

 

S2/4-leaf stage

0

0

 

S3/2-leaf stage

45

3

BRISTOL

S1/6-leaf stage

0

0

 

S2/4-leaf stage

12

0

 

S3/2-leaf stage

19

12

 

Rapeseed stem canker

 

Rapeseed stem canker observations were realized on six plants by stage of contamination and by cultivar. No stem canker was observed at the beginning of winter (table IV). On the contrary, an observation realized at the end of the growing period (maturity) put into light severity of infection on a relatively large scale (from 1.88 to 5.66). This observation revealed a different behaviour whether it was a susceptible and a slightly susceptible cultivars (Bristol and Goeland) or a tolerant cultivar (Vivol).

In the case of the susceptible cultivar (Bristol), the severity of infection was the highest whatever the vegetative stage during the contamination. However, the maximum was observed on the earliest modality of 2-leaf stage contamination. So, this case agreed with the expected result (or supposed) as well on the varietal aspect as on the stage of contamination aspect.

In the case of the tolerant cultivar (Vivol), the results were more unexpected on two levels:

- Vivol reached an unusual level of severity in the case of a 6-leaf stage contamination (4.06). It reached approximately the same level of infection on a field trial close by it.

-it was in the case of an early contamination (2-leaf stage) that the infection was the lower ( 1.88 for a 2-leaf stage contamination against 4.06 for a 6-leaf stage contamination). In the case of the slightly susceptible cultivar (Goeland) an intermediate behaviour was observed with, again, the lowest susceptibility when the contamination was early.

 

Table IV: G2 Note (scale of 0 to 9) at the beginning of winter and at maturity

 

 

Stage at the moment of contaminating stubbles

 

VIVOL

 

GOELAND

 

BRISTOL

G2 NOTE beginning of winter

6-leaf stage

0

0

0

(on the 18/12/96)

4-leaf stage

0

0

0

 

2-leaf stage

0

0

0

G2 NOTE at maturity

6-leaf stage

4.06

4.64

5.02

(on the 23/06/96)

4-leaf stage

3.44

4.36

4.53

 

2-leaf stage

1.88

3.61

5.66

                                                           

CONCLUSION

 

The study of the susceptibility of rapeseed according to its vegetative stages at the date of the contamination aimed at checking the hypothesis (or the report which is generally established) of the highest susceptibility to phoma of the young vegetative stages in autumn, taking into account the varietal aspect (limited approach to 3 cultivars). However, some differences of susceptibility to phoma appeared according to the vegetative stage of contamination and to cultivar:

- the time period of appearance of the first leaf spots in autumn was longer for the earliest stage of contamination. This behaviour was particularly marked on Vivol. The reaction at the intermediate stage of contamination (4 leaves) depends on the cultivar (Vivol and Goeland reacting really faster than Bristol)

- the evolution of the population of the leaf spots was globally more important on plants contaminated at 4-leaf and 6-leaf stages because of the higher number of leaves offering more areas of contamination. Vivol confirmed its tolerant behaviour by the later date of appearance of the leaf spots and by the slower evolution of their number in autumn.

- the greatest susceptibility to the leaf contamination of the early stages (and more particularly of the 2-leaf stage) and this, whatever the cultivar, was revealed by the presence of mycelium at the leaf level. The incubation time was longer in the case of a very young plant.

- the final notation of severity of stem canker (G2 at maturity) gave some results which depended very strongly on the varietal behaviour. In the case of susceptible cultivar (Bristol), it is really the earliest contamination which generates the most important severity of infection at maturity (G2 note). On the contrary, in the case of the tolerant cultivar (Vivol), the result goes against this report; it was the latest contamination (6-leaf stage) which generated the most important severity of infection. Moreover, this observation revealed a certain weakness of Vivol with two disapointing G2 notes which were close to G2 notes of slightly susceptible cultivar.

 

REFERENCES

 

Alabouvette C. and Brunin B., 1974. Recherches sur la maladie du colza due à Leptosphaeria maculans (Desm.) Ces. et de Not. IV- Pouvoir infectieux des pycniospores et sensibilité variétale. Ann. Phytopathol., 6 (3), 265-275.

 

Mac Gee D.C and G.A. Petrie, 1979. Seasonnal patterns of ascospore discharge by Leptosphaeria maculans in relation to blackleg of oilseed rape. Phytopathology 69: 586-589.