STUDIES RELATED TO MATURATION OF LEPTOSPHAERIA MACULANS PSEUDOTHECIA ON RAPESEED STUBBLES INFECTED BY STEM CANKER
Blandine Poisson and André Pérès
CETIOM - Centre de Grignon - B.P.4 - 78850 Thiverval Grignon, France
e-mail : poisson@cetiom.fr
ABSTRACT
CETIOM has observed for 4 years the maturation of Leptosphaeria maculans pseudothecia: one study was carried out on stubbles of different cultivar origins and another one on one year old stubbles. It was respectively shown that the different infected stubbles had slightly the same ability to induce contamination and that stubbles in the following crop year were able to produce new contaminating pseudothecia. These results were involved in the development of a forecast model of pseudothecia maturation.
KEYWORDS : Winter oilseed rape, cultivars, residues, scale, asci.
INTRODUCTION
For several years, the stem canker caused by Leptosphaeria maculans (anamorph = Phoma lingam) has represented the main problem of rapeseed producing regions. The new tolerant cultivars will offer the most efficient protection. In the meantime, fungicidal control is necessary but it requires new knowledges on the biology of Leptosphaeria maculans in order to establish a forecast model of the epidemic risk and to adjust fungicidal treatments better. With this aim in view, two studies have been realised in 1995 (POISSON, 1997): One of them to compare the rapidity of pseudothecia maturation between different cultivar origins and the other one to know the contaminant ability of one year old stubbles.
FIRST STUDY : COMPARATIVE EVOLUTION OF PSEUDOTHECIA MATURATION ON DIFFERENT CULTIVAR ORIGINS OF STUBBLES
MATERIALS AND METHODS
The study was realized at Saint-Pathus (Seine et Marne) on Eurol, Falcon and Samouraï (susceptible cultivars), Goéland, Navajo and Cocktail (slightly susceptible cultivars) and Capitol (tolerant cultivar). Two stubbles of each cultivar were taken in a field trial in 1995. These stubbles were set down on to a lawn on 25 July 1995. The study of pseudothecia maturation began on August 1995 and finished on January 1996. The climatic data were recorded during the study. When pseudothecia appeared, five of them were taken and observed each week. The method of observation consisted to put the pseudothecium into a drop of water on the slide and to take the content of pseudothecium out with 2 fine needles. A drop of blue methyl is added before the slide placed. Then the preparation is observed to microscope and the pseudothecium has been classed in relation to the stage of asci and ascospores maturity according to the scale in table I.
Table I : classification of asci and ascospores pseudothecium maturation
|
CLASS |
ASCI |
ASCOSPORES |
|
A |
undifferentiated (no asci) å presence of pseudo-paraphysoide tissue |
undifferentiated (no ascospores) |
|
B |
differentiated |
undifferentiated |
|
C |
differentiated (mature) |
< 8 spores/ascus |
|
|
|
< 6 cells/spore |
|
|
|
å differentiated in less than 50% of asci |
|
D |
differentiated (mature) |
8 spores/ascus |
|
|
|
6 cells/spore |
|
|
|
å differentiated in more than 50% of asci |
|
E |
empty pseudothecium |
|
+ Affectation in class C, D or E when 50% of asci or ascospores population reached the considered stage.
RESULTS AND DISCUSSION
Differences appeared between susceptible cultivars in the precocity of pseudothecia appearance (table II). We detected first pseudothecia on Eurol when they appeared between three and four weeks later on Samouraï and Falcon. However, pseudothecia of Eurol stubbles were began their differentiation in the same time that pseudothecia of Falcon and Samouraï stubbles. They were became mature the same day (on 20 September 1995) on the three cultivar stubbles. The mature pseudothecia evolution graph of susceptible cultivar stubbles showed an irregular evolution of pseudothecia maturation with a drop recorded about mid november (figure 1). In fact, the weather conditions were less favorable to the evolution of pseudothecia maturation (irregular drop of temperatures). From mid november, we can observed the arrival of a new generation with pseudothecia in class A and/or B probably because of discontinuous rainfall which occured high humidity > 90% (mean of 15 to 16 hours/day).
Table II : Dates of main stages of maturation pseudothecia according to the cultivars susceptibility of stubble rape to phoma
|
CULTIVARS |
Susceptibility of cultivars to Phoma |
Appearance date of first pseudothecia (class A) |
Appearance date of first mature pseudothecia (class D) |
Emergence date of new pseudothecia (Class A/B) |
|
EUROL |
susceptible |
03/08/95 |
20/09/95 |
? |
|
FALCON |
susceptible |
05/09/95 |
20/09/95 |
08/11/95 |
|
SAMOURAÏ |
susceptible |
25/08/95 |
20/09/95 |
24/11/95 |
|
NAVAJO |
slightly susceptible |
25/08/95 |
20/09/95 |
08/11/95 |
|
GOELAND |
slightly susceptible |
25/08/95 |
04/10/95 |
24/11/95 |
|
COCKTAIL |
slightly susceptible |
25/08/95 |
20/09/95 |
15/11/95 |
|
CAPITOL |
tolerant |
25/08/95 |
20/09/95 |
15/11/95 |
The appearance of first pseudothecia carried out the same time on stubbles of slightly susceptible and tolerant cultivars. However, the first mature pseudothecia on Goéland stubbles appeared fifteen days later in relation to others cultivars. The mature pseudothecia evolution graph of slightly susceptible and tolerant cultivars stubbles was irregular as the susceptible cultivar stubbles because of the simultaneous existence of different maturity of pseudothecia on stubbles. The irregular evolution graph was more pronounced on Capitol stubbles. The evolution of mature pseudothecia decreased more about end November on slightly susceptible and tolerant stubbles than on susceptible stubbles. The arrival of new pseudothecia at mid november allowed a new increase of mature pseudothecia rate reaching 100% in January (figure1).
Figure 1 : Evolution of mature pseudothecia percentage by class of cultivars susceptibility in relation with climatic parameters
Globally, these biological data showed a similarity in the mature pseudothecia evolution: - a phase of irregular growth due to the simultaneous existence of different maturity of pseudothecia - a phase of decrease marked by the presence of empty pseudothecia which released their ascospores and the coming of a new generation of no mature pseudothecia - at the final, a new phase corresponding to the asci and ascospores maturation of young pseudothecia. Some differences between cultivars appeared more particularly in the earliness to reach every one pseudothecia maturation stages.
SECOND STUDY : EVOLUTION OF PSEUDOTHECIA MATURATION ON STUBBLES KEPT FOR ONE YEAR IN A COLD ROOM
MATERIALS AND METHODS
The study was realized at Saint-Pathus (Seine et Marne) in autumn 1995. It carried out two stubbles of rapeseed harvested in 1994 and taken on 17 October 1994. After mature pseudothecia observation, stubbles have been conserved in a cold room (mean temperature = 9-10°C, no brightness) during a year. Taking off the cold room on 24 October 1995, stubbles have been set down on to a lawn. Two others batches of two stubbles (one of Saint-Pathus, the other of Nancy) have been observed but so that more irregular in order to compare the results. Five pseudothecia on each stubbles have been observed once a week from end October 1995 until end January 1996 as the same method described in the previous study. Each pseudothecium has been classed according to the stage of asci and ascospores maturity as the scale in table I.
RESULTS AND DISCUSSION
First observations of pseudothecia realized end October on Saint-Pathus stubbles (on 17 october 1994 stubbles) showed the only presence of aged pseudothecia staying on these stubbles. Their content has been only constituted of disorganized and mixed pseudoparaphysoide tissue; the cell wall of asci has been damaged by cold stocking conditions. Often, ascospores presented anomalies (cells number > 6, more globular cells). Several germination tests in malt water revealed no viable spores because no germinated. New pseudothecia have been appeared on 9 November (about fifteen days after the stubbles leaving in natural conditions). The climatic conditions have been particularly favorable: small discontinuous rainfall with important hours number of humidity > 90% and mean temperature increase. Differentiation of asci and ascospores is observed fifteen days after appearance of new pseudothecia. First mature pseudothecia were observed one and a half month after stubbles leaving. Germination tests revealed germinated ascospores. The evolution of mature pseudothecia increased rapidly from end December probably because of climatic conditions (high hours number of humidity > 90%, return of rainfall and mean temperature to 10°C) - Figure 2. We noticed after a high fall of temperatures (from 10°C to 0°C) a fast increase evolution of mature pseudothecia rate. The temperature shock with high humidity seemed to be factors favorating asci and ascospores maturation. Observations realized on the others batches of Saint-Pathus and Nancy stubbles confirmed these results.
Figure 2 : Evolution of % of mature pseudothecia in relation with climatic parameters
It's interesting to note the high activity of pseudothecia in winter where low temperatures (many frosts) seemed not to deteriorate their maturation. So, the stubbles aged a year are able to produce a viable inoculum which can be spread and contaminate plants.
CONCLUSION
These two studies led at Saint-Pathus have given complementary informations on the biology of Leptosphaeria maculans. The evolution of pseudothecia maturation is similar whatever the cultivar origin (first study). Stubbles stayed in a cold room during a year are able to produce new mature pseudothecia in natural conditions (second study). We can suppose that buried stubbles in field putting back superficially by agricultural labour next year would be able to produce pseudothecia and contaminate rapeseed in autumn.
Since 1994, several studies of pseudothecia maturation showed that pseudothecia were present nearly during the whole year on stubbles (PERES et al., 1996). These data allowed to complete the scale of asci and ascospores maturation (table III). Finally, these studies display the important role of climatic factors (temperature, rainfall and humidity) in the formation of pseudothecia and in the asci and ascospores differentiation.
Table III : New classification of maturation asci and ascospores of pseudothecium
|
CLASS |
PSEUDOTHECIUM |
ASCI |
ASCOSPORES |
|
A |
differentiated NO MATURE |
undifferentiated + presence of pseudoparaphysoïde tissue |
undifferentiated
|
|
B |
differentiated NO MATURE |
differentiated |
undifferentiated |
|
C |
differentiated |
differentiated |
differentiated: |
|
|
NO MATURE
|
|
+ < 8 spores/ascus + < 4 cells/spore |
|
D |
differentiated MATURE |
differentiated
|
differentiated: + 8 spores/ascus |
|
|
|
|
+ > or = 4 cells/spore |
+ Affectation in class C and D when at least one ascus and one ascospore reached the considered stage
REFERENCES
PERES A., POISSON B., MAISONNEUVE C., 1996. Comment le champignon progresse dans la plante. OLEOSCOPE n°35, p10-12.
POISSON B., 1997. Etudes relatives à la maturation des périthèces de Leptosphaeria maculans sur les pailles de colza d'hiver nécrosées au collet. ANPP - 5ème Conférence sur les Maladies des Plantes, Tours, 3-4-5 Décembre 1997, Tome I , p345-352.