LEPTOSPHAERIA MACULANS (PHOMA LINGAM) ON OILSEED RAPE :
A SUMMARY OF THREE YEARS OF EPIDEMIOLOGICAL STUDIES (1995, 1996 ET 1997)
André Pérès, Blandine Poisson, Annette Penaud,
Laurent Jain and Etienne Pilorgé
CETIOM, Centre de Grignon, B.P. 4, 78850 THIVERVAL GRIGNON
e-mail : peres@cetiom.fr
ABSTRACT
The main purpose of these Phoma epidemiological studies carried out for 3 years was to get a better understanding of the biology of the fungus, to gather weather and biological data to enable us to develop a forecasting model and define control methods adapted to the new biological context.
Beyond the development of study methods, the main epidemiological acquirements obtained by observations in 3-6 different sites were :
- a wide gathering of biological and weather data from the field and studies carried out in controlled and semi-controlled conditions
- main knowledge allowing us to develop a forecasting model and a decision rule for a preventive control (description and frequencies of different biological stages, definition of the most accurate parameters : temperatures - rainfalls - humidities)
- explanatory and comparative knowledge of varietal behaviors towards disease suceptibility and tolerance
- the underlining of an important variability of the fungus and the characterization of strains (using a wide gathering of isolates)
Taking into account of this knowledge, the main research prospects concern three subjects : carrying on the study of genetic and fungicidal controls, reinforcing the forecasting model and continuing the study of strain characterization.
KEYWORDS : Phoma, rapeseed, epidemilogy, ascospores, pseudothecia, variability, cultivar
INTRODUCTION
Phoma is one of the principal diseases affecting oilseed rape in France. The damage and loss caused vary considerably according to the location and may involve destruction of the crop in extreme cases. Despite the progress made in genetics, chemical control is also necessary. If this is to be efficient, it must be well thought out, since systematic chemical protection may be quite risky. To guarantee the success of protection, crops must be treated at the time of contamination. This approach requires a good understanding of fungal pathogen epidemiology; this has been the objective of epidemiology research undertaken over the past three years (95-96-97). In addition, the acquisition of biological and climatic data should ultimately allow a climatic forecast system to be developed.
GENERAL PROGRAM OF RESEARCH
Research was based on the following topics :
- pseudothecia maturation : the relation between the evolution of pseudothecia maturation, on one hand, and climatic conditions, varietal susceptibility and the age of contaminating stubble, on the other hand
- ascospore releases : the relation between ascospores releases and climatic conditions (methodology refined prior to study)
- suceptibility of oilseed rape to phoma according to its vegetative condition
- an explicative approach to the behavior of suceptible or tolerant varieties with respect to phoma
- the variability of the fungal pathogene
MATERIALS AND METHODS
The methodology of each of these topics was refined.
1 . Pseudothecia maturation
Pseudothecia were observed during the first two months of oilseed rape growth, on stubble naturally infected by the previous year's phoma. Pseudothecia samples were opened and observed under a microscope to determine their maturation stage on a 4-level scale (Poisson and al, 1999). Climatic data were recorded by automatic meteorological station.
2. Ascospores releases
The ascospores were trapped under slides coated with ovalbumin during the first two months of growth of oilseed rape at 3 to 7 geographical locations. In 1995, "passive" type trap were used and, in 1996 and 1997, "dynamic" types (Pérès and al, 1997 and 1999). Climatic data were recorded by automatic meteorological station.
3. Suceptibility of oilseed rape to phoma according to its vegetative stage
The study was carried out using a plastic tunnel, fitted with a fogging system. Three varieties of oilseed rape at three vegetative stages, obtained by planting at three different stages (2 leaves - 4 leaves - 6 leaves) were contaminated at one specific date. They were contaminated by stubble naturally infected with the previous year's phoma (Poisson and al, 1999).
4. Explicative approach to varietal tolerance and suceptibility behaviour
The state of contamination (pathological analysis) and symptoms expressed (leaf spots and necrosis) for 4 phoma-suceptible or tolerant varieties were compared. Pathological analysis was undertaken on plant samples taken from a field every 10 days during the first three months of growth (Pérès and al, 1999).
5. Fungal pathogene variability
It was determined on the basis of isolates collected from 6 locations representing the principal oilseed rape production areas in France. Groups and sub-groups of L. maculans were differentiated by PCR and PCR-RFLP, respectively (Penaud A., 1997).
RESULTS
1 .Pseudothecia maturation
Observation over several years of stubble infected in the current year shows that it is the gradual decrease in post-summer temperatures and the rainy periods prior to and during pseudothecia maturation which condition the release of ascospores (Table 1).
Table 1 : Ascospores maturation and climatical data (Surgères-1997)
|
Months |
Decades |
% of mature pseudothecia |
Days of rainfalls* (days) |
Total mm rainfalls* (mm) |
Average température (°C) |
|
September |
3 |
0 |
0 |
0 |
21 |
|
October |
1 |
0 |
2 |
18 |
19 |
|
October |
2 |
10 |
5 |
52 |
17 |
|
October |
3 |
68 |
8 |
69 |
15 |
|
November |
1 |
62 |
10 |
80 |
12 |
|
November |
2 |
68 |
16 |
158 |
12 |
|
November |
3 |
92 |
24 |
225 |
11 |
* : cumulated data
On the other hand, observation of infected stubble (from the previous year) resulting from a study conducted in natural conditions shows that 1-year old stubble produces active pseudothecia which discharge live, contaminating ascospores (Table 2).
Table 2 : Development of mature pseudothecia on 1-year old stubble
|
Number of days after the beginning of the study |
% of mature pseudothecia |
|
16 days |
1 % |
|
52 days |
10% |
|
73 days |
50% |
|
80 days |
100% |
Finally, a comparative study of pseudothecia maturation was carried out on infected stubble using 7 varieties which behave differently as regards phoma (3 suceptible varieties - 3 slightly suceptible and 1 variety with low susceptibility). No difference in the dates when pseudothecia and ascospores appeared was detected, irrespective of the variety.
2. Ascospores releases
The capture of ascospores over several years regularly shows that ascospore releases are closely linked to principal climatic factors (Pérès and al, 1997 and 1999) :
- the first incidence of ascospores generally appears just after the first significant post-summer rainy-period, accompagnied by a drop in average temperature (Appendix); it strongly coincides with the appearance of the first mature pseudothecia
- the next incidence of ascospores generally coincides with rainy periods or high relative humidity; it may last for 2 to 3 months
- the number of days of rain and the rainfall level required to stimulate the very first release of ascospores are fairly close to a constant value, irrespective of the year and geographical location (Table 3)
Table 3 : Ascospores releases and rainfalls
|
Localities |
Year 1 |
Year 2 |
Year 3 |
|||
|
|
NR(days) |
RL (mm) |
NR(days) |
RL(mm) |
NR(days) |
RL(mm) |
|
St. Florent |
8 |
69 |
9 |
88 |
10 |
69 |
|
St. Pathus |
12 |
67 |
11 |
54 |
10 |
71 |
|
Nancy |
- |
|
15 |
129 |
12 |
93 |
|
Béziers |
- |
|
7 |
130 |
7 |
94 |
NR : number of days of rain from planting until the first release of ascospores
RL : rainfall level (mm) from planting until the first release of ascospores
3. Expression of the disease (leaf spots and collar necrosis)
Since work was principally focussed on the contamination phase (pseudothecia and ascospores), the expression of the disease was studied in less detail. On the other hand, attacks of the disease have not been significant in the past three years. Nevertheless, in a few cases, it has been possible to evidence a relation between early incidence of autumn contamination and the appearance of collar necrosis before the winter, resulting in high severity ratings for attacks at maturity.
4 . Suceptibility of oiseed rape to phoma according to its vegetative stage
Extreme susceptibility to foliar contamination at early growth stages (particularly at the 2-leaf stage) has been indicated by pathological analysis, revealing very fast infection by phoma in early growth (2 to 9 days fot the "2 and 4-leaf" stages, compared with 14 days for the "6-leaf" stage). However, at the end of the growing period (maturity), results vary according to the susceptibility of varieties (Table 4). Vivol, a tolerant variety, for example, is attacked less if contaminated in early stages of growth (2-leaf stage) and more when contaminated in the later stages (6-leaf stage). In the case of Bristol, a suceptible variety, there is no marked difference in suceptibility between the contamination stages.
Table 4 : Notation of phoma gravity at maturity (sclale 1-->6)
|
Contaminated stages |
Vivol (T) |
Goeland (SS) |
Bristol (S) |
|
6 leaves |
4.06 |
4.64 |
5.02 |
|
4 leaves |
3.44 |
4.36 |
4.53 |
|
2 leaves |
1.88 |
3.61 |
5.66 |
T : tolerant SS : slightly suceptible S : suceptible
5. Explicative approach to varietal tolerance and suceptibility behaviour
The results show that, in the case of a tolerant variety (Vivol in this study), all the biological stages of the disease are longer than in a susceptible variety (Synergy and Inca in this study - Table 5). It then follows that, for Vivol, the level and the severity of attacks are lower (for example, in Nancy in 1996, the "phoma" rating for Vivol was 1.9, compared with 7.3 for "Synergy").
Table 5 : Number of days from the first ascospore release until the detection of phoma (Fungus isolation, leaf spot and collar necrosis)
|
|
Synergy |
Inca |
Vivol |
|
1st phoma detection / young leave |
7 |
14 |
25 |
|
1st phoma detection / collar |
7 |
14 |
36 |
|
1st phoma spot / leave |
- |
21 |
21 |
|
1st phoma necrosis / collar |
84 |
84 |
175 |
6.Fungal pathogene variability
The characterization of 104 isolates by PCR revealed the existence of two groups (TOX+ and TOX°) and three sub-groups within the TOX° group (NA1, NA2 and NA3). It seems that their distribution depends on the vegetative sampling site, the geographic location and the time of year when samples are taken (Figure 1).
Figure 1 : Characterization of phoma strains in 1996
St Pathus a = autum, b = dry plant/harvest , P = pseudothecia
CONCLUSION
Due to the acquisition of extensive biological data, the research undertaken has enabled significant progress to be made as far as our knowledge of the contamination mechanism is concerned. Simultaneous acquisition of climatic data has contributed to the refinement of a climatic forecast system. These various studies have taken into account the evolution of the context with respect to fungal variability (characterization of strains) and new varieties of oilseed rape (differentiation of reactions with regard to fungal aggression). This research is currently being continued within the framework of a European project (IMASCORE: Fair contract CT96-1669 - coordinator M. H. Balesdent) - (Balesdent et al., 1998).
REFERENCES
- Balesdent H. and Rouxel T. - IMASCORE : un projet de recherche européen sur le ``Phoma du colza'' - Les Rencontre Annuelles du CETIOM - 1-4/12/1998 - p.30-31
- Penaud A., Jain L., Poisson B., Balesdent M.H. and Pérès A., 1999 : Stucture of populations of Leptosphaeria maculans in France - 10th International Rapeseed Congress 26-29 september 1999 - Proceedings
- Penaud A., 1997: Caractériser les souches de Phoma du colza - CETIOM, Oléoscope n°38, p.31-32
- Pérès A., Auclert B., Fernandès J. and Maisonneuve C., 1997 : Phoma du colza - La lutte efficace passe par la capture de spores - CETIOM, Oléoscope n°38 p.27-28
- Pérès A. et Poisson B., : Phoma du colza - avancées en épidémiologie, CETIOM, Oléoscope n°40, p.37-38
- Pérès A. and Poisson B. 1999 : Explanatory study of a varietal behavior of suceptibility and tolerance to the fungus penetration and the development of attack - 10th International Rapeseed Congress 26-29 september 1999 - Proceedings
- Poisson B. and Pérès A., 1999 : Study of rapeseed susceptibility to primary contamination of Leptosphaeria maculans in relation to the plant vegetative stage - 10th International Rapeseed Congress 26-29 september 1999 - Proceedings
- Poisson B. and Pérès A., 1999 : Studies related to maturation of Leptosphaeria maculans pseudothecia on rapeseed stubbles with stem canker - 10th International Rapeseed Congress 26-29 september 1999 - Proceedings
Appendix
Climatical data and ascospores captures in the field (St. Pathus : autumn 1995)
