PHOMA LINGAM : EXPLICATIVE APPROACH OF A VARIETAL BEHAVIOR OF SUSCEPTIBILITY AND TOLERANCE TO THE FUNGUS PENETRATION AND THE DEVELOPMENT OF ATTACK
André Pérès and Blandine Poisson
CETIOM, Centre de Grignon - BP 4 - 78850 THIVERVAL-GRIGNON
e-mail : peres@cetiom.fr
ABSTRACT
The purpose of this study was to search for biological data to understand a varietal behavior of susceptibility and tolerance to the disease.
These studies, which were carried out on 3-4 varieties in 3 different areas in 1995 and 1996, were based on series of visual observations and in vitro pathological analyses made over the complete vegetative period.
The results showed that in the case of a tolerant type, the most important biological stages of the disease were significantly longer than in the case of a susceptible type (fungus penetration twice as long, later development of necrosis, slower development of attack on the root-collar). It also showed the absence of any new internal colonization of collar tissues after the new start of growth on the tolerant type (unlike susceptible type). This explanatory study of the biological characterization of a tolerant behavior tends to show the existence of defence reactions in the tolerant type ; this hypothesis should be taken into account in the global understanding of the host-fungus relationships.
KEYWORDS : Leptosphaeria maculans , varieties, suceptibility, tolerance
INTRODUCTION
Blackleg caused by Leptosphaeria maculans (Phoma lingam) is one of the main diseases of rapeseed in France. Among the measures of agronomical control, the choice of a tolerant variety is the solution to favour (Pinochet and al, 199 ). To carry out research into this subject, it is necessary to have a good knowledge of the biology of the parasite and of its relationships with the plant. The present study, led during two years (1995 and 1996), aims at trying an explicative approach of the varietal behavior of tolerance and susceptibility as regards phoma in a simple experimental process : some pathological analysis of the plant and some observations of symptoms during all the vegetative cycle. Thus, this elementary work is a simple analytical and descriptive approach which is a complement to the thorough studies on the relation host-fungus and defence reactions notably done by the National Institute of Agronomical Research (Roussel S., 1998).
MATERIALS AND METHODS
The experimental process relies on an experimental device in the field and on observation and analysis work in the laboratory.
In the field, a line of each of these varieties is sown : Synergy or Samouraï (susceptible), Inca (susceptible) and Vivol (tolerant). Every five days to ten days, 15 to 20 plants are taken from the emergence until the rosette-stage in autumn, then in spring, from the shooting until the beginning of flowering. This device is set up on three localities (Nancy, St. Pathus and Béziers) under a medium contaminating pressure.
In the laboratory, the phoma symptoms are quantified on the samples (leaf spots and collar necrosis) and the fungus on the plant is detected by plating pieces of plant tissues on malt-agar medium.
RESULTS
Analysis and observations in autumn and winter (emergence stage - rosette stage)
The comparison of the three varieties in 1995 shows that, in the case of a tolerant variety (Vivol), the settling of the disease is far longer than in the case of a susceptible variety (Synergy and Inca). In particular, one notice a far later appearance of the necrosis on collar in the case of Vivol (Table 1). Concerning the susceptible varieties, the very fast infection of the young leaves and of the collar shown by the pathological analysis implies that, either the two organs are simultaneously contaminated, or the mycelial progress from the leaf to the collar is very quick (Table 1). Some similar results are obtained in 1996.
Table 1 : number of days between the first release of ascospores and the detection of the phoma (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 |
The comparative evolution of the percentages of affected plants on their leaves underlines the tolerance of Vivol and the suceptibility of Synergy and Inca during the first three months of the rapeseed growth (Figure 1).
Figure 1 : Comparative evolution on the percentage of plants with phoma spots (3 varieties)
The studies assessment over the past two years (1995 et 1996) on the time period for the appearance of the necrosis of phoma on collar confirms the contrast between the susceptible type and the tolerant one very well (Table 2).
Table 2 : Time period for the appearance of the necrosis of collar
|
Varieties |
Localities |
Delay of collar necrosis manifestation (days)* |
|
Inca |
St. Pathus |
61 |
|
Synergy |
St. Pathus |
78 |
|
Synergy |
Nancy |
59 |
|
Samouraï |
Béziers |
94 |
|
Average / suceptible var. |
|
73 |
|
Vivol |
St. Pathus |
139 |
|
Vivol |
Nancy |
134 |
|
Vivol |
Béziers |
119 |
|
Average / tolerant var. |
|
131 |
*: Number of days between the first mycelial detection and the appearance of the necrosis on collar
Analysis and observations in spring (shooting - beginning of flowering)
The pathological analysis made in spring on older plants gave just few pieces of information, due to the difficulties to isolate the pathogen at this vegetative stage (strong presence of saprophyte fungi in competition). However, on the locality of Nancy (1995), the internal presence of phoma is shown on some plants of Synergy without external phoma symptoms, whereas no mycelial presence of the pathogen on Vivol is detected (Table 3). This result implies that, in the conditions of the study, any new colonization of the tissues is impossible on Vivol (tolerant), whereas it goes on in the case of Synergy (susceptible).
Table 3 : % of plants infected by phoma (results of the pathological analysis on 20 plants without external symptoms of phoma / locality : Nancy in 1995)
|
Vegetative stages |
% phoma on collar * |
% phoma on collar* |
|
|
Synergy (suceptible) |
Vivol (tolerant) |
|
Beginning of shooting |
28 |
0 |
|
End of shooting |
13 |
0 |
|
Beginning of flowering |
8 |
0 |
|
End of flowering |
12 |
0 |
* : presence of phoma on collar detected by isolation
On the contrary, observation of the symptoms on the attacked plants gives a wider information. Thus, the appearance of the leaf infection, which is the result of late post-winter contaminations, confirms the contrasted behavior of tolerant and susceptible varietal types (Vivol is clearly less attacked than Synergy and Inca - Table 4). The notation of collar necrosis in spring confirms the previous varietal behavior (figure 2)
Table 4 : % of plants with spots of phoma (end of winter / St. Pathus / 1995)
|
Varieties |
|
Dates |
|
|
|
|
|
27/02/95 |
07/03/95 |
13/03/95 |
22/03/95 |
30/03/95 |
|
|
|
|
|
|
|
|
SYNERGY |
27 |
25.8 |
25.8 |
34.8 |
31.1 |
|
INCA |
66.5 |
57.8 |
59.2 |
73.7 |
60.7 |
|
VIVOL |
4.3 |
0 |
1.3 |
0 |
3.9 |
Figure 2 : Development of % of plants with phoma necrosis on collar (spring 1996 / St. Pathus)
CONCLUSION
These studies has enabled an interesting progress as regards methodology on one hand and on their actual results on the other hand.
Concerning the methodology, the importance of a pathological analysis for the detection of the fungus in the vegetative tissues of young plants is emphasized (plants without symptoms of the disease). This method of detection is reliable but heavy. On older plants, the competition between phoma and saprophyte fungi is an obstacle to detection.
Concerning the results, the pathological analysis then the symptomatological observations in time show that the settling of the disease is slow on a tolerant varietal type and quick on a susceptible varietal type. The quickness of the settling of the disease is in concordance with the level of final infection by phoma. Furthermore, in the conditions of one of these studies, a new mycelial colonization turns out to be impossible on the tolerant type from the stage of "the beginning of elongation" contrary to the suceptible varietal type.
These results, only elementary and descriptive, do not allow to explain the inner mechanisms of defence on the adjustment of a resistance against the pathogen, but they imply its existence. The study of these mechanims is precisely the object of the thorough works done on the relation host-fungus (INRA), (Roussel S., 1999)
REFERENCES
Pinochet X., 1996 - Dossier technique CETIOM - Unpublished data
Roussel S., 1999 - Expression de la résistance du colza à Leptosphaeria maculans au stade jeune et au stade adulte - Caractérisation cytologique et épidémiologique - Thèse Université Rennes