Variabilite de Pseudocercosporella herpotrichoides (Fron) Deighton, agent du pietin-verse des cereales

Pseudocercosporella herpotrichoides, l’agent du piétin-verse des céréales, présente une variabilité intraspécifique mise en évidence par plusieurs critères : pouvoir pathogène, morphologie et physiologie in vitro, profils protéiques et enzymatiques, sensibilité aux fongicides. Une synthèse des données est réalisée concernant ces différents critères. L’ensemble de ces caractères permet la différentiation de 2 groupes de souches au sein de l’espèce: souches à croissance normale ou type W ou P h var herpotrichoides d’une part, souches à croissance lente ou type R ou P h var acuformis d’autre part. La variabilité à l’intérieur de ces 2 populations est également importante si l’on se base sur ces mêmes critères d’étude. Les divers aspects de la variabilité de l’agent pathogène sont développés de manière à montrer leur intérêt dans la lutte contre cette maladie, tant au niveau de l’utilisation raisonnée de fongicides qu’à celui de la recherche de sources de résistance.

Pseudocercosporella herpotrichoides, the cause of eyespot in cereal crops, shows variability characterized on the basis of pathogenicity, cultural morphology, physiology, protein and isozyme patterns and sensitivity to fungicides. The species herpotrichoides comprises 2 main pathotypes distinguished by these criteria: fast growing colonies (I or N) correlated with W-type and P h var herpotrichoides; slow growing colonies (II or L) correlated with R-type and P h var acuformis. Growth rate (on agar medium), colony morphology and pigmentation, conidial form allow differentiation into 2 groups. Nevertheless, there are considerable differences between individual isolates in both groups and these criteria do not fully confirm the distinction made on the basis of pathogenicity. The first virulence group is characterised by the W type isolates: their pathogenicity is high to wheat and barley, and is low to rye. The second group is characterized by the R type isolates, which are equally pathogenic to wheat, barley and rye. A type is also described which is pathogenic to couch grass. Apart from these types, 1 type was observed which is pathogenic only to wheat and Aegilops squarrosa. All types are pathogenic to Aegilops ventricosa. There are considerable differences in pathogenicity between individual isolates in both the W and the R type groups. Differences in controlled environment conditions may explain the varying results in the studies on pathogenicity of both types. The penetration of the stem by the W type (or N, or var herpotrichoides) occurs earlier than that by the R type (or L, or var acuformis), which are isolated more frequently at the end of the season. Both types can exist in the same plant or even on the same lesion; interactions between both types can influence the development of eyespot. Electrophoretic studies show that specific patterns (proteins, isozymes) exist for the W type, N type and var herpotrichoides, and also for the R type, L type and var acuformis. Contrary to the former characteristics, high homogeneity is observed within the same type. P herpotrichoides isolates can be clearly and more objectively differentiated by biochemical markers. The development of resistance to fungicides has induced different phenotypes which react in a very specific way to 1 type of compound. Sensitive or resistant isolates to benzimidazole, thiophanate and phenylcarbamate compounds are common in both types. Three types are distinguished according to their sensitivity to ergosterol biosynthesis inhibitors: la or Na which are sensitive to DMI, Ib or Nb and II or L which are less sensitive to DMI. Recently, several prochloraz resistant strains (II or L type) have been isolated on winter wheat in France. These different aspects of the variability of the fungus are discussed to show their importance in eyespot control both for the optimal use of fungicides and for research into host resistance sources. The perfect state in which P herpotrichoides is found, Tapesia yallundae, has only recently been described and its importance is not yet known. It provides a new means for the study of the relationships between isolates types.