HOW MANY PATHOGENS DO CAUSE BLACKLEG DISEASE IN CRUCIFERS

Blackleg in crucifers has become a disease of international importance following the tremendous increase in oilseed rape acreage in Europe, Canada and Australia. The causative agent of the disease has been described as Leptosphaeria maculans (Desm) Ces et de Not, an ascomycete whose anamorph is Phoma lingam (Tode ex Fr) Desm. The pathogen is known to be highly polymorphic in cultural characteristics and pathogenicity. Isolates obtained from Brassica sp in the early 1920s were distinguished as belonging to either a 'pathogenic' pathotype or a 'weakly pathogenic' pathotype. However, the reliability of the morphological, and even the pathogenic, criteria for a discrimination of isolates has been questioned. 'Modern methods' for the study of the polymorphism of fungi, ie isozymic and molecular techniques, usually allow an unequivocal discrimination of the 2 groups. Moreover, these techniques recently demonstrated an important genetic distance between the 2 groups of isolates, suggesting that they may actually belong to 2 distinct species. The 'weakly pathogenic' group can be further separated into 3 genetically distinct subgroups. Moreover, it has been suggested that the prevalent subgroup could be different in Europe, Australia or America. Finally, 4 other L maculans, pathogenic to different species of cruciferous weeds have been described in Canada. Since only pathogenic and morphological criteria were studied, the relationship of these additional L maculans to the 'pathogenic' group or 'weakly pathogenic' groups obtained from Brassica is still unknown. The original discrimination into 2 pathotypes has been highly prejudicial to the understanding and study of this pathosystem. As a consequence, the pathogenicity of each group, or subgroup of isolate to crucifers and its involvement in the blackleg disease has to be reconsidered.