HOW MANY PATHOGENS DO CAUSE BLACKLEG DISE ASE 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 desc ribed as Leptosphaeria maculans (Desm) Ces et de Not, an ascomycete wh ose 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 distingui shed as belonging to either a 'pathogenic' pathotype or a 'weakly path ogenic' pathotype. However, the reliability of the morphological, and even the pathogenic, criteria for a discrimination of isolates has bee n questioned. 'Modern methods' for the study of the polymorphism of fu ngi, ie isozymic and molecular techniques, usually allow an unequivoca l discrimination of the 2 groups. Moreover, these techniques recently demonstrated an important genetic distance between the 2 groups of iso lates, suggesting that they may actually belong to 2 distinct species. The 'weakly pathogenic' group can be further separated into 3 genetic ally distinct subgroups. Moreover, it has been suggested that the prev alent subgroup could be different in Europe, Australia or America. Fin ally, 4 other L maculans, pathogenic to different species of crucifero us weeds have been described in Canada. Since only pathogenic and morp hological criteria were studied, the relationship of these additional L maculans to the 'pathogenic' group or 'weakly pathogenic' groups obt ained from Brassica is still unknown. The original discrimination into 2 pathotypes has been highly prejudicial to the understanding and stu dy of this pathosystem. As a consequence, the pathogenicity of each gr oup, or subgroup of isolate to crucifers and its involvement in the bl ackleg disease has to be reconsidered.