BIOLOGY OF LEPTOSPHAERIA-MACULANS

Phoma lingam (teleomorph Leptosphaeria maculans) has long been recognized as an important worldwide pathogen of crucifer crops. The fungus survives as pycnidia and pseudothecia on colonized crucifer seeds an crop residue and is spread as rain-splashed conidia or airborne ascospores. Much is known about parasitic phases of the life cycle, but little is understood of the survival and trophic relationships of the organism on nonliving substrates. P. lingam has been classified broadly into aggressive (virulent) and nonaggressive (avirulent) strains based on pathogenicity and a number of correlated cultural, chemical, and genetic phenotypes. Increasing evidence, including RFLP analysis, mating behaviour, DNA content and chromosome number, suggests that the two strains represent different species. Variation in pathogenicity exists within the aggressive strain, and isolates have been classified into three groups, PG2, PG3, and PG4, based on differential pathogenicity on Brassica napus cultivars. Intergroup mating and recombinant ascospore analysis has permitted genetic analysis of pathogenicity within the aggressive strain. These studies, together with genetic analysis of differential host interaction phenotypes, should enable the Leptosphaeria-Brassica system to become an informative model for understanding the genetic basis of plant disease.