Variability in the highly virulent type of Leptosphaeria maculans within and between oilseed rape fields

Pathogenicity and random amplified polymorphic DNA (RAPD) markers were used to assess genetic diversity among 93 highly virulent isolates of Leptosphaeria maculans (Desm.) Ces. & de Not. collected from two oilseed rape (Brassica napus L.) fields located 20 km apart in southern Ontario. Using three differential host cultivars of B. napus (Westar, Quinta, and Glacier), isolates were separated into three pathogenicity groups (PG). Eighty percent of the isolates were in PG4, virulent on all cultivars, and 11% were in PG3, virulent on two cultivars. The remaining 9% were tentatively placed into a new PG, PG5, because they showed intermediate virulence on the three cultivars. There was no relationship between RAPD pattern and either PG or isolate collection site. Cluster analysis of RAPD patterns, based on estimates of genetic distance, grouped isolates into two populations, corresponding to the field of collection. Analysis of molecular variance attributed 45.5% of the total variance to differences between populations and 54.5% to differences among isolates. Every isolate was genetically distinct, suggesting that the populations of the fungus were produced mostly by sexual reproduction. We conclude that inoculum in the two fields consisted principally of ascopores, derived almost equally from sources that were common to both fields and sources that were distinct for each field.