The Race Structure of Leptosphaeria Maculans In Western Canada.

Specific resistance genes are an effective means of disease control when the pathogen population is mainly avirulent on the cultivated varieties carrying the corresponding resistance gene. Development of new races of Leptosphaeria maculans (Desmaz.) Ces. & de Not., the species responsible for the most damaging symptoms of blackleg disease of canola (Brassica napus L.), increases the risk that host genetic resistance may be rapidly overcome.  This study aims to elucidate the race structure of L. maculans by determining, under controlled conditions, the frequency of avirulence alleles at eleven avirulence loci in pathogen populations collected from nine locations across western Canada.  The avirulence allele at AvrLm6 was present in all isolates tested (63) from three locations. At eight locations >88% of the isolates (423) carried the avirulence allele at AvrLm2, but only 37% at another location.  For all other avirulence genes (AvrLm1, AvrLm3, AvrLm4, AvrLm7, AvrLm9, AvrLmLepR1, AvrLmLepR2 and AvrLmLepR3), avirulence allele frequency varied from 0 to 99% depending on the loci and the location (300-600 isolates).  Knowledge of avirulence allele frequency and the race structure of L. maculans in canola producing regions of western Canada will be crucial to develop strategies to maintain the efficacy of resistance genes.