Population structure, avirulence profiles and race diversity of Leptosphaeria maculans in western Canada, 2015–2017: implications for R-gene deployment

Understanding the population structure of L. maculans, the causal agent of blackleg disease, is critical for effective resistance management in canola. A total of 1073 isolates collected across the Canadian Prairies from 2015 to 2017 were characterized for 10 avirulence (Avr) genes, race composition and population diversity. AvrLm4, AvrLm6 and AvrLm7 were the most prevalent genes (>50% of isolates), whereas AvrLm1, AvrLm3, AvrLm9 and AvrLep2 were rare or undetectable. Several Avr genes showed temporal and regional variation, including increases in AvrLm2 and AvrLep1 in specific provinces and years. The average number of Avr genes per isolate also increased over time, indicating greater genetic complexity. Forty-four races were identified, of which nine predominant races accounted for 61–86% of the annual population. Races AvrLm4-6–7 and AvrLm6-7 were consistently dominant, although race composition shifted with increased richness over time (Margalef index), while overall diversity remained high and stable (Simpson index). The Avr gene profile was broadly consistent with previous studies in western Canada; however, AvrLmS/AvrLep2 declined further relative to earlier surveys and was nearly undetectable in Alberta and Manitoba, suggesting selection for virulence towards the corresponding resistance gene, LepR2. The low observed frequencies of AvrLm3 and AvrLm9 is likely due to masking by the high frequency of AvrLm7. Overall, L. maculans populations in western Canada maintained a broadly stable Avr profile but exhibited shifts in race composition and specific Avr frequencies. These findings highlight the need for continued surveillance to inform R-gene deployment and sustainable blackleg management.