Analysis of quantitative adult plant resistance to blackleg in Brassica napus

Leptosphaeria maculans L. causing blackleg is a highly evolved fungal pathogen that damages the rapeseed industry in Canada, Australia, and Europe. Advanced backcross populations segregating for intermediate resistance ( BLMR2 ) and susceptible ( blmr2 ) alleles were planted and inoculated using cotyledons. Near iso-genic lines (NILs) containing the intermediate blackleg resistance locus along with the parent lines and other controls were planted in a blackleg nursery at the University of Manitoba. ANOVA indicated a correlated significant difference between the genotypes with BLMR2 and blmr2 alleles at the seedling and adult plant stages. Heterozygous plants with both BLMR2 and blmr2 alleles showed relatively lower disease severity index (DSI) while other plants with only the blmr2 allele induced a higher DSI at seedling and adult plant stages, respectively, on a scale of 0–9. In the field, the NILs with the BLMR2 alleles consistently showed lower stem canker severity index similar to the resistant parent Surpass 400 in three consecutive years. In comparison, lines with known R genes ( BLMR1 and Rlm2 ) showed relatively higher DSI in the field test. This demonstrated that the intermediate resistance locus performed well under severe blackleg disease pressure in the field while the NIL lines with single dominant R genes were ineffective. Because a group of isolates carrying various Avr/avr alleles was used in the field evaluation, our results suggest that the intermediate resistance locus confers horizontal resistance and has excellent potential in blackleg management in western Canadian canola production regions.