Characterization of quantitative resistance to blackleg in Canadian canola cultivars

Most canola cultivars in western Canada are resistant (R-rated) to blackleg disease, although many carry only the resistance (R) gene Rlm1/LepR3 and/or Rlm3, which are not considered effective because the corresponding avirulence genes AvrLm1 and AvrLm3 have rarely been detected, or the effect of AvrLm3 has been masked, since 2010. This indicates that quantitative resistance (QR) contributes substantially to the resistance observed in most Canadian canola cultivars (CCCs). To test this, R-rated cultivars were inoculated under greenhouse conditions with predominant Leptosphaeria maculans (pathogen) races capable of overcoming these R genes. Cotyledon infection, pathogen spread, and blackleg severity at early plant maturity were assessed using rating scales, fluorescence microscopy, and droplet digital PCR (ddPCR). At 14 days post-inoculation, all cotyledons showed symptoms, but R-rated CCCs had lower severity, reduced hyphal growth, and restricted stem colonization compared to the susceptible control Westar. Resistant plants also exhibited delayed mortality and reduced final disease severity. These results demonstrate that QR limits fungal spread from cotyledons (or lower true leaves) into the stem. Moreover, QR levels differed among resistant cultivars and correlated with fungal DNA quantified by ddPCR, supporting its use to compare QR and identify candidates for resistance breeding.