Understanding interactions between quantitative resistance and qualitative resistance for sustainable management of Phoma stem canker (Leptosphaeria maculans ) in oilseed rape (Brassica napus)

Effective control of phoma stem canker (Leptosphaeria maculans) relies on use of host resistance. Two types of resistance to L. maculans have been identified; major resistance (R) gene-mediated qualitative resistance and quantitative resistance (QR). R gene-mediated resistance is race-specific and often rendered ineffective due to pathogen population changes from avirulent to virulent. QR is race non-specific and is considered durable. To investigate the interactions between R genes and QR, eight cultivars with different resistance were used in field experiments at 13 different sites in the 2010/2011 and 2011/2012 growing seasons. Severity of phoma leaf spots in autumn and severity of phoma stem canker in summer were assessed. Results showed that there were effects of background quantitative resistance on the effectiveness of an R gene. The severity of stem canker on DK Cabernet (Rlm1 + QR) was less than on Capitol (Rlm1), suggesting that Rlm1 is more effective when it is introduced into a host background with QR than in one without QR. Similarly, less severe stem canker on Adriana (Rlm4 + QR) than on Bilbao (Rlm4) suggested that Rlm4 is more effective when it is introduced into a host background with QR than in one without QR. Interestingly, cultivars Roxet and Excel both carry Rlm7 but Excel developed less severe phoma leaf spots and stem canker at most experimental sites than Roxet. It is not clear whether this was due to difference between them in host background QR because avirulent AvrLm7 was predominant in L. maculans populations at UK sites.