Strategies for improving Brassica napus resistance to Leptosphaeria maculans, a causal agent of blackleg disease in canola
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Canola (oilseed rape, Brassica napus L.) is a cash crop grown in different countries including Canada, the European Union, China and Australia. The production of this crop is threatened by blackleg disease caused by the fungus Leptosphaeria maculans (Desm.) Ces. & De Not. [anamorph: phoma-like (de Gruyter)]. In order to develop a more effective disease management strategy; there is a need to employ crop rotation, introgress new resistance (R) genes into elite cultivars, test the effectiveness of existing R genes, and predict the emergence of virulent isolates of L. maculans. In this study, sequenced characterised amplified region (SCAR) and derived-cleaved amplified polymorphic sequence (dCAPS) markers linked to the L. maculans resistance gene Rlm6 were developed and their segregation was confirmed by analysis of F2 and F3 progeny. In addition, chromosomal segment substitution lines (CSSLs) harbouring Rlm6 were developed through a combined approach of classical and marker-assisted backcross breeding. The field study suggested a potential effectiveness of a 2-year crop rotation, because of the reduction of disease severity at the end of the 4-year trial. By the end of the study, the R genes LepR1, LepR3, Rlm2, and Rlm4 were effective against L. maculans, whereas Rlm3 was moderately effective. Linear regression analysis suggests an order of effectiveness among the R genes tested: LepR3 > LepR1> Rlm2 > Rlm4 > Rlm3. Moreover, the decreased frequency of effector genes AvrLm2 and AvrLm4 in L. maculans isolates suggested the gaining of virulence toward Rlm2 and Rlm4 within a year. Results from sequencing of the AvrLm2 gene revealed a shift of AvrLm2 to avrLm2 allele due to the accumulation of point mutations. Overall, CSSLs and DNA markers developed in this study may facilitate breeding B. napus varieties resistant to L. maculans in the future. It is predicted that rotation of Rlm2, Rlm3, Rlm4, and Rlm7 could be an opportunity to increase the effectiveness of those R genes under a 2-year crop rotation. However, these studies also suggest development of generic epidemiological models for the occurrence of complex molecular interactions in B. napus-L. maculans pathosystem, allowing plant breeders to select appropriate R genes in the proposed cultivar rotation strategies on the Prairies.