Comparing the effectiveness of R genes in a 2-year canola-wheat rotation against Leptosphaeria maculans, the causal agent of blackleg disease in Brassica species

Blackleg, which is caused by the ascomycete fungus Leptosphaeria maculans, is a major disease of canola in Canada and worldwide. In Canada, canola is usually grown in rotation once every three to four years to limit yield losses. Recently, growers in Canada have begun to grow canola more intensively due to growing market demand driven by crop profitability and improved cultivars. A 4-year study at the Ian N. Morrison Research Station, Carman Manitoba, was conducted from 2014 to 2017 to investigate the effectiveness of five introgression lines (IL) carrying single blackleg resistance genes under field conditions in a 2-year canola-wheat rotation. Blackleg incidence for all tested R genes, except Rlm4, was reduced by a maximum of 16% in 2017 compared to 2014. Disease severity was 31-44% higher in the first year for all tested R genes, except for the Rlm2 when canola was seeded on the same stubble, compared to the fourth year rotation. Relative disease severity, calculated based on the susceptible check Westar, showed that the R genes LepR1, LepR3, Rlm2, and Rlm4 were resistant (<30% of check) against L. maculans after 4 years of rotation, whereas Rlm3 resistance was less effective (35% of check). The results of our 2-year rotation study suggest that canola cultivars harbouring the major resistance genes LepR1, LepR3, Rlm2, and Rlm4 will remain effective against blackleg when grown in the increasingly common 2-year rotation, rather than the recommended 3-4 year rotation. As expected, the regression analysis confirmed that seed yield declined as blackleg severity increased. However, comparatively some ILs were more impacted in terms of yield loss as disease severity increased, and can be ordered from the most to the least resistant as follows: Rlm2 approximate to LepR1 > Rlm3 approximate to Rlm4 > LepR3.

Automated summary

Blackleg, caused by Leptosphaeria maculans, is a major disease of canola worldwide. Canadian growers have been growing canola more intensively, leading to a reduction in blackleg incidence. The study suggests that canola cultivars with major resistance genes LepR1, LepR3, Rlm2, and Rlm4 remain effective against blackleg in a 2-year rotation.