Yield losses and pyraclostrobin sensitivity in blackleg (Leptosphaeria maculans) of canola

Abstract

Blackleg, caused by Leptosphaeria maculans (Desm.) Ces. & de Not., is an important disease of canola (Brassica napus L.) worldwide. In Canada, blackleg is managed mainly by the cultivation of resistant or moderately resistant canola hybrids and the application of fungicides. Field experiments were conducted in central Alberta in 2017 and 2018 to determine the relationship between blackleg severity and yield in two moderately resistant hybrids ‘73-15RR’ and ‘1950RR’. Seed yield per plant was found to decrease as a consequence of L. maculans infection, with regression analysis showing that the relationship between yield and disease severity was best explained by quadratic equations. Sensitivity to the fungicide pyraclostrobin, a strobilurin that is commonly applied as a foliar and seed treatment for blackleg and other diseases, was compared in L. maculans collections made in Alberta in 2011 and 2016. The half-maximal effective concentration (EC50) of pyraclostrobin was determined using agar and microtiter plate assays, and two discriminatory doses of the fungicide were selected to identify highly insensitive isolates in the collections. The mean EC50 was approximately 4× greater for the isolates collected in 2016 versus those collected in 2011. While almost all isolates were still sensitive to pyraclostrobin, this increase in the EC50 suggests that proper fungicide stewardship is warranted for the sustainable long-term management of L. maculans.