Sensitivity of Leptosphaeria maculans to pyraclostrobin in Alberta, Canada

Strobilurin fungicides can be used to manage blackleg (Leptosphaeria maculans) of canola (Brassica napus); however, they have a high risk of selecting for resistance in fungal populations. In 2011, single-spore isolates of L. maculans were obtained from infected canola stubble representing six fields in Alberta, Canada. The isolates were grown on fungicide-amended agar plates and percent growth inhibition was calculated based on colony diameter relative to non-amended controls. Using probit regression analysis, the effective concentration of pyraclostrobin needed to inhibit mycelial growth by 50% (EC50) was determined to be 0.09 mg L−1 based on 13 isolates. To identify highly insensitive isolates, 117 isolates were grown on agar plates amended with a discriminatory dose of 6.25 mg L−1. Isolates in which growth was inhibited <50% were considered insensitive. Growth inhibition values ranged from 66.6% to 100.0% with a mean of 84.3%, indicating that all of the isolates were sensitive to pyraclostrobin. Forty-one of the 117 isolates were also tested via a microtiter plate assay. Based on conidial germination and subsequent growth, the EC50 values obtained for each isolate ranged from 1.0 × 10−4 to 7.4 × 10−3 mg L−1, which were not significantly different from those of the sensitive controls (4.1 × 10−3 and 5.7 × 10−3 mg L−1). While pyraclostrobin insensitivity was not detected in L. maculans populations from Alberta, continued monitoring will be necessary to track future changes.