Assessment of fungicide sensitivity in Cochliobolus sativus isolates from western Canadian barley to propiconazole, fluxapyroxad, and pyraclostrobin

Cochliobolus sativus, the causal agent of spot blotch, poses a major challenge for barley producers in western Canada, causing significant yield losses under favourable conditions. Spot blotch management in this region increasingly relies on fungicides to protect grain yield and quality; however, repeated use can increase the risk of developing insensitivity. In this study, single-spore isolates of C. sativus collected from barley leaf samples in Alberta, Saskatchewan, and Manitoba between 2018–2021 were evaluated for sensitivity to propiconazole, fluxapyroxad, and pyraclostrobin. The mean concentrations required to inhibit fungal growth by 50% (EC50) were determined to be 0.5 mg L−1 for propiconazole, 0.08 mg L−1 for fluxapyroxad, and 0.003 mg L−1 for pyraclostrobin. The sensitivity of 105, 103, and 101 isolates to propiconazole, fluxapyroxad, and pyraclostrobin, respectively, was then evaluated using discriminatory doses. Growth inhibition ranged from 71.7–98.1% for propiconazole, 58.0–100% for fluxapyroxad, and 73.6–100% for pyraclostrobin. Although none of the isolates exhibited complete insensitivity to any of the fungicides tested, 6% displayed intermediate sensitivity (30–70% growth inhibition) to fluxapyroxad. However, those isolates showed clear dose-dependent inhibition at higher fluxapyroxad concentrations, confirming that they remained largely sensitive. Overall, the data provide no evidence of a consistent, population-wide shift in the sensitivity of C. sativus isolates to the fungicides evaluated during the study period. In addition, this study represents the first systematic assessment of sensitivity to propiconazole, fluxapyroxad, and pyraclostrobin in the current western Canadian C. sativus population, providing valuable baseline data for future monitoring and fungicide stewardship.