Fungicide sensitivity of Stemphylium vesicarium isolates from onion ( Allium cepa ) in Nova Scotia, Canada

Stemphylium leaf blight (SLB), primarily caused by Stemphylium vesicarium, is an important foliar disease of onion (Allium cepa) in Nova Scotia, Canada. This study evaluated the in vitro sensitivity of 100 S. vesicarium isolates collected from commercial fields in Berwick, Nova Scotia, to five fungicides representing different modes of action: propiconazole (Fungicide Resistance Action Committee [FRAC] 3), benzovindiflupyr (FRAC 7), pyrimethanil (FRAC 9), fluazinam (FRAC 29) and cyprodinil + fludioxonil (FRAC 9 + 12). Mycelial radial growth assays were conducted on fungicide-amended potato dextrose agar at two concentrations per product. Isolates were classified as sensitive, moderately resistant or resistant based on percentage inhibition compared with untreated controls. High sensitivity was observed for propiconazole (FRAC 3) and cyprodinil + fludioxonil (FRAC 9 + 12), with 82% and 76% of isolates classified as sensitive, respectively. In contrast, reduced sensitivity was observed for benzovindiflupyr (FRAC 7) and pyrimethanil (FRAC 9), for which only 34% and 21% of isolates, respectively, were classified as sensitive. Fluazinam (FRAC 29) showed an intermediate response, with a high proportion (47%) of moderately resistant isolates. These results demonstrate variation in fungicide sensitivity among local S. vesicarium isolates and provide baseline information for future fungicide sensitivity monitoring.