Resistance to multiple SDHI fungicides in Stemphylium vesicarium.

Stemphylium leaf blight (SLB), caused by Stemphylium vesicarium, reduces yield and bulb quality of onion (Allium cepa L.). All onion cultivars are susceptible and fungicide applications do not provide effective reduction in severity. Previous studies reported
fungicide resistance in S. vesicarium to FRAC groups 2, 3, 7, 9 and 11. In the current study, isolates of S. vesicarium collected from Ontario in 2021–2023 were assessed for sensitivity to fluxapyroxad and penflufen (both in FRAC 7). A baseline isolate from 1995 was sensitive to both active ingredients based on mycelial growth, but was resistant to both based on conidial germination. In a conidial germination assay, 99% (n = 79) of isolates were resistant to 100 μg/mL fluxapyroxad and all isolates (n = 51) were resistant to 50 μg/mL penflufen. In contrast, the mycelial growth of 35% (n = 46) of isolates were resistant to 50 μg/mL fluxapyroxad, but 6 of 8 isolates (75%) were resistant to 50 μg/mL penflufen. These results indicate that resistance to penflufen in the S. vesicarium population has increased over time, possibly because of cross-resistance with other FRAC 7 active ingredients, but resistance to fluxapyroxad, also FRAC group 7 is still relatively low.