Fungicide sensitivity of Stemphylium vesicarium in Ontario.

Stemphylium leaf blight (SLB), caused by Stemphylium vesicarium, has become an important disease of onion in Ontario, Canada and the north-eastern USA in recent years. Commercial cultivars are highly susceptible, so growers apply foliar fungicides at 10–14-day intervals to manage the disease. However, the efficacy of several fungicides appears to have declined over time. Isolates of S. vesicarium collected in southern Ontario from onion, asparagus, and leek in 2012–2020 were assessed for sensitivity to the technical grade active ingredients of commonly used fungicides. Two isolates collected from oat in Saskatchewan in 1995 provided an historical baseline. The oat isolates were sensitive to azoxystrobin (FRAC 11), fluopyram (FRAC 7) and difenoconazole (FRAC 3), but insensitive to pyrimethanil (FRAC 9). Sensitivity to azoxystrobin was present in isolates collected from asparagus in 2012, but the population trended towards insensitivity by 2016, with 97% insensitive in 2020. For fluopyram, isolates from 2012 were sensitive, but 37% of the 95 isolates collected in 2018 and 2019 and 100% of 30 isolates in 2020 were insensitive. Most isolates (99%) were sensitive to difenoconazole using a mycelial growth assessment, but even a high rate (100 ppm) did not inhibited conidial germination. For pyrimethanil, 94% of isolates from 2018 and 2019 were insensitive. These active ingredients have been and are being used on onion in Ontario, but no longer provide effective SLB suppression. Identification of alternative management strategies is required.