In vitro and in-field response of foliar fungicides against Stemphylium vesicarium causing Stemphylium leaf blight of onion

Stemphylium leaf blight (SLB) of onion, caused by Stemphylium vesicarium (Wallr.) E.G. Simmons, can cause severe leaf necrosis and small, unmarketable bulbs. To manage SLB, growers typically apply fungicides on calendar-based schedules that do not depend on weather conditions or pathogen biology. This can result in more applications than necessary, which increases the risk of fungicide insensitivity. Isolates of S. vesicarium were collected from onion and leek in southern Ontario to assess sensitivity of the current population to azoxystrobin (a strobilurin fungicide) and pyrimethanil (an anilinopyrimidine fungicide) compared against a baseline (unexposed) isolate collected in 1995. The baseline isolate was sensitive to azoxystrobin at 0.5 µg ml-1 plus 100 µg ml-1 salicylhydroxamic acid (SHAM) and to pyrimethanil at 5.0 µg ml-1, assessed as decreased mycelial growth and conidia germination on amended media. Of 10 isolates collected in 2018, 9 isolates exhibited decreased sensitivity to azoxystrobin and 7 isolates exhibited decreased sensitivity to pyrimethanil. There were no differences in fitness (growth or sporulation) between insensitive and sensitive isolates. A field trial using a weekly foliar spray program of Quadris Top (difenoconazole and azoxystrobin) alternated with Luna Tranquility (fluopyram and pyrimethanil) reduced SLB symptoms by only 10%. This demonstrates that these fungicides no longer provide effective reduction of SLB on onion in southern Ontario, even with repeated application, due at least in part to fungicide insensitivity.