The challenges of managing Stemphylium leaf blight of onion.

Stemphylium leaf blight of onion (SLB), caused by the fungus Stemphylium vesicarium, is an important disease of onion many parts of the world. The disease kills leaves and can reduce yield. Managing SLB is challenging. Field trials were conducted in the Holland Marsh, Ontario in 2022 and 2023 to evaluate fungicides and disease forecasting programs. Biocontrols T-77 (Trichoderma aroviride) and Serifel (Bacillus amyloliquifaciens) were evaluated and fungicides Merivon (pyraclostorbin and fluxapyroxad), Miravis Duo (pydiflumetofen and difenoconazole), Revysol (mefentrifluconazole), Sercadis (fluxapyroxad) and Dithane (mancozeb). Disease severity was low in 2022 and moderate in 2023 and there were no differences in disease severity or yield. Disease forecasting based on temperature and leaf wetness showed no differences in disease severity in 2022 and small differences in 2023. Spray timing based on regular intervals had lower cumulative disease severity than the nontreated control, but was not different from fungicides applied based on forecasting programs. Isolates of the fungus from the region are known to have high resistance to azoxystrobin and fluopyram. Recent assessments found resistance to penflufen and fluxapyroxad. Effective management of Stemphylium leaf blight requires effective fungicides and disease forecasting programs that indicate when there is low risk of disease development.