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. This is the first study in Canada to confirm the species of Stemphylium isolates collected in onion fields using molecular methods and to investigate several elements of the life cycle in North America. The research confirmed that sexual reproduction and survival of overwintering structures on onion leaves occurred in the field and identified six weed species that are alternative hosts: redroot pigweed (Amaranthus retroflexus), marshcress (Rorippa palustris), yellow nutsedge (Cyperus esculentus), perennial sowthistle (Sonchus arvensis), bull thistle (Cirsium vulgare) and purslane (Portulaca oleracea). Using the forecasting models TOMcast and BSPcast to trigger foliar fungicide application reduced applications by one or two sprays. Foliar sprays in combination with fungicide seed treatments reduced SLB severity by 33–48%, but seed treatments or foliar sprays alone did not suppress symptoms. The efficacy of foliar fungicides for the management of SLB has declined over the past 10 years. Isolates of S. vesicarium collected in southern Ontario from onion, asparagus, and leek in 2012–2019 were assessed for insensitivity to the active ingredients of commonly used fungicides via mycelial growth and conidial germination assays. Of the isolates collected in southern Ontario in 2018–2019, 94% were insensitive to azoxystrobin, 61% to pyrimethanil, and 18% to fluopyram. Difenoconazole did not inhibit conidial germination and 1% of S. vesicarium isolates were insensitive in the mycelial growth assay. Weather variables relating to moisture (high daily average relative humidity, low vapour pressure deficit, and increased leaf wetness) were correlated with an increase in air-borne S. vesicarium spores. A model was developed to predict the concentration of air-borne spores but requires validation with more data, especially for years with high SLB severity. Other abiotic factors such as drought may increase disease severity, but a controlled environment experiment did not find a relationship between SLB susceptibility and drought. Future research should focus on new fungicides, alternative products, biocontrol agents, and modifying the integrated pest management program to better indicate when no fungicides are required.