Stemphylium leaf blight, caused by Stemphylium vesicarium, affects onions by creating yellow spots on leaves that expand, causing dieback and reducing yield. The fungus spreads via wind, rain, and contaminated tools/machinery, and it survives in plant debris and soil over winter.

It also affects various crops like garlic, brassicas, pears, and spinach, making management challenging. Effective control involves removing infected plant material early, if possible, rotating crops, and applying fungicides, while managing weeds and alternate hosts to reduce the disease reservoir for the next season. It is important to look for the early signs of disease. Once the tan/brown lesions appear, the fungus has spread via spores.

This fact sheet provides an overview of disease identification, factors affecting Stemphylium, and management.