Cause Stemphylium vesicarium (syn. Pleospora allii), a fungus that typically infects onion already damaged from downy mildew, purple blotch, or thrips. Disease is promoted by extended leaf wetness periods, high relative humidity, and moderate temperature (60°F to 70°F). Infected plants, plant debris are sources for spores that can move on wind and water and be transported in infected fields by equipment and people.
Symptoms Initially, leaves exhibit water soaking and small spots yellow-to-brown in color, lesions turn dark-brown-to-black, coalesce, and lead to blighting of the onion leaves. Bulbs size can be reduced relative to level of leaf infection.
- Practice crop rotations of two years or longer.
- Use pathogen-free seed.
- To minimize high humidity and periods of leaf wetness, manage plant density and spacing as well as irrigation practices.
- Avoid overly wet field locations and promote good soil drainage.
- Minimize plant injury.
Chemical control The disease is not in the Pacific Northwest often enough to warrant an annual spray program. Spray as soon as the disease occurs to prevent further spread.
Reference Schwartz, H.F., and Mohan, S.K. 1995. Compendium of Onion and Garlic Diseases. St. Paul, MN: APS Press.