This article appeared in “Onion World,” March/April 2001.
Disease Identification:
Stemphylium Leaf Blight of Onion
Stemphylium vesicarium
(teleomorph: Pleospora allii)
Initial infections on the leaves and leaf sheaths are small, light yellow to brown, and water-soaked. As the lesions expand, they coalesce, causing extensive blighting of the leaves. Typically, lesions are found in higher numbers on the side of leaves facing the prevailing wind. The centers of lesions turn brown to tan, then dark olive brown, and finally black as the fungus sporulates. Sometimes fruiting bodies called perithecia may appear in infected tissue as small, black, pinhead-like raised bodies.
This fungus normally invades dead and dying onion tissues. However, severe damage can occur on healthy leaves during warm weather when leaves are wet for more than 24 hours. Infection is usually limited to leaves, and does not extend down to the scales of the bulb.
Chemical control with fungicides (Bravo, Rovral) is effective in reducing disease except when weather conditions favor the fungus. Long-term rotation with unrelated crops may reduce losses. Also, good field drainage and reduced plant density may lessen disease severity.
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