How to Manage Pests

Pests in Gardens and Landscapes

Purple blotch—Alternaria porri,
Stemphylium leaf blight —Stemphylium vesicarium

Purple blotch and Stemphylium leaf blight occur primarily on onions as oval-shaped tan and deep purple lesions on leaf blades. Yellow streaks, which turn brown, extend along the blade in both directions from the lesion. Lesions may girdle and kill leaves and seed stems. Concentric zones may develop within the lesions.

Life cycle

Solutions

These diseases are usually only problems during the rainy season. Use disease-free bulbs, sets, and seed. Use a 3-year rotation away from onions, garlic, and related crops where the disease has occurred. Destroy volunteer and diseased plants. Well-drained soil, freely circulating air, and allowing plants to dry between irrigations help prevent the disease. Some varieties may show resistance.

 

Purple blotch lesion on onion leaf blade
Purple blotch lesion on onion leaf blade


Statewide IPM Program, Agriculture and Natural Resources, University of California
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