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

Stemphylium leaf blight and purple blotch are both favored by foggy and rainy weather; optimum temperature for disease development is about 73 to 77°F. The spores are airborne. In California, these diseases are often associated with downy mildew lesions on onions; they occur less commonly on garlic.


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.


Stemphylium leaf blight lesion
Stemphylium leaf blight lesion

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

Statewide IPM Program, Agriculture and Natural Resources, University of California
All contents copyright © 2017 The Regents of the University of California. All rights reserved.

For noncommercial purposes only, any Web site may link directly to this page. FOR ALL OTHER USES or more information, read Legal Notices. Unfortunately, we cannot provide individual solutions to specific pest problems. See our Home page, or in the U.S., contact your local Cooperative Extension office for assistance.

Agriculture and Natural Resources, University of California

Accessibility   Contact webmaster.