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
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.
blotch lesion on onion leaf blade