Onion Foliage Diseases

Downy Mildew

• Downy mildew affects both onion foliage and bulb size. Foliage infected with downy mildew can display purple-tinged fuzzy mildew when wet. Downy mildew is a fungal disease caused by Peronospora destructor. It can attack all members of the Allium family, including onions, shallots and leeks. Leaves on infected plants will initially display yellow lesions before dying. Although the foliage bears the brunt of the damage associated with downy mildew, the mature size of the onion may be limited as well.

Xanthomonas Leaf Blight

• Xanthomonas leaf blight is a bacterial disease initially found in Hawaii in 1978. It is now found to affect onions in many parts of the world. Xanthomonas leaf blight infections will initially cause white spots to develop along onion foliage. The spots spread into larger lesions and become tan or brown and may eventually kill off affected foliage. Although yellow onions are the most susceptible to infection by Xanthomonas leaf blight, chives, garlic, leeks and other onion relatives may be affected as well.

Botrytis Leaf Blight

• Botrytis leaf blight occurs in onions infected by Botrytis squamosa, a fungus. Infected foliage will initially show small yellow or white spots along the length of the foliage. Botrytis leaf blight consumes foliage so quickly that it is sometimes called "blast" by commercial onion growers. Lesions can spread quickly along the foliage and cause complete foliar death, severely stunting the bulb growth of the onion.

Purple Blotch

• Purple blotch is another fungal infection occurring in onion foliage. Infected foliage displays small brown lesions that quickly grow into large, discolored lesions with a bulls-eye appearance. If moist conditions occur during infection, the lesions may develop dark purple mold. Although onion bulbs can continue to develop on infected plants, the onions will ultimately rot during storage.