Pantoea ananatis (syn. Erwinia ananatis), P. agglomerans (syn. E. herbicola)
Peru, Poland, South Africa and USA (Colorado, Georgia, Michigan and New York)
Symptoms first appear as whitish to tan lesions with water-soaked margins, often on interior leaves. Foliar lesions can rapidly coalesce, progressing to wilt and dieback of affected leaves. The pathogen moves from the leaves into the neck and bulb causing yellowish to light-brown discoloration. With severe infections, all leaves can be affected giving a bleached appearance to plants. Secondary bacterial infections rot interior bulb tissue and produce a foul odor. Under conditions favorable to the disease, yield losses may approach 100 percent.
Conditions for Disease Development
Both pathogens are seedborne and can survive on a few reported alternate hosts (corn, cotton, melon, pineapple, rice and sugar cane). They may also survive epiphytically on weeds and crop debris. Spread can occur by wind, splashing water and thrips. Infection is favored by moderate to warm temperatures and rainfall during bulb initiation.
Seed produced in high risk areas should be tested for Pantoea ananatis and Pantoea agglomerans before sowing. Some onion varieties are known to be more susceptible to this disease than others. Avoid planting these varieties where disease pressure is high. Control weeds, volunteer onions and thrips. Consider drip rather than sprinkler irrigation if possible, and avoid working in fields when foliage is wet. Avoid excessive nitrogen fertilization. If applied preventively, copper-based bactericides may provide control under low to moderate disease pressure. Initiate sprays two weeks before bulbing and continue every 5-7 days thereafter. Deep cultivate after harvest to promote decomposition of crop debris. Where this disease occurs, a minimum three-year rotation to non-hosts is recommended.