Older leaves tend to be more susceptible than younger leaves. Symptoms begin as water-soaked lesions that usually have a white center. Edges of lesions become brown to purple and the leaf turns yellow above and below the lesions. With time, dark brown to black concentric rings form throughout the lesions. These are areas of sporulation of the fungus. As the disease progresses, lesions may girdle the leaf causing it to collapse and die. Similar symptoms occur on seed stalks and infected stalks can collapse resulting in shriveled seed development. When bulb infection occurs, it is normally through the neck. If the fungus invades the bulb, the infected area is initially bright yellow, but eventually turns a characteristic red wine color.
Conditions for Disease Development
The fungus over-winters as mycelium in leaf debris and cull piles. Spores are formed during humid nights and leaf wetness periods greater than 12 hours. As the morning dew dries, spores become air-borne and are disseminated to susceptible onion tissue. 1-4 days are needed for symptoms to develop after infection. Disease development is greatest during prolonged periods of leaf wetness.
A fungicide spray program with broad spectrum protective fungicides applied prior to infection can provide good protection. Minimizing leaf wetness by using surface rather than sprinkler irrigation, good field drainage and correct plant spacing can reduce disease development. A rotation out of Allium to unrelated crops for several years can reduce disease as well.