Gray mold develops on tomato under cool, wet conditions, especially when air circulation is poor. Fungicides will not suppress an established infection, but offer some protection if used before infection occurs. If the weather turns warm or humidity drops below 90 percent, these lesions dry up and disease spread is restricted. When favorable blight conditions exist, however, a white spore-producing mildew appears on lower leaf surfaces at the outer edge of the lesion. Infection sources include infected seed stock, blighted tubers in cull piles, volunteer plants growing from infected tubers, and wind-blown spores.
Spores develop on diseased plant parts and are splashed to other parts of the plant or other plants.
Spore germination is favored by warm daytime temperatures. Killing foliage prevents migration of disease from foliage to tubers. Depending on conditions, growers are advised to postpone spraying, to be alert to potential blight favorable conditions, or to spray every 5 or 7 days. By 1987 it was being used on at least 28,000 acres of potatoes in the United States. In tomatoes, late blight can be introduced on transplants or can migrate from potato or other tomato fields as airborne spores.