In damp weather, a whitish moldy growth appears on the underside of infected leaves. This fungus causes a firm rot of tubers in field and storage, later becoming a soft rot. Use a resistant variety when possible. Fungicide applications will help control the disease. Harvest should be delayed about 2 weeks when late blight is present until vines are killed by chemical treatment and infected tubers have decayed completely.
Diseased tubers are spongy and initially discolored around the point of stolon attachment.
Later, they become discolored around the buds and lenticels. They gradually become darker, turning black after about 1 hour. Diseased potatoes have been observed in the Texas High Plains primarily in furrow-irrigated fields with clay soils. Control suggestions are to select sites with good soil drainage and avoiding excessive irrigation late in the growing season. Rough, corky areas that may be round or irregular occur on tubers.