Tubers can also be infected. Affected areas on the surface of the tuber range from pink or purplish to a dark metallic color. The tuber tissues are penetrated deeply and irregularly and have no yellow margin. The infected tubers have a rusty-brown color internally beneath the infected areas. Late blight on leaves.
Late blight survives on infected tubers, including discarded tubers and cull piles.
These provide the source of early season infection. If the average of the maximum and minimum temperatures for five consecutive days is less than 78F and if more than 1.2 inches of rainfall has occurred in the past 10 days, the weather favors late blight. The disease is sporadic in occurrence. Hot dry weather can halt the development of the late blight fungus. Rain can carry the late blight spores down through cracks in the soil to infect the tubers.