Potato late blight is a fungal disease that overwinters on infected potato tubers and surviving plants. Plants growing from volunteer tubers(potatoes left in the ground at harvest time) are a common source of infection. The blight spreads rapidly to new crops in warm damp weather. Spores are washed from leaves down into the soil by rain to infect the potato tubers. Resistant spores which can survive in the soil may also be produced, but it is not known for sure if this is a common occurance.

Susceptible Plants- Potato late blight occurs in potato and tomato.

Symptoms- Late blight is a common and serious fungal problem in warm, wet seasons when it spreads rapidly. It is much less frequent in dry conditions. Potatoes develop dark blotches on leaves, mainly on tips and edges, and on stems. White mold develops  under leaves in humid conditions. The whole plant may collapse quickly.

Infected tubers develop sunken lesions, which become firm and dry. Tubers may decay to a foul smelling mush as a result of bacterial soft rot. When tomato is infected with potato late blight the foliage symptoms are similar to the potato symptoms, but less severe. Green fruit and stems show dark markings, mature fruit quickly develop a dry leathery rot. This may only become evident days after harvest. A gray whitish mold may also develop over the rot.