Late blight (Phytophthora infestans)

Late blight: biology

The late blight fungus overwinters in diseased tubers. Spores are spread by wind from cull piles to growing plants. If slightly infected tubers are planted, the fungus may cause seed-piece decay resulting in gaps in the field or spread up emerging sprouts. Some strains of the fungus can also survive as resting spores in soil. In the crop, spores produced in the mildewy growth are dispersed by wind and rain splash, and may infect tubers through percolating soil water or during harvesting operations.

Late blight is one of the most devastating potato diseases. One infected plant in the entire field is enough to start an epidemic. If left uncontrolled, and weather conditions favour disease development, it can devastate a potato field in five to seven days. In storage, a few slightly infected tubers can lead to extensive storage rot.

In the field, lesions on leaves and stems, which are initially dark green and oily (water soaked), quickly turn brown and become brittle. On stems and undersides of diseased leaves, white mildewy growth appears at the lesion edge during humid weather and may spread rapidly, destroying the foliage.

On tubers, including seed, surface lesions are sunken, irregular and concentrated around the eyes. Internal tissues are blotchy and brown/red up to 2 cm from the surface. Planting infected seed may lead to reduced stand and foliar infections.