How to Manage Pests

Powdery scab—Spongospora subterranea

The powdery scab pathogen infects lenticels and wounds in potato tuber surfaces. Tuber cells are stimulated to divide and enlarge, causing the formation of small, purplish brown, raised lesions. The pimple-like lesions or pustules are about 0.13 - 0.25 inch in diameter. They gradually darken and decay, leaving shallow depressions filled with powdery masses of dark brown spore balls. Roots may also be infected, resulting in the formation of small, white galls that also break down to release masses of spore balls.

Life cycle

The fungus that causes powdery scab survives in the soil as resting spores, which form the spore balls released from tuber and root lesions. Stimulated by the presence of susceptible host roots, resting spores germinate to produce mobile spores that swim through soil water to infect roots or stolons. These infections produce additional spores that spread infection to roots and tubers.

Resting spores survive in the soil for up to 6 years and can survive passage through the digestive tract of animals. The fungus can also infect and produce resting spores on the roots of other potato family plants such as tomato and nightshades. Powdery scab is favored by cool, moist conditions, and symptoms are most severe on white- and red-skinned tubers.

Solutions

Powdery scab can be minimized by using good cultural practices. Use pathogen-free seed tubers, do not use manure from farm animals that have fed on infected tubers, and avoid moving soil from contaminated fields to uninfested areas.