These are only of importance as a source of further disease on the tubers. Tuber infection begins very early, when tubers are only pea-sized, and continues while conditions are suitable. Infected tubers first develop swellings, which later burst, exposing a mass of powdery spores. The fungus is spread on infected seed. Once introduced, it remains in the soil for up to 20 years.
Rotation is ineffective.
Use resistant varieties to control this disease. Katahdin, Exton, Russet Burbank and Nooksack have useful resistance and should be grown where infection is known to occur. Common scab, Actinomyces spp. Unlike powdery scab, this disease is favoured by dry conditions and is most common on spring and summer crops. It is less common on acid soils.