Common scab on potato
Scab is a widespread disease on potatoes and other root crops such as beets, carrots, parsnips, rutabagas, turnips, and radishes. This disease, caused by the filamentous bacterium Streptomyces scabies, is most severe in soils with a pH greater than 5.2 or under drought conditions. The tubers are most susceptible, but stems and roots may occasionally be infected. No above-ground symptoms are apparent on infected potato plants and scab does not adversely affect potato tubers in storage.
The bacterium survives in the soil, on infected tubers and on vegetable debris. Infection occurs when the bacterium penetrates the thin tuber skin or enters through wounds and natural openings such as the lenticels (pores on tubers). Scab infections may appear as either raised or pitted lesions. Raised lesions are dark, rough, corky areas on the tuber surface, while pitted lesions are sunken (1 to 3 mm in depth), dark-colored, corky areas.
Scab is mainly an aesthetic problem and scabby potatoes can be eaten safely. No chemical controls are recommended for home gardeners. Cultural practices should effectively control this disease in home gardens. Choose scab tolerant varieties, such as Norland, Norgold Russet, or Superior for planting, when available. Plant certified disease-free seed in soils of pH 5.2 or less. If the soil has a high pH (alkaline), use acid type fertilizer. Do not use animal manure, wood ashes, or lime in the garden. Keep potato plants well watered, especially during tuber set. Remove and destroy any potato debris and tubers at the end of the season. Try to plant potatoes in the same area of the garden only once every three to four years.
Chad Behrendt, Crystal Floyd