Scab is a common tuber defect in most potato-growing regions, although usually not where soils are very acid. The causal organism has been introduced into most potato soils. It affects only quality, not yield.

Symptoms of Common scab

Several types of lesions develop. They may be superficial or reticular, deep or pitted, or protuberant. They vary in size and shape, but are usually circular and not more than 10 mm in diameter. They may coalesce so that most of the tuber surface becomes affected. Fibrous roots may also be damaged.

Management

Maintain high soil water levels compatible with good potato growth during tuber set and enlargement. Avoid planting scabby seed tubers. Avoid repeated crops of potato or other scab-susceptible plants such as red beet, sugarbeet, radish, rutabaga, turnip, carrot, and parsnip (in these, the disease seldom has economic importance).
Scab-resistant varieties are useful. Maintain soil pH levels at 5 to 5.2 with acid-forming fertilizers or sulfur.
Avoid heavy lime applications and preferably use dolomitic lime where needed. “Acid scab” may be controlled with chemical seed treatments (mancozeb dust, 8%) or soil fumigation.

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