Common scab is caused by Streptomyces scabies, which can cause disease on potato, beet, radish, rutabaga, turnip, carrot, and parsnip. It is economically important only on potatoes, and found in most areas where potatoes are grown. The quality of the crop is seriously decreased, although the yield is almost unaffected. Affected potatoes usually store as well as healthy potatoes.

Symptoms. There are no aboveground symptoms. This disease cannot infect tubers as they approach maturity, unless the tuber is wounded, for example, by insect feeding. When tubers are actively growing, small (5-8 mm) circular spots appear on the tubers. These spots may grow together to make irregularly shaped patches. Spots are usually tan to brown in color and are rough in texture. The roughness may be flat to the tuber (russet scab) or raised up (raised scab). The rough area may be sunken into the tuber (pitted scab), and usually dark brown to almost black in color. The pitted spots may be invaded by scab gnats. If the tuber is sliced open, the flesh underneath the spot is straw colored and somewhat translucent. This is not usually evident under the russet scabs. Brown to tan spots may also be seen on stems and stolons; these are usually oval to round.

Prevention. Plant only disease-free tubers. A 4-year crop rotation, which includes soybeans and alfalfa, can decrease disease severity. This disease is favored by low soil moisture. Proper irrigation can help control it. It is important to keep the field moist when potato tubers are actively growing, especially 4 to 9 weeks after tuber set. Nutrient levels can also affect the development of disease. High ratios of calcium to potassium (K) can increase disease, as can high nitrogen. Keeping the pH of the soil between 5.0 and 5.3 can decrease significantly the level of disease in susceptible varieties. This has been done by adding sulfur in some cases. Resistance and tolerance is available for this disease. Chemical control options are available to help control seed-borne scab. Refer to the latest New England Vegetable Management Guide for chemical options.

By: Pamela S. Mercure, IPM Program Assistant, University of Connecticut, 1998

Updated by: T. Jude Boucher, IPM, University of Connecticut. 2012

References.

• Hooker, W.J. 1981. Common Scab. pp 33-34 in Compendium of Potato Diseases. W. J. Hooker, ed. APS Press, St. Paul, MN.
• Rich, A.E. 1983. Potato Diseases. Academic Press. New York.

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