Written by Amy Ziems, Extension Educator – Plant Pathology
Pathogen
Common scab of potatoes is caused by a soil and seed-borne bacterium, Streptomyces scabies, which is distributed worldwide. This pathogen can also attack many other root crops as wells as fleshy roots of weeds. It has been reported on beet, turnip, rutabaga, radish, carrot and parsnip.
Disease Symptoms
The symptoms of common scab are quite variable and develop on the surface of the potato tuber. Typically, roughly circular, raised, tan to brown, corky lesions of varying size develop randomly across the tuber. The lesions may be raised or warty in appearnce, level with the surface or sunken in the tuber. Superficial russetting, a layer of corky tissue covering large areas of the tuber and deep pitting may occur.
Warty Surface Lesion
Favorable Weather Conditions
Optimum temperature for infection of potato tubers is 68 to 72oF, but the pathogen can attack tubers within a wide range of temperatures from 50 to 88oF in soils. It is most severe in soils with pH above 5.5, however there is a less common form that occurs in soils below pH 5.5.
Control
Cultural
Rotation with other than root crops should be particed if the garden size permits. Avoid planting in the same ground with beans or potato-related vegetables such as tomato, pepper and eggplant.
Plant resistant varieties when available.
If irrigation is available, scab can be reduced by keeping the soil reasonably wet for several weeks while the young tubers are beginning to grow. However, wet soil may help other rooting-type pathogens infect the vine and tubers.
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