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Common Scab of Potato

Ned Tisserat Extension Specialist, Plant Pathology


Potato Scab

Figure 1.  Scab on potato

 

This section was taken from Extension Publication L-511, written by Larry Claflin.

SYMPTOMS

Common scab of potato is caused by a bacterial-like organism called Streptomyces scabies, which is a common soil inhabitant. This same organism is capable of attacking other root crops including beet, turnip, rutabaga, radish, carrot, salsify and parsnip. Beets (red and sugar) also are very susceptible to scab.

Scab lesions first appear as small, reddish or brown spots on the developing tuber. The spots may be raised or slightly sunken and wart-like in appearance. As the tuber develops, lesions become circular in outline and the amount of depression into the tuber is dependent upon the genetic resistance of the variety.

Lesions on susceptible varieties are usually deep and may extend 1/4 inch into the tuber while those on resistant varieties are generally superficial with a corky texture. Scab is easily identified by the rough, corky patches on the skin of the tuber. Scab lesions are sometimes associated with or masked by injuries caused by potato flea beetles or nematodes. Such damage usually appears as corky, winding trails, slightly enlarged bumps and/or small holes in the tuber.

DISEASE CYCLE

Common scab is most prevalent and severe in neutral (pH 7) or slightly alkaline soils. The optimum pH for scab development is from pH 6.0 to 7.5. In general, scab is not a serious problem in soils with a pH of 5.5 or lower.

The scab organism is very sensitive to soil moisture, pH, and temperature. The incidence of the disease is generally greater in sandy or gravelly soils than in heavier soils. The amount of soil moisture at the time of tuber development is very important.

Research conducted in Maine has shown that an average of 500 scab infection sites occurred on each tuber when the soil was too dry for good potato growth. On the other hand, less than 5 infection sites were observed on each tuber when the soil had adequate moisture for maximum potato growth.

The addition of barnyard manure, ashes, and lime generally increases scab severity. The scab organism can persist for many years in soils with heavy applications of manure or where barnyards had previously been located.

CONTROL

Several practices can limit the severity of scab.

  1. Planting resistant varieties is the most feasible and practical control. Varieties showing some level of scab tolerance include: Norchip, Norgold Russet, Reliance, Russet Burbank, Shurchip, Norland, Hi-Plains, Pungo, Redskin, Russet Sebago, Shoshoni, and Superior.
  2. Avoid planting seed exhibiting scab lesions.
  3. Crop rotation in which potatoes will be planted in the same area only once every 3 or 4 years. Avoid rotating with root crops; instead plant grains, grasses, or legumes.
  4. Maintain good moisture levels in the soil while tubers are developing.
  5. Avoid the use of fresh manure just preceding the potato crop.
  6. Chemically treating seed pieces before planting with the fungicide Captan will help reduce seedborne scab inoculum.

 

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Web updated 9/01/06
Web comments to jpierz@ksu.edu