G1940

Common Scab of Potatoes

Protect potatoes from surface scab and pitted scab by using resistant varieties and cultural practices that make conditions unfavorable for scab development.


Amy D. Ziems, Extension Educator — Plant Pathology
Alexander D. Pavlista, Extension Potato Specialist


Common scab of potatoes is caused by a soil- and seed-borne bacterium, Streptomyces scabies, which is distributed worldwide. Infection causes tubers to have a scab-like surface lesion (“surface scab”) which results in lower tuber quality. In severe infections, lesions are dark pits that can be as deep as a quarter inch (“pitted scab”). Streptomyces scabies can also attack other root crops, as well as the fleshy roots of weeds. It has been reported on beet, turnip, rutabaga, radish, carrot, salsify, and parsnip.

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 (Figure 1). The lesions may be raised or warty in appearance, level with the surface, or sunken into the tuber. Superficial russetting, a layer of corky tissue covering large areas of the tuber, and deep pitting may occur (Figures 2 and 3). Tubers with common scab can show additional damage by rodents, white grubs, wireworms, scab gnats, and other pests that are attracted to the damaged tissue. Damage by these organisms can cause the lesions to enlarge further or deepen.

Figure 1. Potato tubers exhibiting different types of the “surface scab” lesion caused by common scab infection.
Figure 1. Potato tubers exhibiting different types of the “surface scab” lesion caused by common scab infection.

Figure 2. Potato tuber exhibiting the “pitted scab” lesion caused by common scab infection.
 
Figure 3. Cross section of a potato with “pitted scab” infection.
Figure 2. Potato tuber exhibiting the “pitted scab” lesion caused by common scab infection.   Figure 3. Cross section of a potato with “pitted scab” infection.

Disease Cycle

Streptomyces scabies is an efficient saprophyte that can overwinter either in the soil or on the surface of tubers and crop residues. Movement of this pathogen commonly occurs via water, wind, seed tubers, or anything that moves soil.

Infection begins when tubers are initially being formed. Optimum temperature for infection is 68 to 72°F, but the pathogen can attack tubers within a range of soil temperatures from 50 to 88°F. Common scab 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.

The pathogen primarily invades lenticels (pores in the tuber skin). Initially the lesions may be so small that they are not visible. In scab-prone varieties, infected cells die and the observed scab is the result of a healing process. The pathogen also secretes a compound that promotes rapid cell division in the living cells surrounding the lesion. This causes the tuber cells to produce several layers of cork cells which isolate the pathogen from surrounding tuber cells. As the tuber cells die above these cork cells, the pathogen continues to feed on the dead tuber cells. This causes the cork cells to be pushed out and sloughed off. The pathogen continues to grow and feed on the additional dead cells which results in the development of the scab lesion. Lesion size varies depending on the timing of initial infection and the genetic susceptibility of the variety. Typically, the earlier a tuber is infected, the larger the lesion will be. The lesion does not develop further after the potatoes are dug, but the organism can be present and remain alive in storage.

Management

Management in a garden setting involves combining host resistance and cultural practices to make conditions unfavorable for scab development.

Table I. Reaction of selected potato varieties to common scab
Type of Potato Potato Variety Reaction to Scab
Blue All Blue Resistant
Red Red Cloud Resistant
Red La Soda Tolerant
Red Norlands Tolerant
Red Pontiac Susceptible
Viking Tolerant
Russet Centennial Russet Susceptible
Goldrush Resistant
Russet Burbank Tolerant
Russet Norkotah Resistant
White Skin Irish Cobbler Susceptible
Kennebec Susceptible
Yellow Skin Yukon Gold Susceptible

This publication has been peer reviewed.


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Index: Plant Diseases
Vegetables
Issued March 2009

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