Scab is a disease of potato tubers resulting in lowered tuber quality due to scab-like surface lesions. There are no above-ground symptoms. Two forms of scab occur. Common scab occurs in all production areas and is most severe in soils with a pH above 5.5. Another less common form, called acid scab, is important in acidic soils (below pH 5.5).

Symptoms:

Scab symptoms are quite variable. Usually roughly circular, raised, tan to brown, corky lesions of varying size develop randomly across tuber surfaces. Russet scab occurs as a rather superficial layer of corky tissues covering large areas of the tuber surface. Pitted scab occurs where lesions develop up to 1/2 inch deep; these deep lesions are dark brown to black, and the tissues underneath are often straw-colored and somewhat translucent. More than one of these lesion types may be present on a single tuber. Although scab symptoms are usually noticed late in the growing season or at harvest, tubers are susceptible to infection as soon as they are formed. Small brown, water-soaked, circular lesions are visible on tubers within a few weeks after infection. Mature tubers with a well-developed skin are no longer susceptible, but existing lesions will continue to expand as tubers enlarge, increasing disease severity throughout the growing season. Scab is most severe when tubers develop under warm, dry soil conditions. Coarse-textured soils that dry out quickly are therefore more conducive to scab than are fine-textured soils.

Several other conditions can be confused with scab. White, enlarged lenticles, which frequently occur on potato tubers harvested from wet soil, can be mistaken for scab. Usually this condition will disappear when tubers are dried. Patchy russeting, checking or cracking of tuber surfaces caused by the fungus Rhizoctonia spp. also may be confused with russet scab. A very different and uncommon disease called powdery scab, caused by the fungus Spongospora subterranea, causes very similar scab-like symptoms. Laboratory examination may be necessary to identify these diseases.

Causal Organisms:

Scab is caused by a group of filamentous bacteria called actinomycetes which occur commonly in soil. In soils with a pH above 5.5, Streptomyces scabies is usually responsible for common scab and is capable of causing all the types of scab lesions described above. It is commonly introduced into fields on seed potatoes and will survive indefinitely on decaying plant debris once the soil is contaminated. The organism can also survive passage through the digestive tract of animals and be distributed.

Control:

Control in the vegetable garden for the most part involves correcting the conditions most favorable for scab production.

• Soil pH and fertilizer choice are very important. Keeping the soil pH at or below 5.2 will suppress scab. Sulfur can be applied to the soil to lower the pH and make it more acidic. Acid-forming nitrogen fertilizers, such as ammonium sulfate and diammonium phosphate, are more effective in reducing scab than ammonium nitrate, while calcium and potassium nitrate can raise soil pH and favor scab development. Fresh barnyard manure applied to the soil can create a scab problem and the pathogen may persist in the soil for many years afterward.

•  Follow a crop rotation schedule when scab is a problem. Plant at least three years of nonsusceptible crops between potato crops. Scab infection builds up following frequent crops of potatoes, beets, radishes, turnips, carrots, rutabagas and parsnips. Rotations including rye, alfalfa or soybeans may reduce scab severity.

• Keep soil moist during early tuber development (for about 2 weeks after the plants emerge from the soil). Avoid overwatering, as it may cause rotting or poor plant growth.

•  Varietal resistance: Plant certified seed potatoes of the russet-skinned varieties, which are more resistant to scab. The variety Nooksack is highly resistant, while Russet Burbank, Netted Gem and Norgold have moderate resistance. A red variety, Red Norland, is also moderately resistant.

• Fungicide Seed Treatment: In situations where growers are planting in ground not planted previously with potatoes or where the field is known to be scab-free, treat seed tubers with a fungicide seed treatment to reduce scab introduction through the seed pieces.

Adapted from the University of Vermont Extension and the Ohio State University Extension, 2000