Potato Scab Control



Scab is a disease of potato tubers resulting in lowered tuber quality due to scab-like surface lesions. Scab is caused by a group of filamentous bacteria called actinomycetes which occur commonly in soil.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).
HOST PLANTS: Common in potatoes but can also be found in beets, radish, turnip, carrot, rutabaga, and parsnips.

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.

FAVORED ENVIRONMENT: Streptomyces scabies overwinters in fallen leaves and in the soil. The organism can survive indefinitely in slightly alkaline soil but is relatively scarce in highly acid soils. It is transmitted to plants by infected seed tubers, wind and water. The organism is also spread in fresh manure, since it can survive passage through the digestive tract of animals.S. scabies enters through pores (lenticels) in stems, through wounds, and directly through the skin of young tubers. This should be kept in mind when considering a crop rotation schedule.

Note: S. scabies can survive in the soil for many years in the absence of potato..

POTATO SCABS CONTROL: Plant resistant varieties whenever possible. We suggest using the russet-skinned varieties, since they have more resistance to the disease. Rotate root crops by planting in alternate locations to limit this disease. Potato scab is most prevalent in dry, alkaline soils. Decrease soil pH by adding elemental sulfur. The disease is controlled or greatly suppressed at soil pH levels of 5.2 or lower. Keeping soil moist during early tuber development may have a dramatic effect on common scab infection. Maintain proper soil moisture for about 2 weeks after the plants emerge from the soil. Avoid overwatering.

Tip: If you will be planting potatoes in soil where tubers have not been grown before or where the area is known to be scab-free, treat seed potatoes with sulfur fungicides to reduce scab introduction.

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