Common Scab of Potato

Common Scab of Potato

Common scab is found in most areas where potatoes are grown. It is not a serious disease in terms of yield loss but reduces the quality of tubers produced. The disease is caused by Streptomyces scabies (Thaxter) Waksman & Henrici. This pathogen is an actinomycete which by definition is a filamentous bacteria. These organisms have also been classified as fungi (Fungi Imperfecti) because of their filamentous nature. S. scabies is a soil borne pathogen that infects below ground portions of the plant only. In addition to potato, it also infects beets (table and sugar), radish, rutabaga, turnip, carrot, and parsnip. The organism can survive as a saprophyte on plant debris (including tubers) and the roots of living plants. It can survive passage through an animal's digestive tract so can also be found in feed lots and heavily manured fields. Introduction to a new field usually occurs via infected seed tubers although inoculum can also be disseminated in infested soil on implements or windblown and by water. Increased incidence occurs when potatoes are continuously cropped, especially with susceptible varieties. Soil pH also plays an important role in disease development. Common scab occurs in soils with a pH range of 5.5-7.5. S. scabies is also more of a problem in dry soils that are lacking bacteria antagonistic to the organism. A similar scab called acid scab occurs on potatoes grown in a lower pH soil in the eastern U.S. The disease cycle begins when vegetative mycelium is converted into sporogenous hyphae. The hyphae is pinched off, forming spores. These spores invade young tubers through lenticels, stomata or wounds. Older tubers with a well developed skin are not susceptible but if they were infected early, scab lesions may continue to expand.

Symptoms

No above ground symptoms are visible with this disease and tuber symptoms are variable. In general, they consist of irregular raised tan to brown spots scattered randomly on the tuber. The spots have a corky texture and are often superficially. One variation of the symptoms is russet scab which consists of large areas of superficial corky tissue rather than individual spots. In contrast, pitted scab produces lesions up to ½ inch deep that are dark brown to black. The tuber tissue beneath these lesions is straw-colored and translucent. These two distinctly different types of lesions as well as the smaller, superficial circular lesions can occur on the same tuber. The lesions may not be detected until late in the growing season but symptom expression begins much sooner. In addition to tuber, stolons may also become infected.

Plant Health Management

Use resistant varieties

Do not use scab infected tubers for seed

Crop rotation to a nonhost crop

Lengthen the period between successive potato crops.

Maintain adequate soil moisture

References

Agrios, G.N. 1997. Plant pathology, 4^th ed. Academic Press, San Diego, CA. 635 pp.

Hooker, W.J., ed. 1983. Compendium of potato diseases. American Phytopathological Society. St. Paul, MN. 125 pp.

in R.C. Rowe, ed. Potato health management. APS Press. St. Paul, MN.

University of California, Division of Agriculture and Natural Resources. 1986. Integrated pest management for potatoes in the western United States. ANR Publication 3316.

Useful Links

Oregon State University

Ohio State University

Highlighted Area

Caveat

This description is presented for information only and no endorsement is intended for products listed, nor criticism meant for products not mentioned. Always consult the product label before purchasing and using any pesticide.

Material contained on the Links from the page are the responsibility of the linked page's author(s).

This page was drafted by: Jane Christensen, Department of Plant Pathology, University of Nebraska-Lincoln

Disease images were provided by: Dr. David Wysong, Department of Plant Pathology, University of Nebraska-Lincoln

Questions, Comments, Complaints and Complements?

This page is authored and maintained by:
Dr. J.E. Partridge, Department of Plant Pathology, University of Nebraska-Lincoln

E-Mail Home Page