The Bug Box: Common scab

Wayne Jones

Cornell University

This bug is creeping around your property. He may be friend or foe.

Name: Streptomyces scabies

Alias: Common scab. This bacterium lives in the soil and occurs worldwide. It is a problem in both gardens and commercial potato fields. Symptoms on potatoes are mostly limited to the tubers. S. scabies infects young, developing tubers through the lenticels and occasionally through wounds. Early symptoms consist of browning and cell expansion. As the disease progresses, the lesions on the tuber become circular to irregular. The lesions may be above the surface of the tuber, even with the surface or sunken. At harvest, the lesions are usually rough in texture and tan to dark brown. The pathogen sporulates inside of these lesions and some of the spores are released into the soil, where they overwinter. S. scabies can live for several years, if not indefinitely in the soil without a potato crop. It can be introduced to soils from contaminated potato seed.

Crimes: While common scab does not reduce yields, it does reduce the marketability of the crop. It also attacks radishes, beets, carrots and other crops.

Redeeming Qualities: Several beneficial antibiotics have been isolated from these types of organisms, such as streptomycin sulfate.

Sentence: This is a difficult disease to control. Avoid planting scabby seed tubers and wait three or four years before planting potatoes in the same area. There are differences between potato varieties, so if you are struggling with scab, you could try another variety of potato. Common scab likes high-pH soils, which we have here, so making the soil more acidic may be beneficial. But do not add manure, as that may increase the incidence of scab. Maintain high soil moisture for four to six weeks when the tubers first start to grow. There are seed-piece treatments that may provide some control of the disease.

For more information on dangerous and beneficial bugs, call agent Wayne Jones at the Bonneville County Extension Office at 208-529-1390.