Streptomyces scabies, bacteria; no foliar disease
Infection:
Tubers get infected during early bulking by attack from the bacteria in the soil. After attack, the wound heals and a repeated attack is possible. There is disagreement on how well soil can be infested by seed-borne scab and how important is seed-borne scab.
Symptoms:
Light infection causes a rough (corky), circular, tan to brown surface blemish. With mild infection, blemishes may be raised. Severe infection causes dark brown pits to form which are wider, shallower and lighter than those of black pit. No rot occurs.
Control Practices:
Grow scab-resistant varieties on scabby ground. Irrigate to maintain soil moisture above 90% 2-3 weeks after tuber initiation and maintain this soil moisture level until mid to late bulking. Application of ammonium sulfate in-furrow or just before tuber initiation has been reported to reduce scab by half. In acid soil, acidify further by adding sulfur to below pH 5.4. Recent research suggests that a few insecticides have lessened scab infection by affecting soil insects that may play a role. Avoid manure. The similarity of potato common scab and the common scab infecting sugar beets and beans is not clear. Avoid growing potatoes the year after these crops.
Tuber Blemishes