Common scab

Common scab (Streptomyces scabies) is a disease of potatoes, but may also affect beetroot, radish, swede and turnips. It has has very little affect on storability and edibility, but makes the tubers appear unsightly. It is most severe on light soils under dry conditions.

Symptoms
Disease is only skin deep. Lesions may be circular or angular and may coalesce into large irregular areas. Severity can range from sparse colourless, corky lenticels to dark brown, raised or pitted scabs covering the tuber surface.

Common scab might easily be confused with powdery scab. However, powdery scabs tend to have a more round character and when coalescing they tend to merge as discrete eruptions rather than one large scab.

Treatment
None.

Prevention
Dig in compost or try growing in leaf mould, but do not lime before planting. Grow resistant varieties (e.g. Carlingford, Swift, Wilja).