There are two forms of scab that affect potatoes – common scab and powdery scab. Both are pathogenic micro-organisms and cause rough, scabby patches. Scabs appear during summer and persist on harvested tubers throughout storage.
Common scab is most serious on potatoes, but also affects beetroot, radishes, swedes and turnips. Common scab is worse when soil conditions are dry when tubers form. Powdery scab is worse under wet conditions and also infects tomato roots.
Common scab shows as raised, rough patches of skin on the tuber surface. Powdery scab shows as irregular brown depressions containing masses of dusty brown spores on the surface of tubers.
To help prevent scab always choose your seed potatoes carefully. Do not plant those which show obvious signs of scab. Liming soil to prevent club root in brassicas makes the soil alkaline which may trigger common scab. You should also not allow the soil to dry out during tuber development so you need to water regularly in dry weather when the plants appear.
Powdery scab is worse in wet conditions which allow the spores that cause the disease to become prolific.
There is no chemical cure available to the allotment gardener so rotation, soil conditions and good standards of general cultivation are important.