Common scab: how to recognise and treat it

Disease

Symptoms

Brownish, rough, corky spots on the skin of potato tubers, flat or slightly raised.
Spots may merge or sink into pitted corky areas; severe attacks crack or deform the tubers.
The damage is mostly skin-deep and the leaves look healthy, so scab is usually noticed only at harvest; beetroot, radish and turnip roots can show the same spots.

Nothing cures scab on formed tubers; lightly scabbed potatoes are fine to eat once the scabby skin is peeled off.
Keep the soil moist while tubers form — water if needed from 2–3 weeks after the shoots emerge, for about four weeks.
Raise the organic-matter content of the soil with well-rotted material to help it hold water.

No chemical controls are available to home gardeners; rely on watering, soil pH and resistant cultivars.

Reference information, not a guarantee. Read and follow the label of any product: dose, safety and the days to wait before harvest.

Plant seed potatoes that show no scab.
Do not lime before potatoes; scab is worse on alkaline soil and is suppressed below about pH 5.2, so apply lime after the potato part of the rotation.
Do not use fresh manure or wood ash on ground for potatoes or beetroot.
Choose cultivars with some resistance; russet-skinned potatoes are usually less affected than smooth-skinned ones.
Rotate with crops that are not hosts; this reduces but does not eliminate the bacteria.

Figures checked against these sources.