Potato common scab

What you see

Rough, corky, raised or pitted patches on the tuber skin. The flesh underneath is perfectly sound. Yields are unaffected; it is purely cosmetic damage to the skin.

Conditions

Dry soil during the few weeks when tubers are forming, alkaline or freshly limed ground, and light sandy soil. Ironically it is worse in a dry year on good soil than a wet year on poor soil.

Treatment

Nothing to do to a crop already affected - and nothing needs doing. The potatoes are entirely fine to eat once peeled.

Prevention

Keep the soil consistently moist for the three to four weeks after the plants flower - that is the whole trick. Do not lime the potato bed and do not use fresh manure. Grow resistant varieties and add organic matter to hold moisture.

Months: June, July, August

Plants

Potato, Beetroot, Radish, Carrot

Remedies

Mulch

The thing people get wrong

Almost everyone treats this as a disaster and it is nothing of the kind - peel the potato and it is perfect. What it really is, is a moisture diary: scabby tubers tell you the bed dried out during tuber initiation. Water consistently for those three weeks after flowering and the same variety in the same soil comes up clean the following year.