Common scab

Streptomyces scabies

also called potato scab, common potato scab

Rough, corky, scabby patches on the skin of potatoes — unsightly, but only skin-deep, so the tubers still peel and eat perfectly well.

Bacterium Disfiguring Soil-borneSeed-borne Spreads to neighbors

What you'll see

Raised, rough, corky patches or scabs scattered over the tuber skin.
Scabs may be superficial and net-like, or form deeper, star-shaped pits.
The blemishes are confined to the skin; the flesh beneath is sound.
Root vegetables such as beetroot and radish can show similar corky lesions.
No effect on the leaves or haulm above ground — the tubers alone are marked.

Why it happens

A soil-dwelling bacterium (strictly an actinomycete) that lives in the ground and on infected seed tubers. It infects the developing tuber through the young skin and pores, roughening the surface but not rotting the crop.

Dry soils around the time the tubers are forming make it far worse, as does light, sandy, free-draining ground and soil limed to a high pH. Fresh manure and alkaline conditions favor the bacterium.

Treating it

Keep the soil evenly moist while the tubers are swelling — watering through dry spells greatly reduces scab.
Accept that scab is cosmetic: simply peel the tubers, which are perfectly good to eat.
Do not lime ground before planting potatoes, as this encourages the disease.
Lift and store affected tubers as normal; scab does not spread in store.
Don’t replant scabby tubers as next year’s seed.

Keeping it away

Grow scab-resistant varieties on light or chalky soils.
Water generously and consistently as the tubers form.
Work in plenty of well-rotted organic matter to hold moisture, but avoid fresh lime.
Rotate potatoes and use certified seed tubers.
Keep the soil slightly acidic rather than alkaline where scab is a recurring nuisance.

Who gets it

Potatoes above all, with beetroot, radish, turnip and other root crops sometimes affected.