Potato Scab: Rough Skin Patches and How to Prevent Them
Scab shows up as rough, corky, raised or pitted patches on the skin of otherwise sound potatoes. It's a cosmetic and storage nuisance rather than a danger — peel the affected skin and the flesh underneath is fine to eat — but heavy scab makes for ugly, hard-to-store tubers. It's driven by soil conditions during the few weeks when young tubers are forming.
Soil too alkaline
What's happening
The organism behind common scab thrives in neutral to alkaline soil and is suppressed in acidic conditions. Liming the bed or growing in naturally high-pH soil pushes conditions in the scab-causing direction.
How to confirm
Tubers show scattered corky lesions, and a soil test reads above pH 6.0; the problem often follows a recent liming or appears in known high-pH ground.
How to fix it
There's no cure for the current crop — peel scabby skin before cooking. Adjust the soil for next season rather than this one.
Prevent it
Keep potato beds on the acidic side, ideally pH 5.0–6.0; don't lime where potatoes will grow, and choose scab-resistant varieties.
Dry soil during tuber formation
What's happening
Young tubers are most vulnerable to scab infection in the few weeks just after they begin to form. Soil that dries out during this window lets the organism attack the developing skin.
How to confirm
Scab is worse following a dry spell around flowering and tuber set, and shows up most on plants that weren't watered evenly.
How to fix it
Keep the rest of the crop watered steadily; affected tubers are still usable once peeled.
Prevent it
Water deeply and consistently — about 1–2 inches per week — especially from flowering through tuber bulking, and mulch to hold soil moisture even.
Infected seed or contaminated soil
What's happening
The scab organism persists in soil for years and can also ride in on infected seed potatoes or in fresh manure, building up where potatoes are grown in the same ground repeatedly.
How to confirm
Scab recurs in the same bed season after season, or appears on a crop grown from uncertified or grocery-store potatoes.
How to fix it
Lift and use the crop as normal after peeling; plan to break the cycle next year rather than treating this one.
Prevent it
Always plant certified disease-free seed potatoes, rotate so potatoes don't return to the same bed for 3–4 years, and avoid adding fresh manure before planting.
When to worry (and when not to)
Scab is almost never worth panicking over — the potatoes are perfectly good to cook once the rough skin is peeled away, and light scab barely affects yield. Worry only about storage: badly scabbed tubers keep less well and can let in rot, so use the worst ones first. If scab is heavy or returns every year, treat it as a soil-and-rotation problem and fix the pH, watering, and crop rotation for next season.