Why do my potatoes have rough scabby patches?
Common scab is a skin blemish, not a rot — how to tell it from powdery scab and from blight, whether the potatoes are still good, and the watering that prevents it.
- Difficulty
- beginner
- Time
- 12 min
- Read
- 3 min
Short answer
Rough, raised, corky patches on otherwise firm potatoes are common scab. It is cosmetic — peel them and they are fine. It is worst on dry, limy, freshly limed soil, so the practical prevention is to keep the soil moist while the tubers are forming and not to lime the potato bed.
Scab looks alarming and is almost always harmless. It is caused by a soil-borne organism that attacks the developing skin of a tuber during a specific window a few weeks after tuber initiation, and its severity is driven far more by soil conditions than by anything you have done wrong.
Step by step
- Confirm it is common scab and not something worse.Common scab makes rough, raised, corky, irregular patches on a firm tuber; the flesh underneath is normal. Cut one open — if the flesh is sound, that is scab.
- Rule out powdery scab.Powdery scab produces more regular round pustules that burst to release a brown powder, and it is associated with wet, heavy soils rather than dry ones. It matters because the management is the opposite: improve drainage rather than water more.
- Rule out blight and slug damage.Blight in a tuber shows as reddish-brown staining spreading into the flesh from a sunken patch, and it rots in store. Slug holes are clean round entry points with tunnelling inside. Neither looks corky on the surface.
- Use the crop normally.Scabbed potatoes peel perfectly well and taste the same. They store as well as clean ones as long as the flesh is sound. The only real cost is peeling waste.
- Water through the tuber-forming window next season.The vulnerable period is roughly the few weeks after tubers start to form — around and just after flowering for earlies. Keeping the soil consistently moist through that window is the single most effective control there is.
- Do not lime the potato bed.Scab thrives as pH rises. Potatoes tolerate slightly acid soil happily, so if your rotation applies lime for brassicas, apply it to the brassica bed and let the potato ground come round to it two or three years later.
- Add organic matter rather than ash or lime.Well-rotted compost and manure hold moisture and support the soil life that competes with the scab organism. Wood ash raises pH and makes scab worse.
- Choose a scab-resistant variety if it is a recurring problem.Varietal tolerance to common scab varies a lot and seed catalogues list it. On light sandy soil that dries fast, this is the most reliable long-term answer.
Tips
- Light, sandy, free-draining soil and a dry June is the classic scab combination. On that soil, mulching the row keeps the moisture steady far more cheaply than watering does.
- Scab does not spread in store, so there is no need to separate scabby tubers from clean ones.
- Growing potatoes in bags or pots does not prevent scab if the compost is allowed to dry out at the wrong moment.
- Do not save your own seed from a badly scabbed crop — you carry the problem forward.
Common mistakes
- Throwing the crop away — Common scab is a skin defect. The potato underneath is perfectly good food.
- Liming to 'cure' it — It does the opposite. Higher pH favours the organism; potatoes are one of the few crops that prefer the acid end.
- Applying fresh manure to the potato bed just before planting — Fresh, strawy manure creates air pockets and dries the soil around the developing tubers, which is exactly what scab wants. Well-rotted material dug in earlier is fine.
Questions people ask
Is it safe to eat scabby potatoes?
Yes. Common scab affects the skin only. Peel them, check the flesh is firm and normal, and use them exactly as you would clean potatoes. Discard any where the flesh underneath is discoloured, soft or smells sour — that is something else.
Does scab stay in the soil?
Yes, it persists and it has a wide host range including beetroot and other roots, so rotation helps but does not clear it. Managing moisture and pH, and choosing a tolerant variety, does more than moving the bed.
Why did I get scab this year and not last year?
Almost always the weather in the few weeks after tuber formation. A dry June gives scab; a wet one does not. That is why the same bed and the same variety can produce very different results.