Scab & black scurf (potato)
Corky raised or pitted scabs (common scab) or hard black soil-like crusts (black scurf) that blemish the tuber skin and cut its market value.
- Pathogen
- Streptomyces scabies / Rhizoctonia solani
- Spreads by
- Infected seed tubers; soil-borne
- Favoured by
- Scab: dry/alkaline soil · Scurf: cool, wet soil
- Main risk
- Blemished skin; lower grade & market value
Overview
Common scab and black scurf are two tuber-surface diseases of potato that mainly damage the skin and appearance, cutting grade and market value (and, for scab, occasionally digging into the tuber). Common scab, caused by the bacterium-like Streptomyces scabies, makes rough, corky, brown scab-like lesions on the tuber skin — raised, superficial or pitted — that are only skin-deep but disfigure the tuber. Black scurf, caused by the fungus Rhizoctonia solani, sticks hard, black, irregular crusts (the fungus’s sclerotia) onto the tuber skin that look like soil but do not wash off; the same fungus can also cause stem cankers and kill sprouts, thinning the stand.
Both are carried on infected seed tubers and survive in the soil. Common scab is favoured by dry soil during tuber formation and by alkaline soils; black scurf by cool, wet soils and slow emergence. Because they are seed- and soil-borne and mostly cosmetic (scab) or stand-affecting (scurf), management rests on clean, treated seed tubers, rotation, and managing soil moisture and pH — rather than any cure once the tubers are blemished.
How to identify it
- Common scab: rough, corky, brown scab-like lesions on the tuber skin — raised, superficial or pitted — that are skin-deep but disfiguring.
- Black scurf: hard, black, irregular crusts (sclerotia) stuck to the tuber skin, looking like soil but not washing off.
- Black scurf on the stem: brown-black cankers on underground stems and stolons that can kill sprouts and thin the stand.
- Both mainly damage skin/appearance and grade rather than yield (though scurf stem cankers reduce stand).
What causes it and when
- Common scab: the bacterium-like organism Streptomyces scabies; black scurf: the fungus Rhizoctonia solani.
- Both are carried on infected seed tubers and survive in the soil on debris.
- Common scab is favoured by dry soil during tuber initiation and by alkaline (high-pH) soils.
- Black scurf is favoured by cool, wet soils and slow emergence that keep the sprouts vulnerable longer.
Cultural & organic control
- Plant clean, certified, disease-free seed tubers and treat the seed where recommended.
- For common scab: keep the soil moist during tuber formation (avoid drought stress) and avoid over-liming to very alkaline pH.
- For black scurf: plant into warm enough soil for quick emergence, avoid deep planting in cold wet soil, and harvest promptly.
- Rotate away from potato for a few years and destroy infected debris and volunteers.
Chemical / seed treatment
- Treat seed tubers with a recommended fungicide (against black scurf sclerotia) before planting.
- There is no cure for already-blemished tubers — control is preventive via clean seed and soil management.
- Manage soil moisture and pH (for scab) and planting conditions (for scurf) as the main levers.
- Confirm the current seed treatment and package for your area with your local KVK.
Prevent it coming back
- Make clean, treated seed tubers and rotation the routine, and destroy infected debris and volunteers.
- Keep soil moist during tuber formation and pH from getting too alkaline (scab); plant into warm soil for quick emergence (scurf).
- Harvest promptly and avoid carrying infected tubers/soil into clean fields.
Crops most at risk
Where this problem is called out in our crop guides — open a crop for the full package of practices.
Frequently asked questions
My potatoes have rough corky scabs / hard black crusts that won’t wash off. What is it?
Rough, corky, brown raised or pitted scabs on the skin are common scab (Streptomyces scabies); hard black crusts stuck to the skin that look like soil but won’t wash off are black scurf (Rhizoctonia). Both are seed- and soil-borne and mainly blemish the skin, cutting grade and market value. There is no cure for blemished tubers — use clean, treated seed, rotate, and manage soil moisture/pH (scab) or plant into warm soil for quick emergence (scurf).
How can I reduce common scab in my potato crop?
Common scab is worse in dry soil during tuber formation and in alkaline soils, so the main levers are keeping the soil evenly moist during tuber initiation (avoiding drought stress at that stage) and avoiding over-liming to a high pH. Add clean, certified seed tubers and rotation away from potato, and destroy infected debris. Since scab is only skin-deep and cosmetic, these preventive steps to reduce it are more worthwhile than chasing a cure that does not exist.
Do the black crusts of black scurf harm the potato inside?
The black crusts themselves are the fungus’s sclerotia sitting on the skin — they blemish the appearance and cut grade, but do not rot the flesh inside. The bigger harm from the same fungus is on the underground stems and stolons, where it causes cankers that can kill sprouts and thin the stand, reducing yield. So control both the skin blemish and the stand loss with clean, treated seed tubers, warm-soil planting for quick emergence, prompt harvest and rotation.
Symptoms and control measures follow ICAR, KVK and state agricultural-university package-of-practices; any named chemical is a registered use. Always read the product label and confirm the dose for your crop and zone. editorial policy.