Potato scabs

Common scab and powdery scab are bacterial and 'fungal' diseases of potato skins, causing rough, scabby patches.

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Common scab of potato. Image: RHS, Horticultural Science

Quick facts

Common name: Common scab and powdery scab
Scientific name: Streptomyces spp. and Spongospora subterranea f. sp. subterranea
Plants affected: Mainly potatoes
Main symptoms: Rough, scabby patches on tubers
Caused by: Filamentous bacteria and an organism closely related to the slime moulds.
Timing: Summer

What are potato scabs?

Common scab is caused by a number of different species of Streptomyces, and powdery scab by Spongospora subterranea f. sp. subterranea. Both are pathogenic micro-organisms and cause rough, scabby patches. Scabs appear during summer and persist on harvested tubers throughout storage.

Common scab is most serious on potatoes, but also affects beetroot, radishes, swedes and turnips. Common scab is worse if soil conditions are dry when the potato tubers form.

Powdery scab is worse under wet conditions and also sometimes infects tomato roots.

Symptoms

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