An infection on the skin of the potato. Raised scab-like patches grow and can split the surface. The flesh of the potato is not usually affected and after cooking the scab pulls away with the skin.
The cause is Streptomyces scabies which resembles a bacterium but produces a mycelium, like a fungus. It is usually found in sandy soil with a high lime content and in recently cultivated areas. Low moisture levels during hot dry summers promote its development.
Do not add lime to ground where potatoes are to be grown. Areas where brassicas have grown recently are more prone to scab. Adding a few handfuls of grass clippings when planting should reduce the damage to the new potatoes, possibly because it increases the acidity as it decomposes. There are resistant varieties, eg. 'King Edward' and ones which are more prone to infection, eg 'Maris Piper' and 'Desiree'. There are no chemical treatments and as the damage is only cosmetic, Scab does not usually cause great concern.
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