Scab, common (Streptomyces scabies)

This is a bacterial disease that favours high pH (>5.2) and dry conditions. It overwinters in the soil or in tubers which were missed at harvest, infecting new tubers as they grow. The plant reacts with a corky scab which eventually bursts then regrows, leading to the characteristic raised and layered appearance of the scabs produced. Common scab is usually less serious than powdery scab, the damage being largely cosmetic - tubers are still completely edible - although in severe cases, where the skin cracks, it can affect storage.


  • Raised rough corky brown patches on tubers

Prevention & control

  • Resistant varieties offer some protection
  • Good plot hygiene, making sure you lift all of last year's crop
  • Keep well-watered in dry spells, particularly as the tubers begin to form (I.e. from a fortnight after emergence for a month or so)
  • Keep soil pH low:
    • Don't lime for brassicas until after a potato crop
    • Apply brimstone (sulphur) which lowers pH by creating a weak acid
    • Or you could try a 5cm thick layer of grass cuttings, covered by 2.5 cm of soil at the bottom of the potato trench before planting
Top tips
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Common Potato Scab (administrator - 16:57 26/01/2015) Report inappropriate tip
1. Choose and plant resistant varieties; 2. Practise good crop rotation;3. Maintain an acid soil so do not add lime or alkaline ashes or poultry manure; 4. If you line the planting trench with grass clippings or wilted comfrey then cover with soil before adding your potatoes the decomposing vegetation will locally reduce pH (increase acidity)..and provide some food for the growing crop.
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