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powdery scab

powdery scab

© Sandra Jensen

Type: Diseases

Scab, powdery (Spongospora subterranea f. sp. subterranea)

Powdery scab is caused by a single-celled organism related to amoeba. Transmission from tuber to tuber involves a swimming stage so wet conditions favour infection. While unsightly, the potatoes are perfectly edible after peeling. However, secondary infections can enter the lesions and reduce storage life.


  • Initially, small purple/brown marks
  • These develop into irregular depressions with raised edges. These pustules contain the powdery/dusty spores which go on to infect other tubers

  • Prevention & control

    • Resistant varieties are the best defence against powdery scab
    • Good long rotations will at least reduce the problem, although spores can survive up to 10 years
    • Good crop hygiene, removing all traces of any previous crop
    • Avoid cold, heavy soils, adding organic matter to improve aeration
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