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Potato tuber blemishes: understanding & diagnosis

Pitted or raised symptoms

Pitted or raised symptoms on tubers  can be:

  • Shallow, raised or deep lesions surrounded by corky tissue, they may be circular or sometimes star shaped > Common scab,
  • Purplish Brown pustules filled with dry dark brown powder, in general small (0.2 to 5 mm diameter) but sometimes they expand in depth and width; presence of cankers > Powdery scab,
  • Small black or dark purple pustules (1 to 3 mm diameter) slightly raised and surrounded by slightly depressed ring> Skin spot,
  • Raised dark brown to black structures called sclerotia, irregular in size and in shape, tightly attached to the periderm> Black scurf,
  • Cavities up to several mm in deep and 3-6 mm in width, filled with black sclerotia> Dry core,
  • Small circular holes (< 0,5 cm) surrounded by brown margins, the cavities go right into the tuber flesh> Wireworm,
  • Wide circular holes (> 0,5 cm), the cavities are irregular and sometimes extended> Slug

Other pitted or raised symptoms can be encountered and are due to other biotic (viruses, fungy, pests) and abiotic causes.

Click on a picture to learn more about each pitted or raised symptoms