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Potato tuber blemishes: understanding & diagnosis

Black dot

Fungus, Adelomycete = Deuteromycete

Latin name : Colletotrichum coccodes

Other names : Anthracnose

Diagnostic characters for visual diagnosis

  • Diffuse brown  lesions with black microsclerotia regularly spaced.
  • The lesions are irregularly shaped with less well-defined margins than silver scurf. Lesions may coalesce to form larger areas
  • Sclerotia develop into acervuli producing conidia
  • Black dot symptoms may be present at harvest but can develop during storage
  • Sunken lesions may occur after cold storage



Notes on confirmatory diagnostics

  • Incubating infected tubers at high relative humidity, microsclerotia produce silks with a sea urchin appearance observable under a hand  lens
  • For isolation, pick up tissue from edge of lesion after surface sterilization
  • Malt & PDA are good isolation media                                                                                                  


Specific reference :

Glais-Varlet & Andrivon (2004). Plant Pathology, 53, 254

more symptoms

Possible confusing symptoms :