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Potato tuber blemishes: understanding & diagnosis

Skin spot

Fungus, Ascomycete

Latin name: Polyscytalum pustulans / Oospora pustulans


Diagnostic characters for visual diagnosis

  • Small black or dark purple pustules (1-3 mm diameter)
  •  Randomly scattered across tuber surface but may be clustered or found associated with eyes
  •  Spots are slightly raised and surrounded by slightly depressed ring
  •  Lesions rarely penetrate more than 1-2 mm and easily removed by potato peeler
  •  On occasions may develop into deeper lesions
  •  Symptoms rarely develop until 3 months after storage 


Notes on confirmatory diagnostics 

  • On incubation at 12-15 °C and high humidity, characteristic chains of single-celled spores (2-3μm x 6-18 μm) develop from spots
  • Fungus is very slow growing and spores may not develop and be visible for microscopic examination for 7-14 days
  • Confirmation can be made by isolation onto potato dextrose agar
  • Sequencing the fungus ITS or using species specific primers may help identification of P. pustulans

Specific references:

Carnegie SF & Cameron AM (1987). PotatoResearch 30, 79-88

Lees et al (2009). JournalofPhytopathology, 157, 154-158

Nagedy GA & Boyd EW (1965). EuropeanPotatoJournal 8, 200-214

more symptoms

Possible confusing symptoms :