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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). Potato Research 30, 79-88

Lees et al (2009). Journal of Phytopathology, 157, 154-158

Nagedy GA & Boyd EW (1965). European Potato Journal 8, 200-214

Powdery scab

Other pitted or raised symptoms