Skin spot

Skin spot (Polyscytalum pustulans) is a fungal disease of potato tubers which is generally symptomless until after approximately 2 months of storage, when the infected tissue begins to show spots on some varieties. This unsightly blemish reduces market value, but even processing crops might require extra peeling to remove well developed symptons .

Symptoms

On stems, small shallow light brown lesions coalesce into larger brown patches and deep longitudinal cracks develop. Stems might also become brittle and show stress fracturing. Skin spots on tubers tend to be dark grey and slightly raised. The mature fungal lesion frequently forms a sunken ring around a raised centre. Lesions may form either singly or in clumps and be distributed at random or aggregated around eyes, stolons and damaged skin. Cutting through a skin spot shows a dark thickening, which may extend up to 2mm into the tuber. Tubers incubated in a moist container for 3 days at room temperature may develop sporulating mycelia, visible as white fluff. Skin spot may be confused with immature  powdery scab before they erupt. Lenticel discolouration is sometimes mistaken for the disease, but skin spot rarely has such a regular distribution.