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.
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.
Skin spot is more of a problem with heavy soils in cool, wet seasons, especially when harvesting wet conditions. In cold storage conditions lesions often penetrate deeper and in damp stores infections can spread by spores. Early applications of CIPC sprout suppressant may exacerbate skin spot by halting natural skin curing. In seed tubers skin spot can lead to necrotic buds in eyes and lead to gappy emergence.
The fungus can survive for 8 years on plant debris up to 4 years in the soil. Therefore long rotations effectively limit bridging from one potato crop to the next. Disease-free seed is essential for susceptible varieties, see the variety database for more information. Lift in warm, dry conditions and harvest early to minimise development of the disease. In store, dry cure and avoid condensation. Stores with previous problems should be cleaned and disinfected.