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Skin spot – Polyscytalum pustulans
Symptoms Discrete light brown lesions develop on infected roots, stolons, and belowground parts of stems. The lesions enlarge, darken, and crack transversely. In storage, purplish black, slightly raised spots, up to 2mm in diameter, form on tubers. These symptoms are normally detected in ambient stores from January onwards. The spots are distributed at random over the surface of affected tubers or aggregated around eyes and stolon scars. Buds in eyes may be killed and tubers become blind. When seed tubers with damaged eyes are planted, plant emergence is delayed giving an uneven crop.
Sources and Factors Favouring Development
Seed tubers are the main source of inoculum for infection of stolons, roots and daughter tubers, even if symptoms are not obvious Infection is encouraged by low temperatures and high soil moisture, particularly during lifting and the first 3 months of storage. Varietal susceptibility, related to skin thickness, plays a significant role in determining the extent of symptoms.
Controls Planting disease free tubers will result in a healthy crop, but inoculum can be wide spread on farms, and the crop is at risk of infection through out storage of the healthy crop, during crop growth, and during storage of the harvested crop. Early harvesting of susceptible varieties followed by curing at 12-15°C for a period of 7-10 days has proved effective. Fumigation of seed with 2-aminobutane (2AB) shortly after lifting can have a very high degree of control, as well as being effective against Gangrene and Silver scurf. 2-AB may cause damage if used on immature or damaged tubers. At present it is only permitted for essential use until 31st of December 2007 by the Pesticide Safety Directorate (PSD).