Causal agent(s) and transmission
Skin spot is caused by the fungus Polyscytalum (formerly Oospora pustulans) encountered in cold and damp regions of potato cultivation, such as Great Britain and Scandinavia, where heavy soils are prevalent.
The infection can come from the soil or from the seed tuber. The disease may survive for several years in the soil in the form of sclerotia.
Skin spot is a minor disease of potato tubers. However, in heavy and cold soils, such as in northern European countries, it can have an economic impact for the ware potato industry, when tubers are washed before selling.
Symptoms on tubers
The symptoms of skin spot on tubers consist of small dark pustules, 0.5 to 4 mm in diameter, which develop on the surface of tubers, either isolated (photo 1), or grouped around the eyes or at the heel-end (photo 2). The lesions are prominent conical pustules which are usually surrounded by a depressed ring (photos 2 and 3). The underlying tissue is corky and detaches itself easily.
The symptoms can develop during the first weeks of storage if drying is insufficient.
- Symptoms on sprouts, roots and stolons
On growing crops, superficial brown and cracked lesions similar to those found with black scurf can develop on the roots and stolons. On emergence, sprouting tubers can be infected and the bud tips can be completely killed by the fungus, resulting in poor emergence or reduction of the number of stems.
The disease is favoured by cold, humid conditions both during the growing period and storage. In such a favourable environment, short crop rotations with susceptible cultivars can lead to frequent skin blemish problems.
Skin spot is not very common in most potato growing areas. In case of disease pressure, the following control measures should be followed:
- the use of healthy seed tubers (and, if possible, use tolerant cultivars);
- the choice of long rotations between potato crops;
- harvesting in warm, dry conditions and thorough drying and wound healing after harvesting.