Rhizoctonia solani is found very commonly in soil, but exists as a number of different strains, not all of which can attack potatoes. Infection can arise from the soil or by the planting of affected seed tubers. The strains that attack potatoes are likely to increase rapidly in the soil if the crop is grown in successive years or in a short rotation. The black scurf is a resting structure of the fungus, known as a sclerotium.
Attacks on the sprouts are usually most damaging if their growth through the soil is delayed by cold, dry weather. Stem canker and black scurf also tend to be more common on lighter soils. Development of black scurf on the tubers increases rapidly as the crop dies down naturally at the end of its growth.