Rhizoctonia solani is a fungus that attacks tubers, underground stems, and stolons of potato plants. Although it probably occurs wherever potatoes are grown, it causes economically significant damage only in cool, wet soils. In temperate production areas, losses from R. solani are sporadic and occur only when weather is cold and wet in the weeks following planting. In northern areas, where growers often must plant in cold soils, Rhizoctonia is a more consistent problem. Poor stands, stunted plants, reduced tuber number and size, and misshapen tubers are characteristic of the Rhizoctonia disease.