This fungus attacks the plant beneath the ground and in severe infections kills sprout before they reach the surface. More commonly it produces rusty-brown lesions on the underground main stem and white mold on the stem just above the soil line. The number of tubers may be increased and their size decreased. Rhizoctonia is present on seed and in soil, infecting a wide variety of plants. Seed with a great deal of black scurf increases infection.
Crop rotation is the best control.
Viruses are carried in the seed from season to season. In the field they are generally spread by aphids or leafhoppers. If clean seed is used, little infection will occur. Early control of insects can be used to prevent spread in the field, especially if seed is to be saved. Scab is caused by a soilborne bacterium that infects tubers and produces russetted areas or small, brown spots that later become large, corky pits.