The margin between healthy and diseased tissue in the interior is sharply defined. The rotted root tissue is firm but may become soft as secondary fungi and bacteria invade. This sexual stage is rare but occasionally observed on the underside of infected petioles late in the season. R. solani occurs throughout the world in agricultural soils and infects many plant species. Although R. solani does not produce spores, its vegetative hyphae is quite characteristic and easily identified microscopically.
Young hyphae are pale, becoming brown with age.
They branch mostly at right angles, usually with a constriction at the branch. With age, dark brown thick-walled, barrel shaped hyphal cells aggregate to form dry, hard, compacted structures called sclerotia. These are visible to the naked eye and allow the pathogen to survive adverse conditions. The various AG types represent genetic differences within the species. AG-3 causes damage in potato but is not pathogenic to sugarbeet.