Damage caused by Rhizoctonia can range from yield losses due to poor stands and stunted plants to marketable quality concerns as a result of misshapen and discoloured tubers.
Rhizoctonia symptoms are found on both above- and below-ground portions of the plant. Early in disease development, reddish-brown to brown lesions develop on sprouts, stolons and young stems. Rhizoctonia lesions that pinch off sprouts before plant emergence result in severe crop damage. The secondary sprouts that develop from the pinched off sprouts are less vigorous and emerge much later, causing irregular, uneven stands. Early infection of stolons often results in pruning before tuber formation or interrupted development of newly formed tubers. As the stem lesions mature they become cankers. Cankers on young stems cause the emergence of weak plants.
Mid- and late-season infections result in long, deep, sunken cankers on the stems, the formation of aerial tubers and deformed daughter tubers. Aerial tubers may indicate that the plant has no tubers of marketable quality below ground.
A white to grey mat of fungal mycelium develops at the base of the stem. The mycelium is easily rubbed off. The affected stem area remains green.
The most noticeable sign of Rhizoctonia is black scurf. During this phase of the disease, black sclerotia form on the skin of daughter tubers. Sclerotia are hard, black structures of irregular shape and variable sizes tightly attached to the tuber skin. They are commonly called ‘the dirt that won’t wash off.’
