rhizoctonia disease noun alsorhizoctonia (-s) Etymology: New Latin Rhizoctonia 1.: a disease caused by fungi of Rhizoctonia or closely related genera 2.: a disease of potatoes caused by a fungus (Pellicularia filamentosa syn. Rhizoctonia solani) and characterized especially by black scurfy spots on the tubers — called also little potato, rosette, russet scab, stem canker; compare damping-off