Effect of temperature on virulence of Rhizoctonia solani and other Rhizoctonia on potato

Pathogenicity of 47 isolates from various geographical locations and host plants representing 11 of the 12 known anastomosis groups of Rhizoctonia solani and 12 isolates representing other multinucleate and binucleate species of Rhizoctonia was determined on sprouts and roots of emerging potato plants at 10, 15.5, and 21.1 C. Isolates of R. solani AG-3 killed significantly more sprouts than any other group. Isolates of most groups killed no sprouts. Whereas isolates of R. solani AG-3 and AG-5 damaged sprouts significantly more than other groups, most damage to roots was caused by isolates of R. solani AG-8 and AG-3. Other isolates, including those representing other anastomosis groups of R. solani, R. oryzae, R. zeae, and binucleate Rhizoctonia caused a minimal amount of damage to sprouts and roots. Isolates of R. solani AG-3 heavily damaged sprouts at 10, 15.5, and 21.1 C but caused significantly more damage at 10 C. Isolates of AG-3 also damaged roots at all three temperatures. Isolates of AG-5 damaged sprouts at 15.5 and 21.1 C but caused minimal damage to roots. Isolates of AG-8 caused heavy damage to roots but minor damage to sprouts at all three temperatures. It is apparent that at cool temperatures isolates of R. solani AG-3 are more virulent. At warmer temperatures, isolates of AG-8, AG-5, and perhaps, representatives of other groups of Rhizoctonia may be more important in the etiology of Rhizoctonia disease of potato. We know of no previous reports of AG-8 in association with potato plants, but these data indicate its potential to be a very damaging pathogen