Characterization and Pathogenicity of Binucleate Rhizoctonia spp. from Azaleas and Other Woody Ornamental Plants with Web Blight. T. A. Frisina, Graduate Research Assistant, Department of Plant Pathology, North Carolina State University, Raleigh 27695. D. M. Benson, Professor, Department of Plant Pathology, North Carolina State University, Raleigh 27695. Plant Dis. 71:977-981. Accepted for publication 9 June 1987. Copyright 1987 The American Phytopathological Society. DOI: 10.1094/PD-71-0977.

Fungi with Rhizoctonia-like mycelia were isolated from foliage of azaleas and other woody ornamentals grown in production nurseries in North Carolina and other southeastern states. Isolated fungi were identified as either binucleate Rhizoctonia spp. or Rhizoctonia solani on the basis of hyphal characteristics and nuclear number. Representative isolates of binucleate Rhizoctonia spp. and R. solani were characterized for anastomosis, temperature optima in vitro, and virulence on azalea. Isolates of binucleate Rhizoctonia spp. (BN8 and BN21) were assigned to Ceratobasidium anastomosis group 7 (CAG-7) or were assigned tentatively to CAG-3 (BN2 and BN6) on the basis of hyphal fusion reaction with tester isolates. Three binucleate Rhizoctonia spp. (BN10, BN17, and BN22) did not anastomose with any of the tester isolates. Isolates of R. solani were assigned to R. solani anastomosis group 1 (AG-1). The optimum temperature range for growth of binucleate Rhizoctonia spp. and R. solani was 24–28 C. Growth rate of R. solani at optimum temperatures was more rapid than for binucleate Rhizoctonia spp. Most isolates of binucleate Rhizoctonia spp. and all isolates of R. solani caused a foliar blight on azalea in the greenhouse. However, isolates of binucleate Rhizoctonia spp. produced symptoms that more closely resembled typical symptoms of web blight observed on azalea in nurseries. Disease severity was correlated with the growth of aerial mycelia in the foliage.