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Characterization of Rhizoctonia spp. Causing Root and Stem Rot of Miniature Rose

Root and stem rot of miniature rose (Rosa hybrida L.) was observed in commercial glasshouse-grown roses in Gifu prefecture, Japan, during the summer and fall of 1997 and 1998. One hundred and fifty-three isolates of Rhizoctonia spp. were obtained from infected roots and stems. Of the 153 isolates, 9 had binucleate and 144 had multinucleate vegetative hyphal cells. Binucleate Rhizoctonia failed to anastomose with tester isolates of anastomosis groups (AG)-A through -S (not including AG-J and AG-M). Of 144 isolates identified as R. solani, 83.3% were AG 2-2 IIIB and 16.7% were AG 4 HG-I. Five isolates from each group caused severe rot and mortality on cuttings during rooting. Pathogenicity of Rhizoctonia spp. varied on three different ages of miniature roses cv. Silk. Isolates of AG 4 HG-I caused root and stem rot and mortality on 15-, 25-, and 40-day-old plants, whereas isolates of AG-2-2 IIIB caused root and stem rot and mortality on 15- and 25-day-old plants, but light root rot on 40-day-old plants. Isolates of binucleate Rhizoctonia caused root and stem rot and mortality only on 15-day-old plants.