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 comme rcial glasshouse-grown roses in Gifu prefecture, Japan, during the summer a nd fall of 1997 and 1998. One hundred and fifty-three isolates of Rhizocton ia spp. were obtained from infected roots and stems. Of the 153 isolates, 9 had binucleate and 144 had multinucleate vegetative hyphal cells. Binuclea te Rhizoctonia failed to anastomose with tester isolates of anastomosis gro ups (AG)-A through -S (not including AG-J and AG-M). Of 144 isolates identi fied as R. solani, 83.3% were AG 2-2 IIIB and 16.7% were AG 4 HG-I. Five is olates from each group caused severe rot and mortality on cuttings during r ooting. 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 a nd 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.