Characterization of mycorrhizal isolates of Rhizoctonia solani from an orchid, including AG-12, a new anastomosis group

Isolates of Rhizoctonia solani collected from mycorrhizal orchid (Pterostyl is acuminata) plants and adjacent leaf litter were characterized. Of 23 sel ected isolates, 20 were members of a new anastomosis group (AG-12) and the rest were members of AG-6. There were no bridging anastomosis reactions obs erved between AG-12 and other AGs of R. solani. Among the 20 isolates of AG -12 evaluated, 18 vegetatively compatible populations were detected, indica ting diversity within the AG. Mature cultures were dark brown, as were matu re sclerotia. Some cultures produced alternating dark- and light-colored co ncentric rings, with sclerotia forming in the darker rings. Most cultures w ere appressed to the agar surface. In tests run to characterize pathogenic potential, selected mycorrhizal isolates of AG-12 and AG-6 did little damag e to potato and barley seedlings, moderate damage to head lettuce seedlings , and more extensive damage to seedlings of cauliflower and radish. Isolate s of AG-12 have not been observed to fruit in nature, and all attempts to i nduce formation of the teleomorph (Thanate-phorus cucumeris) in the laborat ory by selected isolates of AG-12 failed.