Identification and Pathogenicity of Rhizoctonia spp. Isolated from Apple Roots and Orchard Soils.

ABSTRACT Rhizoctonia spp. were isolated from the roots of apple trees and associated soil collected in orchards located near Moxee, Quincy, East Wenatchee, and Wenatchee, WA. The anastomosis groups (AGs) of Rhizoctonia spp. isolated from apple were determined by hyphal anastomosis with tester strains on 2% water agar and, where warranted, sequence analysis of the rDNA internal transcribed spacer region and restriction analysis of an amplified fragment from the 28S ribosomal RNA gene were used to corroborate these identifications. The dominant AG of R. solani isolated from the Moxee and East Wenatchee orchards were AG 5 and AG 6, respectively. Binucleate Rhizoctonia spp. were recovered from apple roots at three of four orchards surveyed and included isolates of AG-A, -G, -I, -J, and -Q. In artificial inoculations, isolates of R. solani AG 5 and AG 6 caused extensive root rot and death of 2- to 20-week-old apple transplants, providing evidence that isolates of R. solani AG 6 can be highly virulent and do not merely exist as saprophytes. The effect of binucleate Rhizoctonia spp. on growth of apple seedlings was isolate-dependent and ranged from growth enhancement to severe root rot. R. solani AG 5 and AG 6 were isolated from stunted trees, but not healthy trees, in an orchard near Moxee, WA, that exhibited severe symptoms of apple replant disease, suggesting that R. solani may have a role in this disease complex.