Characterization of a New Anastomosis Group (AG-W) of Binucleate Rhizoctonia, Causal Agent for Potato Stem Canker.

Two binucleate Rhizoctonia (BNR) isolates were recovered from potato cankered stems in Heilongjiang Province, northeastern China. Their cultural appearance on potato dextrose agar remained whitish as the cultures aged. White monilioid cells formed in the fluffy aerial hyphae, whereas no sclerotia appeared during the incubation. The two isolates could anastomose with each other, but they failed to anastomose with reference strains of BNR from AG-A to AG-Q, and AG-U. Analyses of restriction fragment length polymorphism (RFLP) of internal transcribed spacer of ribosomal DNA (rDNA-ITS) regions confirmed that these two isolates differed from the reference strains. The phylogenetic tree based on the sequences of rDNA-ITS regions showed that they were located in a distinct clade from other BNR AGs. These collective results suggested that the isolates recovered from potato in this study belonged to a new BNR AG designated as AG-W. Pathogenicity tests under glasshouse conditions revealed that both isolates were able to cause brown, dry, and slightly sunken lesions on potato subterranean stems. To our knowledge, this is the first report of the AG-W causing potato disease in China as well as worldwide.