A Binucleate Rhizoctonia anastomosis group (AG-W) is the causal agent of sugar beet seedling damping-off disease in China

Two binucleate Rhizoctonia (BNR) isolates (HLJ-21 and HLJ-57) were recovered from diseased sugar beet seedlings in Heilongjiang Province, Northeastern China in May, 2015. The two isolates were identified as members of the anastomosis group (AG)-W based on morphological characteristics, hyphal anastomosis, polymerase chain reaction restriction fragment length polymorphism (PCR-RFLP), and phylogenetic analysis of the internal transcribed spacer of the ribosomal DNA (rDNA ITS). The optimum temperature for hyphal growth was 25 °C and the mycelia retained a whitish appearance on potato dextrose agar as cultures aged. No sclerotia, however, were observed during the period of culture. Both of the isolates exhibited anastomosis with AG-W, but failed with other reference strains of BNRs. Restriction fragments resulting from the eight enzymes indicated that HLJ-21 and HLJ-57 isolates had the same restriction pattern as AG-W isolates. Phylogenetic analysis also indicated that the rDNA ITS region of the HLJ-21 and HLJ-57 isolates clustered with other AG-W isolates in the same clade. The AG-W isolates from sugar beet and potato were the causal agent of damping-off disease on sugar beet seedlings as well as root disease on wheat and corn seedlings. The AG-W isolates from sugar beet could also induce stem canker disease on potato plants. Notably, the isolates from sugar beet seedlings had a higher virulence on all of the tested crops than those isolates obtained from potato. This appears to be the first report of a AG-W BNR causing sugar beet seedling damping-off disease in China, and perhaps globally.