Abstract：Current classification within Rhizoctonia solani is largely based on grouping of the population into anastomosis groups(AG), but does not always appear to accurately reflect the genetic variability among population. Hyphal anastomosis reactions and internal transcribed spacer(ITS) sequences were integrated to evaluate the genetic diversity of isolates Rhizoctonia solani AG2-1 and AG3 from potato(Solanum tuberosum). Anastomosis reactions indicated that the complete fusion reaction occurred among the different isolates of AG3. Either the complete fusion reaction or incomplete one occurred among the traditionally defined isolates of AG2-1. Therefore, AG2-1 was grouped into two sub-groups by incomplete fusion reaction. Phylogenetic tree constructed by ITS sequences showed that the AG3 formed only one separated clade, whereas AG2-1 clustered into the two different sub-clades. The horizontal lengths of the branches indicated that AG2-1 has been appearing obvious genetic differentiation because the relative genetic diversity of AG2-1 or the two subgroup AG2-1-1 and AG2-1-2 were higher than that of AG3. Based on ITS sequences, the two sub-clades within AG2-1 were consistent with the two sub-groups based on the incomplete anastomosis reaction. So the incomplete fusion reaction can be used as a referent character for genetic differentiation within an anastomosis group of Rhizoctonia solani.
