Comparison of rDNA-ITS Sequences between Potato and Tobacco Strains in Rhizoctonia solani AG-3

Genetic diversity among 51 isolates of Rhizoctonia solani AG-3, representing potato and tobacco populations, was inferred from the sequences of the internal transcribed spacer (ITS) and 5.8S ribosomal RNA (rRNA) gene. The 5.8S rDNA sequence was completely conserved not only in AG-3, but across all the AG isolates examined, whereas the rDNA-ITS sequence was found to be variable among the isolates. The nucleotide sequence similarity in the ITS 1 region was high (96-100%) for isolates within each of the two populations, but was 91-92% for isolates from different populations. The AG-3 isolates had 56 to 91% sequence similarities in the ITS 1 region with R. solani isolates of the other AGs. Phylogenetic analysis based on the ITS-5.8S rDNA sequence data indicated that the different populations in AG-3 are distantly related to each other. Genetic divergence between the two populations was also supported by the results of DNA-DNA hybridization studies. This study suggests that AG-3 consists of two genetically isolated groups corresponding to separate subgroups: AG-3 PT (potato type) and AG-3 TB (tobacco type). Specific primer sets for the detection of the two AG-3 subgroups were developed from the aligned rDNA-ITS sequences.