Characterization of a new subgroup of Rhizoctonia solani anastomosis group 3 (AG-3 TM) associated with tomato leaf blight

Abstract

Ten Rhizoctonia solani AG-3 isolates obtained from blighted tomato leaves [nine isolates, collected from three regions of Japan (Akita, Okayama, and Fukushima) and the United States] and from soil (one isolate, collected from Australia) were examined for sequence variations in the rDNA-ITS, rDNA-IGS1 regions, and tef-1α gene. Three loci of 16 AG-3 PT (potato type) and eight AG-3 TB (tobacco type) isolates were also sequenced to evaluate genetic relationships with these 10 isolates. In the sequence comparisons of these loci, all 10 AG-3 isolates shared high sequence similarity (99.4–100% for rDNA-ITS, 99.1–100% for rDNA-IGS1, and 98.2–100% for tef-1α) to each other, but a comparatively low similarity (95.4–98.8% for rDNA-ITS, 94.7–98.3% for rDNA-IGS1, and 93.5–97.2% for tef-1α) to AG-3 PT and AG-3 TB isolates. The 10 AG-3 isolates were genetically closer to AG-3 PT than to AG-3 TB on the basis of rDNA-ITS and tef-1α. A phylogenetic tree based on these three loci indicated that the 10 isolates form a new clade separated from AG-3 PT and TB populations. These results indicate that the 10 isolates from tomato and soil represent a new subgroup distinct from AG-3 PT and TB. New subgroup AG-3 TM (tomato type) is proposed. This study is the first report on the phylogenetic analysis of R. solani isolates based on the combined data from the rDNA-ITS, rDNA-IGS1 regions, and tef-1α gene.