Comparison of rDNA-ITS Sequences, Vegetative Compatibility, Biological Characteristics, and Thifluzamide Sensitivity of Rhizoctonia solani AG-3 PT, TM, and TB Subgroup Isolates Collected from Potato and Tobacco in China

Potato black scurf and tobacco target spot are significant agricultural diseases primarily caused by Rhizoctonia solani AG-3, which is further classified into PT (potato type), TM (tomato type), and TB (tobacco type) subgroups, with the naming of these subgroups originally indicating their respective infection hosts potato, tomato, and tobacco. This study determined the subgroup classification of 129 R. solani AG-3 isolates recovered from tobacco leaves exhibiting target spot disease in major tobacco-cultivating regions of China, as well as 80 R. solani AG-3 isolates obtained from potato stem cankers and tuber sclerotia in Liaoning, Jilin, and Heilongjiang provinces, and Inner Mongolia autonomous region of China. Sequence analysis of the internal transcribed spacer (ITS) regions revealed that all potato isolates belonged to the R. solani AG-3 PT subgroup, whereas the tobacco isolates were classified into TM and TB subgroups. Specifically, 51 strains from the northeast (Liaoning, Jilin, and Heilongjiang provinces) belonged to the TM subgroup, while the others belonged to the TB subgroup. Phylogenetic analysis indicated a closer relationship between AG-3 TM and AG-3 PT compared to AG-3 TB. Population genetic analysis highlighted distinct genetic variations among the subgroups, with the AG-3 TB subgroup isolates exhibiting fewer ITS1 variable sites and the AG-3 PT subgroup displaying a higher number of ITS2 variable sites. Furthermore, comparisons were made regarding macroscopic vegetative interactions, biological characteristics, pathogenicity to potato and tobacco, and susceptibility to thifluzamide among the three subgroups. The results indicated no macroscopic somatic interactions among the subgroups, with the highest frequency of somatic incompatibility observed within AG-3 PT, followed by AG-3 TM, and AG-3 TB. Additionally, the three subgroups differed in colony color, sclerotium morphology, production time, and distribution location on potato-dextrose-agar (PDA) medium, with AG-3 TM being the least likely to produce sclerotia. Regarding temperature, optimal growth conditions varied among the subgroups. Regarding pathogenicity, the AG-3 PT subgroup strains were more pathogenic on tobacco and potato stem bases compared to the AG-3 TB subgroup, whereas the AG-3 TB and TM subgroup strains were more pathogenic on leaves. Notably, AG-3 TM exhibited remarkable virulence towards both the stem base and leaves. The AG-3 PT subgroup strains exhibited the highest susceptibility to thifluzamide, with a susceptibility baseline of 0.037±0.013 mg·L -1 , followed by the AG-3 TM subgroup strains with a baseline of 0.111±0.034 mg·L -1 , and the AG-3 TB subgroup strains with a baseline of 0.137±0.035 mg·L -1 . This study contributes to a deeper understanding of the biology and etiology of the three subgroups of R. solani AG-3, providing a valuable theoretical basis for the scientific control and management of tobacco target spot and potato black scurf diseases in China.