Global-Scale Patterns of Host Transitions Suggest Contrasting Levels of Specialization Within Rhizoctonia solani

The ability of plant pathogens to infect multiple host species has substantial adverse effects on food security and ecosystem health. Rhizoctonia solani is a globally distributed, soil-borne fungal pathogen capable of infecting a wide variety of economically important crops. Its broad distribution across geographic regions and host taxa is accompanied by high genetic diversity. Historically, the R. solani species complex has been classified into 14 anastomosis groups (AGs) based primarily on hyphal anastomosis compatibility. Despite being widely perceived as a generalist, R. solani's host range has not been studied in a comprehensive cross-AG framework to evaluate global patterns of pathogen diversity and host associations. We analyzed a global dataset of 4119 R. solani rDNA-ITS sequences collected over the past 70 years, including newly sampled taxa from Nebraska, and used a smaller concatenated ITS and TEF-1α dataset comprising 162 R. solani sequences to assess whether broad phylogenetic patterns were consistent across markers. Phylogenetic reconstruction suggests that most AGs formed well-supported clades that largely corresponded to reported AG designations. Host family-level transition inference suggests that generalist AGs may experience more transient host associations, while specialist lineages show stronger host-associated genetic differentiation that contributes to the observed genetic structure. Our ancestral host-state reconstruction analysis further suggests that the AG-3 lineage likely originated from an ancestor with a broader host range and subsequently became predominantly associated with Solanaceae, with no inferred reversal in the sampled dataset. These findings suggest that host association and host-transition patterns are important features of the generalism-specialization continuum and contribute to population structure in a globally distributed fungal pathogen.