Trichoderma reduces the relative abundance of pathogen-associated fungi while maintaining fungal community stability in Rhododendron soils

Soil fungal communities are shaped by host-mediated and environmental filters, yet it remains unclear whether microbial inoculants can override these processes. Trichoderma is widely used as a biocontrol agent, but its effects on fungal community assembly and functional guild structure in woody plant systems are not well understood. We tested whether Trichoderma alters soil fungal communities or acts within host-structured systems to influence functional groups. A factorial greenhouse experiment was conducted using three Rhododendron species, four Trichoderma treatments (control, T. asperellum, T. harzianum, and a combined treatment), and four soil treatments (three live conspecific soils and one sterilized soil). Plant performance traits were measured, and soil fungal communities were characterized using internal transcribed spacer 2 high-throughput sequencing. Community composition, alpha diversity, and functional guild abundances were analysed using multivariate and linear modelling approaches. Fungal community composition was structured primarily by host species identity, with soil origin exerting a secondary influence, while Trichoderma inoculation had no detectable effect on community composition or diversity. Despite this, Trichoderma increased plant relative growth rate and reduced the relative abundance of pathogen-associated fungi in a host-dependent manner, with the strongest reduction observed in R. yedoense, while saprotrophic and ectomycorrhizal fungi remained unchanged across treatments. These findings demonstrate that Trichoderma does not restructure fungal communities in host-structured soils but instead acts as a functionally selective agent, reducing pathogen-associated fungi under specific ecological contexts. The effectiveness of microbial inoculants therefore depends on host identity and baseline abundance of pathogen-associated fungi, emphasizing the importance of ecological context in shaping plant-microbe interactions.