Trichoderma reduce disease symptoms on Phytophthora cinnamomi exposed Rhododendron

Abstract Soil microorganisms strongly influence plant performance, yet their effects are often context dependent and can be non-additive when multiple taxa interact. This study tested whether inoculation with the beneficial fungi Trichoderma will enhance Rhododendron’s performance in the presence of the root pathogen Phytophthora cinnamomi, whether co-inoculation of Trichoderma asperellum and Trichoderma harzianum cultures act additively or non-additively, and whether responses differ among Rhododendrons. The experiment was conducted under controlled greenhouse conditions at Squire Valleevue Farm, Case Western Reserve University, Cleveland, Ohio, USA, using three Rhododendron species differing in susceptibility to P. cinnamomi. Seedlings were grown in a factorial design crossing pathogen presence/absence with single- and dual-species Trichoderma inoculations. Performance traits (survival, leaf damage, shoot and root biomass, relative growth rate, root-shoot ratio, and specific root length) were measured, and contrasts tested for non-additive effects. Pathogen inoculation reduced survival and growth, but Trichoderma mitigated these effects, increasing total biomass under pathogen presence and reducing leaf damage. Species-specific responses were evident, with Rhododendron molle showing the strongest growth benefits, while R. maximum and R. schlippenbachii were less responsive. Leaf damage, shoot biomass, total biomass, and growth showed no added benefit from co-inoculation of Trichoderma species while survival, root biomass, and root allocation traits showed additive responses. These results demonstrate that the benefits of Trichoderma are not uniform across hosts and that combining strains can constrain protection. This study contributes to evidence that microbial non-additivity can shape plant-pathogen interactions, emphasizing the need to consider host identity and microbial composition in understanding plant-microbial ecology.