The enemy of my enemy is my friend: Trichoderma fungi reduce root rot disease symptoms caused by Phytophthora cinnamomi in Rhododendron

Abstract Enemies of plant pathogens could provide plants with substantial indirect benefits. Soil microorganisms strongly influence plant performance, yet their effects are often context dependent and can be non-additive when multiple taxa interact, complicating biocontrol strategies. This study tested whether inoculation with beneficial fungi Trichoderma enhances Rhododendron performance in the presence of the root pathogen Phytophthora cinnamomi , whether co-inoculation of Trichoderma asperellum and T. harzianum acts additively or non-additively, and whether responses differ among Rhododendrons. We conducted a factorial greenhouse experiment at Squire Valleevue Farm, Case Western Reserve University, Cleveland, Ohio, USA, using three Rhododendron species differing in susceptibility to P. cinnamomi , crossing pathogen presence/absence with single- and dual-species Trichoderma inoculations. Performance traits (survival, leaf damage, 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 and reducing leaf damage in the presence of the pathogen. Species-specific responses were evident: R. molle showed the strongest growth benefits, while R. maximum and R. schlippenbachii were less responsive. Leaf damage, biomass, and growth showed no added benefit from co-inoculation, while survival and root allocation traits showed additive responses. These results show that Trichoderma benefits are not uniform across hosts and that combining strains can reduce protection. This less-than-additive response suggests competition among Trichoderma , indicating that co-inoculation may be counter-indicated. This study contributes to evidence that microbial non-additivity can shape plant-pathogen interactions, highlighting the need to consider host identity and microbial composition.