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.
Folorunso et al. (Tue,) studied this question.