Agathis australis Root‐Associated Fungi Inhibit Phytophthora Pathogens In Vitro and Demonstrate Tolerance to Phosphite

ABSTRACT Phytophthora pathogens are responsible for many plant diseases across agricultural and natural systems. While phosphite‐based treatments have proved effective at mitigating the damage to plants infected by Phytophthora , the impacts of phosphite treatments on broader plant‐associated microbial communities remain poorly understood. In particular, little is known about how phosphite applications in natural forest ecosystems may affect beneficial plant‐associated microorganisms, such as fungal endophytes, that can confer benefits to plant health and immunity. In this study, we investigated how phosphite affects the growth and pathogen inhibition of fungal taxa isolated from kauri ( Agathis australis ) roots in vitro. We identified eight fungal isolates from four genera that significantly inhibited Phytophthora pathogens in vitro, which we then tested in individual cultures and dual cultures against Phytophthora species with varying phosphite quantities. We provide novel evidence for how phosphite affects non‐target plant‐associated microorganisms. We found that all fungal isolates tested exhibited significantly greater tolerance to phosphite than the Phytophthora taxa tested, except at the highest phosphite concentrations. Further, we found that the competitive abilities of these fungi were differentially affected when phosphite was added: Trichoderma strains remained strong Phytophthora inhibitors in low phosphite conditions, whereas Talaromyces and Ilyonectria strains were significantly less effective at inhibiting Phytophthora growth when phosphite was present. These results indicate that phosphite applications could affect the community dynamics of plant‐associated microbial organisms, and that a better understanding of how plant health‐promoting microbial organisms function when phosphite is applied will be informative for effective, sustainable management of Phytophthora ‐driven plant diseases.