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The soil-borne pathogen Phytophthora cinnamomi causes a deadly plant disease. Phosphite is widely used as an effective treatment to protect plants from Phytophthora cinnamomi. Phosphite as a common fungicide might influence the composition of soil fungal communities. However, whether the belowground mechanisms of phosphite-mediated protections are direct or indirectly mediated through soil biota are unknown. Therefore, exploring belowground mechanisms could contribute to the evaluation of the sustainability of phosphite use and tests hypotheses about direct versus indirect mechanisms in pathogen response. Our greenhouse pot experiment on Rhododendron species had either an after-pathogen or a before-pathogen use of phosphite to compare and evaluate plant and soil fungal responses to phosphite and the presence of an oomycete pathogen phytophthora cinnamomi. The factorial experiment also included with and without pathogen and soil biota treatments, for a test of interactive effects. High throughput sequencing analyzed the soil fungal communities, and we measured the diversity, evenness and richness of soil fungi. Phosphite effectively increased survival of Rhododendron species. It altered the composition of soil fungal communities, and the timing of using phosphite determined the way in which the fungal communities changed. Trichoderma taxa also responded to soil phosphite and Phytophthora cinnamomi. The benefits of antagonistic fungi such as Trichoderma are context-dependent, suggesting protection against pathogens depends on the timing of phosphite application. This study provides the first evidence that phosphite-mediated pathogen protection includes both direct benefits to plants and indirect effects mediated through the soil microbial community. 

Phytophthora cinnamomi, also known as root rot, is an oomycete that is particularly damaging to the plant world. Infecting the root of plants, Phytophthora cinnamomi inhibits water uptake in plants, leading to increased rates of plant mortality. Rhododendron species are not impervious to the infestation of root rot, so, as a popular plant among gardeners, decreasing susceptibility to and identification of Phytophthora cinnamomi is beneficial to plant longevity. In this study, phosphite treatment and soil microbial communities are used to potentially prevent root rot from infecting the eight tested Rhododendron species. It is hypothesized that the phosphite treatment will directly attack the oomycete, as well as improve the defense system of the plants themselves. Rhododendrons treated with the live soil microbiota are predicted to be less susceptible to root rot due to increased resilience to disease from the presence of soil biota, potentially including mutualists such as mycorrhizal fungi. Since Phytophthora cinnamomi primarily affects the roots of plants, it is difficult to detect without uprooting those suspected of being diseased, which causes unnecessary and potentially fatal stress on the plant. This is why we used color analysis software to find a link between root rot infection and leaf color. Since Phytophthora cinnamomi decreases water uptake, plants that are infected will begin to wilt, and their leaves will begin to change color. Discovering a significant link between leaf color in Rhododendron species and Phytophthora cinnamomi infection has given a new diagnostic measure that will cause significantly less stress to the plant and will lead to better plant longevity outcomes. Our data also suggests both preventative measures and treatment options for certain Rhododendron species infected with P. cinnamomi, through the use of a combination of phosphite treatments and live soil biota presence. Our results differ by species, which we further analyzed through the utilization of specific leaf area measurements. Using this data, we were able to link our results to current theory, such as growth-defense tradeoffs and implications of tolerance versus resistance.