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Background and aims Plant interactions with soil microbial communities are critical for understanding plant health, improving horticultural and agricultural outcomes, and maintaining diverse natural communities. In some cases, disease suppressive soils enhance plant survival in the presence of pathogens. However, species-specific differences and seasonal variation complicate our understanding of the drivers of soil fungal communities and their consequences for plants. Here, we aim to describe soil fungal communities across Rhododendron species and seasons and as well as the test for fungal indicators of species and seasons in the soil. Further, we tested for correlations between fungal community composition and prior experimental quantification of disease suppressive soils. Methods We conducted high throughput sequencing of the fungal communities found in soil collected under 14 Rhododendron species and across 2 seasons (April, October) at two sites in Ohio, USA. We described these soils and used phylogenetic analyses to ask whether fungal community composition correlated with increased plant survival with the addition of whole soil communities from a prior greenhouse experiment. Results We found effects of Rhododendron species and season on fungal communities. Fungal community composition correlated with survival following exposure to whole soil microbial communities, though this result depended on the presence of R. minus. We identified 45 Trichoderma taxa across our soil samples, and some Trichoderma were significantly associated with particular Rhododendron species in indicator species analyses. Conclusion The correlation between plant responses to soil biotic communities and fungal community composition, as well as the presence of potential beneficial taxa such as Trichoderma and mycorrhizal fungi, are consistent with fungal-mediated survival benefits from the pathogen Phytophthora cinnamomi. 

Background and Aims 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 effects of phosphitemediated protections are direct or indirectly mediated through soil biota are unknown. Therefore, exploring belowground effects could contribute to the evaluation of the sustainability of phosphite use and tests hypotheses about direct versus indirect effects in pathogen response. Methods 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. Results 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. Conclusions 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 evidence that phosphite-mediated pathogen protection includes both direct benefits to plants and indirect effects mediated through the soil fungal community.