The phylogenetic structure of plant communities drives the belowground transmission of fungal pathogens

Biodiversity is known to influence disease risk, yet the pathways of pathogen transmission within plant communities remain poorly understood, especially belowground. In particular, how soil-borne pathogens move from resident vegetation and soil to colonize new hosts is unresolved. We traced belowground pathogen transmission using phytometer seedlings of two plant species planted in a long-term grassland biodiversity experiment. After 3 months, we characterized the fungal communities of phytometer roots, resident plant roots, and soil using high-throughput sequencing and the FungalTraits database to identify associations between pathogen taxonomy and plant families. Next, we related pathogen abundance to phytometer growth. The phylogenetic similarity of phytometers with resident plant species strongly predicted the relative abundance of pathogens that were considered family-specific, but not of pathogens without a clear host preference. However, neither pathogen abundance in phytometers nor resident plant biomass affected phytometer growth, which was best explained by the resident communities' species richness. Combining sequencing of fungal communities with in situ field manipulations enabled us to track the associations between multiple soil-borne pathogens and plant hosts within the full complexity of plant-soil systems. While pathogen dynamics were readily detectable, their consequences for plant performance may only become apparent over longer ecological timescales.