Host-specific soil oomycete communities are spatially repeatable across natural grasslands.

Soil oomycetes contain some of the most devastating plant pathogens, and are widespread within the soil microbiome, yet how individual plant species structure oomycete communities in natural soils, and whether any such structuring is consistent across space, remains poorly resolved. Because host-specific pathogen accumulation is a central prediction of plant-soil feedback theory but is rarely tested outside greenhouse conditions, we focused on whether the host-specific fraction of these communities is preferentially pathogenic. Using oomycete-specific ITS amplicons, we sequenced rhizosphere-associated and paired bulk soils from four grassland species across 25 sites on the Canadian Prairies spanning more than 200,000 km², totalling 1800 samples. Each host supported a distinctive core of taxa, with two species more likely to host pathogenic taxa unique to their rhizosphere. This enrichment was decoupled from richness of oomycete taxa in the rhizosphere, indicating selective accumulation rather than a by-product of resource-rich environments. Community composition also differed among hosts after accounting for site level edaphic variation. Host plant identity therefore leaves a detectable, spatially repeatable imprint on soil oomycete communities, one expressed most strongly through the selective accumulation of pathogenic taxa.