Plant neighbours determine fungal root pathogen dynamics in grasslands. Plant neighbours and fungal root pathogen dynamics

Plant diversity has shown to control disease outbreaks in natural ecosystems, which are otherwise frequent in agricultural monocultures. However, although diversity often reduces the risk of pathogen transmission within plant assemblages (the so-called ‘dilution effect’), the opposite phenomenon (‘amplification effect’) has also been observed. Here, we aimed to find which factors determine the dilution or amplification of soil-borne fungal pathogens in temperate grassland communities. In a common garden biodiversity experiment comprising monocultures and mixed communities of temperate grassland species, we tracked the build-up of fungal pathogens in roots using Illumina MiSeq amplicon sequencing. We identified three major pathogenic taxa with different host range breadths, showing preferential colonisation of roots of forbs or grasses, or a generalistic occurrence. We found that, for all three pathogens, the identity of plant neighbours is the major determinant for pathogen build-up, over other community characteristics such as taxonomic or functional plant diversity. Roots, including those of presumed non-host species, accumulated pathogens when co-existing with certain host neighbour species, suggesting that non-host species can act as pathogen reservoirs. Our results suggest that the current classification of host versus non-hosts is insufficient to predict the impact of neighbours on pathogen pressure in ecosystems.