OOS 22-5 - Plant-pathogen networks in complex plant communities

Fungal pathogens are a ubiquitous component of the microbiomes of plants, and nearly all leaves are colonized by a diverse assemblage of fungal symbionts. Fungi can play a critical role in shaping plant communities, and disease interactions are themselves shaped by plant community structure. The key factor that mediates both how pathogens affect and are affected by community structure is pathogen host range. Despite their ubiquity, we know little about the diversity, composition, spatial distribution, and structure of plant mycobiomes in complex plant communities.

We characterized foliar fungal communities of all plant species across three distinct, geographically adjacent plant communities in central coastal California: a mixed-evergreen coastal forest, a grassland, and an organic farm. We used PacBio fungal metabarcode (ITS) sequencing to identify 1078 fungal taxa (OTUs) from 308 plant species; we identified 367 of these fungi as likely plant pathogens. Plant species showed more habitat specificity than did fungal taxa, with a third of the fungi found across all three habitats, providing potential epidemiological links across hosts in different habitats.

Bipartite network analysis of plant-pathogen interactions revealed similar network structures in the three plant communities despite large differences among them in plant species number. In all networks, fungi had fewer plant partners than plants had fungal partners; that asymmetry was strongest in the forest network. Modularity (Q) and nestedness (WINE) were nearly identical in the farm and meadow networks, with the forest network showing slightly greater modularity and less nestedness.

Host specialization varied among pathogen taxa; a small number of pathogens were broad host generalists (6 were found on >100 host species), while around a third were found on a single host species (this includes both host-specialists and rare taxa). Most fungal pathogens were found on a limited subset of several host species. The hosts in this subset were more closely related to each other than expected by chance, showing a phylogenetic signal in host range. We also found that fungal pathogen communities were structured by phylogenetic relatedness of their hosts: more distantly related host pairs showed greater beta diversity between their pathogen communities.