Exotic plants accumulate generalist fungal taxa that are known to cause plant disease
Fungal pathogen richness did not differ between native and exotic plants, with individual plants hosting approximately 8 OTUs (± 0.4 s.e., χ2=0.84, p>0.36) identified as pathogenic on average. Fungal pathogen richness did not change with the proportion of exotic plants planted in the community or differ in home vs. away soil (proportion of exotics: χ2=1.05, p=0.30; soil treatment: χ2=2.67, p=0.10, Table 1). Fungal pathogen relative abundance did not differ between native and exotic plants at the individual plant level (χ2=1.31, p=0.25, Table 1), but pathogen relative abundance in communities increased along the exotic plant gradient (χ2=5.17, p<0.02, Table 1, Fig. 1A). The soil treatment had no effect on pathogen abundance in communities (χ2=0.27, p=0.60) and no interaction with the proportion of exotics (χ2=3.31, p=0.07, Table 1). This gain in pathogen relative abundance along the exotic gradient was not simply due to reductions in other fungi, as total fungal biomass (using phospholipid fatty acid [PLFA] biomarkers) did not change along the exotic plant gradient (χ2=0.16, p=0.69, Table 1).
On average, plants interacted with an increasing proportion of available pathogens along the exotic plant gradient, as indicated by plant-normalized degree (χ2=37.97, p<0.001, Table 1, Fig. 1B). Likewise, more generalist pathogens inhabited the roots of plants in exotic-, compared with native-dominated communities, as indicated by the normalized degree of fungal pathogens (χ2=45.12, p<0.001, Table 1, Fig. 1C).
When growing in 100% native or 100% exotic mesocosms, exotic plants were more generalist than native plants in their interactions with pathogens, as indicated by normalized degree (χ2=21.80, p<0.001, Table 1, Fig. 2A). Further, exotic plants were more central in the network, as indicated by mean closeness centrality values for natives vs. exotics (χ2=36.57, p<0.001, Table 1, Fig. 2B), suggesting that exotic plants had a greater ability to indirectly affect others in the community by spreading pathogens, compared with native plants.