Soil microbes mediate the effects of nitrogen supply and co-inoculation on Barley Yellow Dwarf Virus inAvena sativa

Summary Nutrient supply rates to hosts can mediate host–pathogen interactions. In terrestrial systems, nutrient supply to plants is mediated by soil microbes, suggesting a potential indirect effect of soil microbes on plant–pathogen interactions. Soil microbes also may affect plant pathogens by inducing plant defenses. We tested the role of soil microbes, nitrogen supply to plant hosts, and co-inoculation on infection by aphid-vectored RNA viruses, Barley Yellow Dwarf Virus (BYDV-PAV) and Cereal Yellow Dwarf Virus (CYDV-RPV), in a grass host grown in soil microbes collected from a long-term nitrogen enrichment experiment. BYDV-PAV incidence declined with high nitrogen supply, co-inoculation, or presence of soil microbes exposed to long-term low nitrogen enrichment. However, when combined, the negative effects of these treatments were sub-additive: nitrogen and co-inoculation did not reduce BYDV-PAV incidence in plants grown with the soil microbes. While soil microbes impacted leaf chlorophyll, they did not alter biomass or CYDV-RPV incidence. Soil microbes mediated the effects of nitrogen supply and co-inoculation on infection incidence and the effects of infection on host symptoms. Thus, soil microbial communities can indirectly control disease dynamics, altering the effects of nitrogen enrichment on plant–pathogen and pathogen–pathogen interactions in terrestrial systems.