Amplification of disease by nutrient addition: Testing mechanisms from individual to community levels

Nutrient supply can amplify disease epidemics through mechanisms from individual to community levels. Within host individuals, nutrient addition can drive pathogen replication or growth. Across a host population, nutrient addition can drive disease transmission by increasing host growth and abundance relative to defense. Furthermore, such effects may be influenced by pathogen species interactions. Understanding how nutrients impact disease epidemics requires a framework that integrates these mechanisms across biological levels. To build such a framework, we conducted a field experiment in an old field on tall fescue, Lolium arundinaceum, and used structural equation models to integrate multiple hypothesized mechanisms. Nutrient addition (NPK fertilizer) increased brown patch disease but was best modeled as a direct path and not mediated by host abundance. To expand our framework, we also re-analyzed a previous experiment. That experiment reproduced the direct path from nutrients to disease, and added an indirect path mediated by host population abundance. Nutrient addition also increased foliar nitrogen, consistent with individual-level mechanism, but this did not increase disease. Brown patch decreased with burden of another disease, anthracnose, independently of nutrients. These results partially support both individual- and population-level hypotheses, emphasizing the importance of considering multiple biological levels underlying impacts of abiotic change.