An ongoing challenge in ecology is understanding how disease epidemics are driven by seasonal shifts in environmental conditions. This issue grows urgent as anthropogenic climate change alters seasonal patterns of environmental conditions, and thus potentially alters disease transmission and seasonal epidemics. Predicting changes in disease transmission requires understanding ecological mechanisms. Seasonal epidemics can be shaped both by environmental conditions and by species interactions. Species interactions are relatively well studied within host individuals, but less so at the level of the host population, the level at which epidemics occur. Here, we present a framework for how seasonal environmental conditions can drive species interactions within host individuals, and how within-host species interactions can scale up to disease epidemics. We present results of investigations conducted using an experimentally tractable system: fungal pathogens infecting the leaves of the widespread and abundant grass species, tall fescue, in the piedmont region of North Carolina.
This system is characterized by seasonal epidemics of multiple diseases, with an anthracnose epidemic starting in spring, a brown patch epidemic in summer, and a crown rust epidemic in the fall. While this seasonal pattern of pathogen community assembly in the host population has been broadly repeated each year, more detailed analyses reveal important variation among years. Specifically, after beginning each spring in a similar state, subsequent assembly of the pathogen community then follows a different trajectory each year, diverging further over summer and then fall. These interannual shifts in pathogen community assembly were correlated with interannual shifts in environmental conditions that are key for fungal reproduction: precipitation and humidity.
Experiments are revealing the nature and potential importance of within-host interactions among these pathogens. Within host individuals that were inoculated in the lab with an anthracnose- and/or brown patch-causing pathogen species, then deployed into the field, pathogen community trajectories diverged from each other, with magnitude of divergence depending on the species composition of the host’s initial inoculation. Lab experiments have documented that within-host pathogen species interactions are subject to priority effects depending on the temporal sequence of inoculation, and have begun to test effects of abiotic environmental conditions. With mathematical models, we are exploring when within-host interactions can scale up to disease epidemics. Together, these results suggest important roles for historical contingency, both within host individuals and host populations. More broadly, they suggest that seasonal disease epidemics can be governed by a combination of abiotic environmental conditions and pathogen species interactions.