Host resistance and pathogen aggressiveness vary along an elevation gradient in a wild plant-pathogen interaction

Diversity in host resistance and pathogen virulence is ubiquitous in nature, determines infection outcomes, provides the raw material for antagonistic (co)evolution, and is fundamental to epidemiological dynamics. Yet empirical data on spatiotemporal variation of host and pathogen abundances together with measures of host resistance and pathogen virulence are rare. To address this gap, we surveyed host abundance and pathogen prevalence in four populations of a plant host and its specialist fungal pathogens along an elevation gradient. Using one of the pathogen species, we experimentally tested whether host resistance, pathogen virulence and aggressiveness vary among the study populations. Our measurements of infection success and severity across several hundred plant-pathogen pairs showed significant variation in both host resistance and pathogen aggressiveness among the populations, with resistance decreasing and aggressiveness increasing with elevation. Further, the high-elevation pathogen population produced the most severe infection on their local host population, whereas the low-elevation pathogen population produced the least severe infections on their local host populations. Our results reveal contrasting patterns between pathogen prevalence and key host and pathogen traits among populations along an environmental gradient, highlighting that shifts in pathogen distributions may reshape epidemiological and evolutionary dynamics between hosts and pathogens.