Order of arrival and nutrient supply alter outcomes of coinfection with two fungal pathogens

A pathogen arriving on a host typically encounters a diverse community of microbes that can shape priority effects, other within-host interactions, and infection outcomes. In plants, environmental nutrients can drive tradeoffs between host growth and defense and can mediate interactions between coinfecting pathogens. Nutrients may thus alter the outcome of pathogen priority effects for the host, but this possibility has received little experimental investigation. To disentangle the relationship between nutrient availability and coinfection dynamics, we factorially manipulated nutrient availability and order of arrival of two foliar fungal pathogens (Rhizoctonia solani and Colletotrichum cereale) on the host grass tall fescue (Festuca arundinacea) and tracked disease outcomes. Overall, C. cereale infection facilitated infection by R. solani, increasing its infection rate regardless of their order of inoculation. Additionally, simultaneous and C. cereale-first inoculations decreased plant growth and - in plants that did not receive nutrient addition - increased leaf nitrogen concentrations compared to uninoculated plants. Nutrient addition did not influence infection rates, severity of infection, or plant biomass. These findings highlight the importance of understanding the intricate associations between the order of pathogen arrival, host nutrient availability, and host defense to better predict infection outcomes.