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 the 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.