Interactions among symbionts operate across scales to influence parasite epidemics

Parasite epidemics may be influenced by interactions among symbionts, which can depend on past events at multiple spatial scales. Within host individuals, interactions can depend on the sequence in which symbionts infect a host, generating priority effects. Across host individuals, interactions can depend on parasite phenology. To test the roles of parasite interactions and phenology in epidemics, we embedded multiple cohorts of sentinel plants, grown from seeds with and without a vertically transmitted symbiont, into a wild host population, and tracked foliar infections caused by three common fungal parasites. Within hosts, parasite growth was influenced by coinfections, but coinfections were often prevented by priority effects among symbionts. Across hosts, parasite phenology altered host susceptibility to secondary infections, symbiont interactions and ultimately the magnitude of parasite epidemics. Together, these results indicate that parasite phenology can influence parasite epidemics by altering the sequence of infection and interactions among symbionts within host individuals.