Disease resistance is more costly at younger ages: An explanation for the maintenance of juvenile susceptibility in a wild plant.

High juvenile susceptibility drives infectious disease epidemics across kingdoms, yet the evolutionary mechanisms that maintain this susceptibility are unclear. We tested the hypothesis that juvenile susceptibility is maintained by high costs of resistance by quantifying the genetic correlation between host fitness and age-specific innate resistance to a fungal pathogen in a wild plant. We separately measured the resistance of 45 genetic families of the wild plant, Silene latifolia, to its endemic fungal pathogen, Microbotryum lychnidis-dioicae, at four ages in a controlled inoculation experiment. We then grew these same families in a field common garden and tracked survival and fecundity over a 2-y period and quantified the correlation between age-specific resistance and fitness in the field. We found significant fitness costs associated with disease resistance at juvenile but not at adult host stages. We then used an age-structured compartmental model to show that the magnitude of these costs is sufficient to prevent the evolution of higher juvenile resistance in models, allowing the disease to persist. Taken together, our results show that costs of resistance vary across host lifespan, providing an evolutionary explanation for the maintenance of juvenile susceptibility.