Disease resistance is more costly at younger ages: An explanation for the maintenance of juvenile susceptibility

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 juvenile resistance by quantifying the genetic correlation between host fitness and age-specific innate resistance to a fungal pathogen in a wild plant. We found significant fitness costs associated with disease resistance at juvenile but not at adult host stages, and show that the magnitude of these juvenile resistance costs are sufficient to prevent the spread of juvenile resistance in models. Our results provide the first direct evidence that costs of resistance decrease with host age, and provide an explanation for the maintenance of epidemiologically important juvenile susceptibility in natural populations.

Methods
Two data sets are included:

Results from greenhouse inoculation studies on 45-full S. latifolia plants at 4 ages (9, 19, 78, and 104 days)
Measurements of survival, flowering, and fruiting over 2 years for 721 plants from the same full sib families planted in a common garden in Beltsville, MD

We include both the raw data, and a combined data set with both the resitance measures and survival and flowering field data.  

Scripts for the analsyis are also included. These are descripted in the manuscript.