OOS 22-1 - Why are seedlings more susceptible than mature plants? Quantifying the costs of resistance across host age in the wild plant, Silene latifolia.

Seedlings are typically more susceptible to disease than older plants, a phenomena referred to as ‘mature plant resistance’ in agricultural settings. This juvenile susceptibility carries important ecological consequences for pathogen transmission, in some cases allowing the persistence of disease within the population. But why are juvenile plants more susceptible? From an evolutionary perspective high susceptibility prior to reproduction appears maladaptive. Here we tested the hypothesis is that disease resistance is more costly at juvenile stages than at adult stages. The herbaceous plant Silene latifolia is commonly infected with the sterilizing fungal pathogen Microbotryum-lychnidis-dioicae, which causes anther-smut disease. Previous work has shown that seedling are significantly more susceptible to infection than adult flowering plants. We measured the resistance of 45 full-sib families of S. latifolia collected from a natural metapopulation in Southern Virginia at four different ages (ranging 3 weeks to 4 months) in a greenhouse inoculation experiment. Concurrently, we planted 16 individuals from each family in an anther-smut free common garden in Beltsville Maryland and measured their survival and flowering for two years. We used integrated ASTER models for life-history to estimate perennial fitness in the absence of disease and used linear regression to examine the relationship between greenhouse-measured resistance at each age and integrated fitness in the field. We found a significant negative relationship (indicating that resistance is costly) at all ages except for the second oldest group. However, the slopes did not differ significantly across ages, refuting the hypothesis that seeding resistance is more costly than adult resistance. However, when our results are put in the context of recent theoretical models of age-specific resistance, we show that the modest costs of juvenile and adult resistance we observed can help explain the patterns of age-specificity found in nature.