Infection Patterns of and Effect on Host Survival and Reproduction in a Wild Population of

spp. are biotrophic parasites that cause white rust in Brassicaceae species, with significant crop losses. The generalist and the specialist infect , and the pathosystem Albugo-Arabidopsis is a model for research in molecular genetics of plant-pathogen interactions. The occurrence of infection by Albugo in wild populations of Arabidopsis and data on the genetics of resistance-susceptibility are compatible with a hypothesis of host-pathogen coevolution. However, the negative impact of Albugo infection on Arabidopsis-a requirement for coevolution-has not been shown under field conditions. To address this question, we analysed the demography and the dynamics of Albugo infection in a wild Arabidopsis population in central Spain and measured plant fitness-related traits. Infection increased mortality by 50%, although lifespan, the fraction of plants that reproduced and seed production were reduced only in plants from the spring cohorts. Despite these negative effects, simulations of demographic dynamics showed that the population growth rate remained unaffected even at unrealistically high infection incidences. The lack of negative effects in autumn-winter cohorts suggests compensatory mechanisms in longer-lived plants. Results support the hypothesis of Albugo-Arabidopsis coevolution.