"The goal of the proposed research is a comprehensive understanding of how evolutionary potential of pathogen populations interacts with epidemiological dynamics in natural populations. The empirical work will be conducted on the specialist fungal pathogen Podosphaera plantaginis and its host plant Plantago lanceolata in a large network of host populations. I will address the key theories of pathogen evolution, involving life-history trade-offs, competition for resources under multiple infection, and sexual reproduction. This project takes advantage of the exceptional research opportunities offered by the focal study species to test models that have not been validated with respect to realized population dynamics and the persistence of pathogen populations.I have studied the coevolutionary dynamics between P. plantaginis and P. lanceolata for several years. Unique epidemiological data have been collected annually on the occurrence of the pathogen in a network of 4000 host populations since 2001. Recently, I have generated an EST library for the pathogen that allows and facilitates genetic studies. In the planned research, I will combine laboratory experiments with large-scale population surveys, genetic studies and mathematical modelling to achieve the objectives of this proposal.The proposed research has potential for groundbreaking results on pathogen evolution and epidemiology through: i) Simultaneous study of multiple forces driving pathogen evolution and their importance in natural populations, with direct connections to epidemiology. ii) Development of new methodology for the study of obligate parasites like P. plantaginis. iii) Construction of a stochastic, spatially explicit epidemiological model predicting pathogen occurrence from one season to the next with applicability to a wide range of pathogens. iv) Identifying critical life-history stages and mechanisms for virulence evolution yield much needed insights and tools into the battle against disease."