Spatial and temporal dynamics of epidemics of the rust fungus Uromyces valerianae on populations of its host Valeriana salina

1 Analysis of the dynamics of rust disease caused by Uromyces valerianae in 30 discrete populations of Valeriana salina occurring on an archipelago of small islands in the Gulf of Bothnia, central Sweden, showed strong temporal and spatial effects. 2 Over a 13-year period, the population dynamics of the pathogen varied across the metapopulation, with disease incidence (presence/absence), prevalence and severity all showing strong population and year effects, indicative of heterogeneity among years and host populations in the suitability of conditions for the pathogen. While some individual populations were infected for the entire study period, others were infected for only 1 or 2 years. Local pathogen population extinction and recolonization events were relatively common, with annual recolonization rates of previously healthy populations ranging from 0% to 13.3%. 3 The incidence of disease within individual host populations was significantly affected by host population size, the prevalence of disease in the previous year and the proximity of neighbouring populations that were infected in the current year. The prevalence of disease in infected populations only depended on the prevalence of disease in the previous year. There was little to choose between disease prevalence and severity in predictive power. 4 Overall, the dynamical behaviour of this set of pathogen demes best fit that predicted for a metapopulation with considerable asynchrony in epidemiological patterns between different demes, despite evidence of among-population migration.