Short-term epidemic dynamics in the Cakile maritima-Alternaria brassicicola host-pathogen association

1 Studies combining within- and among-population processes are crucial for understanding ecological and co-evolutionary dynamics in host-pathogen inte ractions. We report on work over an 18-month period involving multiple beac h populations of the plant Cakile maritima and its fungal pathogen Alternar ia brassicicola along the southeast coast of Australia. 2 Results from permanent transects replicated on several beaches show that disease incidence and prevalence vary significantly with plant age, class a nd distance from the sea, as well as time during the season. Plant density is also positively related to disease levels. 3 Results from three subregions indicate that disease persistence depends o n survival of infected plants behind the foredunes of protected beaches. Po pulation extinction was more likely on beaches with greater wind exposure a nd sea access, with the latter also related to colonization consistent with the dispersal of Cakile seeds via ocean currents, 4 Although disease dynamics during the epidemic were similar across subregi ons, the severity of the epidemic varied significantly among these areas, s uggesting that large-scale environmental factors may influence the timing a nd development of the epidemic. 5 Estimates of synchrony in disease dynamics indicated that populations wit hin a subregion were significantly correlated with respect to epidemic deve lopment. There was, however, no evidence for spatial synchrony in disease d ynamics based on among-population covariances in disease prevalence and int erbeach distances. Populations within a subregion were thus often at quite different phases of the epidemic at any given time.