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

Summary1 Studies combining within‐ and among‐population processes are crucial for understanding ecological and co‐evolutionary dynamics in host–pathogen interactions. We report on work over an 18‐month period involving multiple beach populations of the plant Cakile maritima and its fungal pathogen Alternaria brassicicola along the south‐east coast of Australia. 2 Results from permanent transects replicated on several beaches show that disease incidence and prevalence vary significantly with plant age, class and 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 on survival of infected plants behind the foredunes of protected beaches. Population extinction was more likely on beaches with greater wind exposure and 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 subregions, the severity of the epidemic varied significantly among these areas, suggesting that large‐scale environmental factors may influence the timing and development of the epidemic. 5 Estimates of synchrony in disease dynamics indicated that populations within a subregion were significantly correlated with respect to epidemic development. There was, however, no evidence for spatial synchrony in disease dynamics based on among‐population covariances in disease prevalence and interbeach distances. Populations within a subregion were thus often at quite different phases of the epidemic at any given time.