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Large‐scale spatial dynamics of a specialist plant pathogen in a fragmented landscape

1 Spatial population structure may have profound consequences for the stability and evolution of host–pathogen interactions. We used a large‐scale 4‐year dataset to study the spatial dynamics of Podosphaera plantaginis, an obligate pathogen of Plantago lanceolata in the Åland Islands in south‐west Finland, where the host populations are mostly very small and have a scattered spatial distribution. 2 The fraction of host populations infected was low but highly variable among years, ranging from 1% to 4.7%, with frequent local extinction and colonization events. Demographic stochasticity associated with regular seasonal declines and severe drought was responsible for the high extinction rate. Frequent colonizations of host populations close to the pathogen source populations suggest that there is substantial gene flow in the pathogen at spatial scales up to 1 km. 3 Occurrence of the pathogen was strongly and positively correlated with the logarithm of host population size, the proximity of a road to the host population (presumably facilitating dispersal) and the proximity of the coastline (presumably due to microclimatic conditions favouring pathogen development). 4 The high turnover rate of the pathogen, distance‐dependent dispersal and regional variation in host–pathogen encounter rates have consequences for the coevolutionary dynamics between Plantago lanceolata and Podosphaera plantaginis.