The recently discovered ubiquity of the plant pathogen Pseudomonas syringae in headwaters and alpine ecosystems worldwide elicits new questions about the ecology of this bacterium and subsequent consequences for disease epidemiology. Because of the major contribution of snow to river runoff during crop growth, we evaluated the population dynamics of P. syringae in snowpack and the underlying leaf litter during two years in the Southern French Alps. High population densities of P. syringae were found on alpine grasses, and leaf litter was identified as the main source of populations of P. syringae in snow-pack, contributing more than the populations arriving with the snowfall. The insulating properties of snow foster survival of P. syringae throughout the winter in the 10 cm layer of snow closest to the soil. Litter and snowpack harboured populations of P. syringae that were very diverse in terms of phenotypes and genotypes. Neither substrate nor sampling site had a marked effect on the structure of P. syringae populations , and snow and litter had genotypes in common with other non-agricultural habitats and with crops. These results contribute to the mounting evidence that a highly diverse P. syringae metapopulation is disseminated throughout drainage basins between cultivated and non-cultivated zones.