Soil water flow is a source of the plant pathogen Pseudomonas syringae in subalpine headwaters

The airborne plant pathogenic bacterium Pseudomonas syringae is ubiquitous in headwaters, snowpack and precipitation where its populations are genetically and phenotypically diverse. Here, we assessed its population dynamics during snowmelt in headwaters of the French Alps. We revealed a continuous and significant transport of P.syringae by these waters in which the population density is correlated with water chemistry. Via in situ observations and laboratory experiments, we validated that P.syringae is effectively transported with the snow melt and rain water infiltrating through the soil of subalpine grasslands, leading to the same range of concentrations as measured in headwaters (10(2) -10(5) CFU l(-1) ). A population structure analysis confirmed the relatedness between populations in percolated water and those above the ground (i.e. rain, leaf litter and snowpack). However, the transport study in porous media suggested that water percolation could have different efficiencies for different strains of P.syringae. Finally, leaching of soil cores incubated for up to 4 months at 8°C showed that indigenous populations of P.syringae were able to survive in subalpine soil under cold temperature. This study brings to light the underestimated role of hydrological processes involved in the long distance dissemination of P.syringae.