Surface waters provide irrigation, influence soil fertility and humidity, and capture run-off that alters their chemistry and microbiology. In stark contrast with knowledge about human and animal pathogens in water, little is known about plant pathogens in the surface waters that flow through a watershed. With sufficient knowledge bases about plant pathogens in surface waters, it would be possible to create applications—similar to those to assess risks of river water for human health—to estimate risks for plant health resulting from irrigation with river water. We have mapped the abundance and diversity of Pseudomonas syringae (Ps) in a 270-km stretch of the 300- km reach of the Durance River in Southern France. Situated in a fruit and vegetable production region, this river drains >14,000 km2 of which 20% is agricultural production. Water collected at 21 sites in fall, winter, spring and summer over a 2-year period resulted in ca. 5,000 putative isolates of Ps whose identification was confirmed by partial sequences of the citrate synthase gene. Ps was detected in nearly all samples from the fall, winter and spring, whereas populations were below the detectable level in ca. 25% of summer samples at sites almost exclusively in the southern half of the watershed. Population densities ranged from 103–105 cells /L with Ps phylogroups 2, 13, 10, 7, and 1 detected most frequently (in descending order of frequency) and harboring novel phylogroups. Current work aims to reveal the most risky lines of Ps for local crops and their origins and trajectories throughout the basin.