Surface waters provide irrigation but little is known about the presence of plant pathogens in these waters. 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 > 14000 km2 of which 20% is agricultural production. Water collected at 21 sites over 4 seasons in 2016 and 2017 resulted in 5436 putative isolates of Ps whose identification was confirmed by partial sequences of the citrate synthase gene (cts). Ps was detected at 10^3 – 10^5 cells /L in nearly all samples from the fall, winter and spring but was often undetectable at water temperatures > 25°C. Populations were consistently dominated (10% of all Ps) by a cts haplotype that has been observed worldwide and involved in numerous epidemics of various crops.