Pseudomonas syringae pv. tagetis (Pst), a phytopathogenic bacterium, was evaluated as natural biological control agent for Canada thistle [Cirsium arvense (L.) Scop.]. Canada thistle patches exhibiting symptoms of Pst infection commonly occur along roadsides in association with perennial grasses and a grass litter layer. Field experiments were conducted to determine if grass and litter provide an environment that supports Pst infection of Canada thistle or if grass, litter, and soil collected from infected Canada thistle patches act as inoculum sources for Pst infection of Canada thistle. This experiment provides evidence that grass and litter are important components of the landscape that support the natural Pst infection of Canada thistle, and perennial grass competition has potential to manage Canada thistle in roadside rights-of-way and wetland restoration sites. A previously published Pst specific primer set was determined to require high Pst populations for detection.