Biological Control of Canada Thistle in Wetland Prairie Restoration

Project summary:

Canada thistle [Circium arvense (L) Scop.] is a primary noxious weed in Minnesota, which by law, must be controlled on land managed by the Department of Transportation (MnDOT). Infestations of this invasive species are very common along roadsides and in newly created wetland mitigation sites throughout the state. Recent changes in the Wetland Conservation Act require the implementation of five-year vegetation management plans on wetland mitigation sites, and Canada thistle is one of the most frequently cited species that must be controlled. Current control methods consist of the use of herbicides and mowing, which can be effective but are expensive and in many cases damage native species. Herbicide treatments are somewhat effective on Canada thistle infestations along roadside but are often not desirable on wetland mitigation sites because the herbicides that are used to control Canada thistle also eradicate desirable species that MnDOT has planted.

Pseudomonas syringae pv. tagetis (Pst), a phytopathogenic bacterium, was evaluated as a natural biological control agent for Canada thistle. 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.