A non-chemical strategy for getting rid of Canada thistle

Researchers at the University of Wisconsin investigate a bacterial 'herbicide' to control thistle in pastures

By Katherine Friedrich

February 7, 2005: In an effort to discourage the growth of Canada thistle, one of Wisconsin's legally “noxious” weeds, scientists at the University of Wisconsin-Madison recently explored biological control techniques for non-cultivated fields. According to Research Brief #65 from the Center for Integrated Agricultural Systems (CIAS), “more work [is] needed” before their method can be successful.

Building on the work of John Gronwald of the University of Minnesota/USDA, agronomy professor Jerry Doll and graduate assistant Ryan Tichich harvested thistle foliage infected with the naturally occurring bacteria PST (Pseudomonas syringae pv. tagetis). The scientists blended and distilled a liquid to transfer the bacteria and applied it to healthy thistles. Although the thistles showed signs of infection, the toxic effects must be strengthened if PST is to become an effective biological control agent.

Canada thistle, a tenacious invasive plant which originated in Europe, plagues livestock producers because it is tough and non-edible. According to the CIAS, Canada thistle propagates through an extensive root and rhizome system that can span 15 feet and extend 6-15 feet below ground. The thistles also produce over 5,000 seeds each, which are dispersed by the wind and remain vital for years.

The state of Wisconsin has placed Canada thistles on its “least wanted” list. In his article, “‘Noxious’ Weeds in Wisconsin,” Doll explained that state law requires landowners to “destroy all noxious weeds” on their property. However, Doll noted that the law is not enforced consistently.

Doll and Tichich described the way that PST affects wild and cultivated plants in a 2003 paper presented at the Fertilizer, AgLime and Pest Management Conference. In addition to thistles, they wrote, PST infects marigolds, annual sunflowers, common groundsel, horseweed, woolyleaf bursage and several species of ragweed. The bacteria cause the host plant’s foliage to lose its chlorophyll and turn yellowish white, weakening the plant’s root reserves. In a well-maintained pasture, infected thistles lose their competitive edge.

If PST could slow the spread of Canada thistle, Doll and Tichich asked, could this effect be increased by applying PST in higher concentrations and volumes? Would multiple applications increase the effect? What was the best time of year and moisture level for PST growth?

Unfortunately, the bacteria did not suppress thistle growth during the study period. Doll and Tichich concluded that the infection rate would need to be increased from 25-40 percent to 70-85 percent. Increasing the concentration and spray volume of PST didn’t increase the incidence or severity of disease observed on the plants. Introducing the bacteria during mid-July was more effective than during June and August, because of heavier rainfall. (PST enters leaves more effectively during rainy periods.) Applying PST to the thistles multiple times led to increased incidence of infection, but this method increases costs for farmers.

Doll explained the obstacles that the researchers faced. “We need to find methods that increase the disease incidence of a single application, either by producing more toxin per cell or by increasing the number of PST cells on the leaf,” he recommended. In the future, he believes, scientists should focus on understanding how leaf surfaces absorb the bacteria.

In 2004, after several years of abundant rainfall, Doll found that some of the thistle plots that he studied had almost disappeared. Within the next few years, other scientists may develop a reliable way of using PST to deter the hardy Canada thistle.

Katherine Friedrich is a graduate student at the University of Wisconsin-Madison in Life Sciences Communication (formerly known as Agricultural Journalism).