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
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
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).