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
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
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
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).