Summit County uses fungus to target invasive Canadian thistle |

Summit County uses fungus to target invasive Canadian thistle

Colorado Department of Agriculture biocontrol specialist Joel Price spreads rust fungus spores on a Canada thistle test plot at the Paisade Insectary in Palisade in October 2014. He said the spores successfullly infected the noxious thistles despite thinly spread dosages of the fungus.
Courtesy Colorado Department of Agriculture |

The silent Summit County invaders are gaining even more territory.

Noxious weeds are non-native plants that out-compete native species, form monocultures and cause local ecosystems to lose their stability and biodiversity.

They negatively impact agriculture, wildlife and the outdoor recreation-based economy; and, according to the county’s 2015 Weed Management Plan, they increase in population by 15 percent in a typical year if no measures are taken to stop or slow their spread.

County Commissioner Karn Stiegelmeier said that growth could be up to 50 percent in years with this much rain, making addressing the weeds on the thousands of public and private acres in Summit already infested even more important.

This year, a new warrior — rust fungus — will enter the fray.


In late June, two biocontrol specialists with the Colorado Department of Agriculture visited Summit, and, working with county weed manager Ben Pleimann, they chose a site full of Canada thistle — one of the state’s top three noxious weeds.

“No matter where you’re standing in the state, you can probably throw a rock and hit a Canada thistle plant,” said Dan Bean, CDA director of biological pest control.

Canada thistle is native to Eurasia and was brought to North America and other parts of the world hundreds of years ago. The plant spreads with seeds and creeping, horizontal roots.

“Canada thistle is up there on the list of terrible, terrible noxious weeds that every property owner is required by law to manage or destroy,” Stiegelmeier said.

Unlike other noxious thistles, which have a lone flower at the end of a tall stem, Canada thistle has small clusters of three to five flowers, which are usually pink or purple. Infestations are found in lawns, gardens, riparian areas, cultivated fields, pastures, rangeland, forests and roadsides.

Typical treatment of the plant has been herbicides, which he said are labor-intensive and generally ineffective.

Researchers have found a control method proven to be successful elsewhere around the world and are testing it for the first time at more than 100 sites around Colorado with a $390,000 multi-year grant from the U.S. Department of Agriculture.

Rust fungus is a host-specific pathogen, meaning it only kills Canada thistle; it doesn’t even kill other thistle species. The fungus causes the infected plant to produce yellow spores, which later turn a rusty red-brown color and spread to other plants.

“In areas the fungus is well-established, the Canada thistle is dying back,” Bean said. “We want it spread as far and wide in Colorado, so that Canada thistle becomes less of a problem here.”

One problem with the fungus, however, is it kills thistles slowly, so that it can first use its host to reproduce.

“The idea of this project is to speed up the whole process,” he said. “We think it’ll take and it’ll work — we just don’t know how well.”

Researchers are trying to figure out better ways to establish and spread the fungus as well as establish how long it takes to either kill or significantly shrink Canada thistle patches.

“It’s very exciting because it’s passive. Once you put it out there, it just continues on its own,” Stiegelmeier said. Plus, “it eliminates the need for all the herbicides, and the places it’s been used it’s been extremely effective.”


At the Summit site off Highway 9 and Tiger Road near the Highland Greens neighborhood, biologists Karen Rosen and Joel Price placed markers 12 meters apart and strung a tape between them.

They walked along the tape and placed a measuring square on the ground every two meters and counted the thistles in the square. They found 47.

“That’s not even a very thick patch,” Rosen said.

In the fall, when temperature and humidity are right, she and Price will return to apply the rust fungus.

They produce the treatment by collecting infected Canada thistle leaves with spores, drying the leaves and grinding them into a powder with a blender. They mist the plants in test patches with water before applying the powder, so it will stick, and they are trying to find more efficient ways to apply the fungus over large areas.

Pleimann told the researchers they can monitor the Summit site for the next four years, and it won’t be sprayed with chemicals.

“The rust fungus needs healthy thistle to carry out its life cycle, so it can kill it,” Rosen said, and destroying a Canada thistle patch takes a few years.

Property owners interested in trying the method on their land can contact the Palisade Insectary, where the researchers work. For now, the control method is free, and Rosen said she and Price might be able to visit the property to apply the fungus, or they can instruct landowners how to do it themselves.

People can learn how to tell Canada thistles from native beneficial thistles through the state thistle identification and management guide available on the Summit County Weed Department website.

The state agriculture department also recently released a noxious weed identification app available through iTunes and for Android devices, but the app doesn’t tell users how to manage the weeds.

For more information about the rust fungus program, call the Palisade Insectary at 970-464-7916, or email program directors Karen Rosen at or Joel Price at

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