Canada thistle rust fungus as a biological control


By Ethan Proud

PREVIEW Columnist

Puccinia punctiformis is an obligate fungal pathogen of Canada thistle and is better known as Canada thistle rust fungus. It is a naturally occurring fungus that can only grow on Canada thistle and cannot complete its life cycle on other plants, making it a very safe biological control. Some biocontrols have adapted post-release to feed on native plants and thus their distribution has been canceled.

The rust fungus is in its infant stages as a biological control release in Colorado and the inoculation protocol is still being refined. Experimental plots are being set up across the state with several in Archuleta County.

Last year, during the 2018 spray season, natural populations of the fungus were harvested and used to inoculate experimental plots. The rust fungus is most effective in areas that receive adequate moisture. The best results from experimental plots set up by the state cite complete control within three years, though not every plot will see those success rates.

Canada thistle rust fungus is easily identified in the spring when it shows up on young shoots as orange blobs called spermagonia. It has a sweet smell that can be detected when it is present in large quantities.

During this stage, the plant should be left alone so the spores can cross, which will give rise to aeciospores. Aeciospores coat the undersides of the leaves and spread throughout the late summer. They have an appearance similar to cocoa powder, but may be mistaken for lacebug frass.

Lacebug infestations may occur alongside rust fungus infections. Lacebugs also appear to have a detrimental effect on Canada thistle, but more observations will be necessary. Mowing Canada thistle while the fungus is in the aeciospore stage will aid in spore dispersal, but it is important to note that the fungus needs a living host and too aggressive of mowing may hinder its progress.

In the fall, teliospores form and infect new healthy plants. These spores can be observed affecting the underside of fall regrowth surrounding the shoots infected with the aeciospores. In the winter, basidiospores form on the roots when the plant is dormant.

The teliospores may be collected to inoculate new plants. Cut the infected shoots at the surface and allow them to dry for several days. Strip the leaves from the stem and either hand crush them or use a blender. Use gloves to do this as the spines are hard to dig out of your hands. The blended-up leaf and spore mixture may be spread on plants (about a tablespoon per plant) on new rosettes in the late fall. If dew is present, inoculate in the morning. Misting the plants at night and inoculating is an acceptable alternative if it is a drier year.

Rust fungus can be acquired from the Colorado Department of Agriculture’s Request-a-Bug Program, though the spores are in short supply and there is a long waiting list. Contacting the Archuleta County Weed and Pest to be put on a list will increase the likelihood that you receive spores, though it is not guaranteed.

Biocontrols are part of the Archuleta County Weed and Pest’s Integrated Management Plan to reduce our dependency on herbicides to promote a healthy native environment.

Archuleta County Weed and Pest is your local resource for managing noxious weed populations and controlling other pests.

Upcoming events

July 13: Archuleta County Annual Weed Tour.

Aug. 1-4: Archuleta County Fair. Do you quilt or sew, can vegetables or fruit, grow hay crops, veggies or flowers? Maybe you do leather or wood work? Possibly brew beer or make wine? Or, maybe you have a hidden crafting talent that you would like share with us? If so, then you can enter the Archuleta County Fair Open Classes. Go to to find out how to enter.

CPR and first aid classes

CPR and first aid certification classes are offered monthly by the CSU Extension office on the second Monday and Wednesday of each month from 6 to 10 p.m. Anyone needing to receive or renew certification can register by calling the Extension office at 264-5931.