Canada Thistle, Watch Out!

Canada thistle is a non-native noxious weed. It is highly aggressive, and when left unchecked, often forms large, monotypic infestations where it outcompetes all other vegetation. It is a serious invader of irrigated croplands and wet meadow pastures in Las Animas County. This is a noxious weed that has serious economic implications for agricultural production and ecological diversity.

Meet Puccina puncitformis, a mighty microscopic Canada Thistle contender.​​

In mid-September the Purgatoire Watershed Weed Management Collaborative, in collaboration with the Colorado Department of Agriculture's (CDA) Biological Pest Control Division and landowners in the Upper Purgatoire River Watershed, released a biocontrol agent on several large Canada thistle infestations. The biocontrol agent is a host-specific pathogenic rust fungus, Puccina punctiformis. According to the CDA, recent studies have shown this agent to be an effective means of controlling Canada thistle in Colorado.

In order to inoculate Canada thistle with the root fungus, CDA staff first harvests infected shoots of Canada thistle plants. The infected shoots are dried and then finely chopped in blenders. The chopped rust fungus-infested shoot material is then dusted on healthy Canada thistle rosettes (young plants that have not flowered) with the spray-apply-spray (SAS) method, in which the leaves are lightly sprayed with water before and after dusting to create high humidity (a critical factor for inoculation). Within a week of application, the fungus will travel to the roots of the plant where it will overwinter. In the early spring, infected shoots will emerge that will eventually release spores to infect other nearby Canada thistle plants. Ideally, application of Canada thistle rust fungus should be done in the fall several weeks before nighttime temperatures dip below freezing and day time temperatures are 80+ degrees F.​

Biocontrols are heavily vetted and permitted by USDA APHIS to ensure host-specificity, meaning the biocontrol agent will only affect one type of plant. Puccina punctiformis meets this criteria, exclusively targeting Canada thistle as a host. However, unlike many other biocontrol agents that normally do not kill their host, Puccina punctiformis has been found to actually eradicate Canada thistle infestations in some instances. The CDA has reported Canada thistle stands experiencing an 80% to even 100% reduction of Canada thistle patches within five years in some locations after inoculation with the root fungus.

To read more about the life cycle and effectiveness of Puccina punctiformis, please visit the CDA website:

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Trinidad, CO