Thistle Rust: A Potential Biocontrol Agent to Help in the Management of Canada Thistle

Authored by Tim Seipel, Jed Eberly, Melissa Maggio, Elizabeth Hecker, Daniel Chichinsky, and Patrick Carr

Figure 1: Illustration of Canada thistle root growth, showing roots that can grow horizontally

Figure 1. The extensive roots of Canada thistle make it difficult to control.

Canada thistle is a perennial noxious weed that is well adapted to a wide range of environmental conditions. It is a major problem in annual croplands, and perennial vegetation. In Montana, dense infestations of Canada thistle drastically reduce yields of crops and forage. Control methods for Canada thistle include cultivation, competitive cropping, mowing, controlled burns, grazing, and herbicide application. Management of Canada thistle is difficult, however, because of its extensive rhizomatous root system (Figure 1), in addition to its ability to produce up to 5,000 seeds per stem.

The difficulty in controlling Canada thistle has led to interest in biological control. The use of the fungal pathogen Puccinia punctiformis, thistle rust, has shown potential as a Canada thistle biocontrol agent. Thistle rust exists in temperate climates with its only known host Canada thistle (Cirsium arvense), where the pathogen lives as a systemic parasite in roots. Infected individuals develop dark red pustules on the leaves and stem, causing stem dwarfing and tissue necrosis, although plants can remain asymptomatic for one to two years after inoculation.

There have been some successful field trials using thistle rust as a biocontrol agent but the use of thistle rust as part of an integrated weed management program in agricultural systems is just now being explored by our team. The aim of our work is to determine if Canada thistle can be reduced using thistle rust as part of an integrated weed management program on organic farms in the Northern Great Plains.

Figure 2: Under-leaf photo showing a dense covering of orange spores, second photo shows scattered small red spores under a leaf

Figure 2. (A) The orange (spermagonia) spores in summer produce a fragrance similar to blooming Canada thistle flowers then (B) dark red “freckles” develop during late summer to early fall.

The rust fungus does not spread far which limits its efficacy as a natural biocontrol method. For do-it-yourselfers the fungus can be spread to uninfected patches following these steps. First, identify diseased plants in the spring (Figure 2a). Look for thistle plants that are yellowing and have black or yellow spores on the leaves. Infected plants at this stage will have a fragrance similar to thistle flowers. Mark infected plants with a flag or stake. In mid- to late-summer return to the marked sites and look for surrounding plants infected with uredinia and telia (types of the fungal spores; Figure 2b). Collect the above ground material and put it in a paper bag. Let it thoroughly air dry and prevent it from rotting, then grind it with a blender (Figure 3). Save ground infected thistle in a cool, dark, and dry place until fall when the temperatures begin to cool. Apply a teaspoon to healthy rosettes when the weather is stable, temperatures are between 50-65° F and there is potential for high relative humidity but no rain. For more information, visit the Canada Thistle Control website.

Figure 3: Photos of a sifter that contains leaf matter, then a bag of leaf matter, and finally a leaf on the plant

Figure 3. Blend infected leaf matter for fall inoculations. Apply in fall to rosettes.

If you need a positive identification of the fungus contact your local county agent or submit a sample to the Schutter Diagnostic Lab. Our research team is interested in identifying locations to collect more thistle rust and documenting its occurrence and impact on Canada thistle. Please reach out to us at MSU Extension Cropweeds and the Montana Biological Weed Control Coordination Project.