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What is Canada thistle?
Cirsium arvense (Canada-, California-, Creeping thistle) is an invasive plant known to invade open, disturbed habitats. It colonizes new areas through the production of nearly 1,500 seeds per stem. Once established, it spreads through clonal underground shoots.
How to Identify Canada thistle
Canada thistle is a perennial weed that can grow over 6 ft. tall (more commonly ~3 ft.). Each branch of the plant typically bears 3-5 (dandelion-sized) flowers that range from white to purple (more commonly pink). Canada thistle leaves are rigid and glossy, and stems are virtually spineless.
Effects of Canada thistle
It was introduced to North America in the 1600s from Europe. Today, it is distributed throughout temperate regions of the globe and has become one of the most problematic weeds of crop, range, and pasture lands. Canada thistle is generally unpalatable to grazing animals and continues to cause severe economic losses. Canada thistle can form monocultures that outcompete and replace native plants, invade critical riparian habitat, dominate lawns, and congest roadsides.
Since the 1970s, a Canada thistle gall fly (Urophora cardui) and a stem mining weevil (Hadroplontus litura) have been used to control Canada thistle in North America. However, these two insects have been very limited in field settings and are ineffective overall. Recent research now allows us to utilize a host-specific pathogenic rust fungus (Puccinia punctiformis) that was likely introduced with early Canada thistle infestations and is present in nearly every location where Canada thistle occurs.
Life Stages and Cycle of Rust Fungus
In early spring, diseased Canada thistle shoots emerge that appear unusually tall, sparse, and are covered with yellow speckling (spores) on the underside of the leaves (A-B). During this time the diseased stems emit a sweet floral fragrance. In late spring to early summer, diseased shoots will cross with other nearby diseased shoots and the spores on leaf tissues will turn a rusty red-brown color (C). Spores from diseased shoots from the spring will infect neighboring Canada thistle stems throughout the summer via wind-blown spores. Finally, during late summer or fall, diseased stems die and the leaf tissue falls on fall emergent rosettes (D) that allow the fungus to quickly move to the roots where it overwinters (E).
- Will the rust kill the thistle?
Unlike many classical biological control agents that limit or control the spread of an infestation, the Canada thistle rust fungus has the potential to significantly decrease infestations. This rust fungus has evolved to be very effective at causing thistle mortality. In past trials, the worst case was a 45% reduction of thistles over five years, while the best case was 100% after 18 months…largely dependent on the current density of thistle and surrounding vegetation.
- Is the rust fungus safe?
The rust fungus is host-specific, meaning; Canada thistle is the only organism the rust fungus can attack. It is safe to use around water, livestock, and other plants. It does not even attack other thistle species. Once the Canada thistle is gone, the rust fungus dies with it.
- How long will it take to control my Canada thistle?
Because the rust fungus is so effective at killing its host plant, it spreads slowly to ensure new thistles will be present to infect in later years. However, after nearly 100 years of study, we have recently determined the best way to quickly spread the disease. Significant declines in the number of thistle stems can be seen 1-2 years after establishment. By the 3-4 year mark, the thistle is often just a small percentage of its original infestation. Do not be concerned if there appear to be very few diseased plants after rust spore application. The real killing work is done in the root system, so above-ground symptomatic stems are typically not an accurate representation of the infection rate overall.
- What is the best release site?
Because the rust fungus lives within plant tissues, environmental conditions are unlikely to affect the rust fungus. As long as Canada thistle is present, the rust can establish. Shorter vegetation does allow spores to spread faster. Because it is an organism with a complex life cycle, sites with rust fungus are best left untouched (no herbicide, heavy grazing, or short mowing).
More About Canada Thistle
Rust Fungus Inoculation
The basic technique for fall inoculation of Canada thistle (Cirsium arvense) patches using the rust fungus (Puccinia punctiformis).