My experiment with biological control of Canada Thistles in prairie restorations is showing modest signs of progress. This involved finding thistles with signs of a bacterial infection (Pseudomonas syringae pv. tagetis or pst) that produces chloresis, turning the thistle to the color of beeswax and generally sickening the shoot. After collecting two infected thistle shoots in 2006, I liquified them in a blender, added water and a few drops of Dawn dishwashing detergent to act as a surfactant and sprayed the liquid on two small colonies of thistles in the restoration. Several thistles developed the chloresis symptom. The following year there were several dozen shoots in the test area with chloresis, so I gathered more of them and repeated the process, spraying thistles randomly throughout the prairie restoration. I repeated application of the spray on June 28, 2008 under wet and overcast conditions with cool temperatures. I used about a half dozen infected plants gathered from throughout the prairie restoration.
Below is shown an individual Canada Thistle shoot with the chloresis symptom of infection. The chloresis appears to start near the top of the shoot and to work its way down the plant.
About mid-June of this year I noted that there are now pallid thistle shoots to be found throughout the field. Several colonies were intensely affected to the point that it looked like the colony might die out. Few of the shoots were setting seed heads in the most affected groups. Other areas showed thistles turning white, but still growing. Most of the chloresis seems not to appear until around mid-June, but I saw isolated affected shoots earlier.
Canada Thistles grow in colonies. The shoots are connected by the underground root system. In theory if you infect one shoot, the entire colony should become infected. Below is a photo of a colony in which many of the shoots are showing the signs of infection. Note that some shoots still appear healthy.
I have, by no means, won the battle against Canada Thistles. While there are now hundreds of ill shoots in the field, there are hundreds of healthy looking thistles for every sick one, and I've been systematically going through the field to remove their tops with a weed whacker before they produce seed.
However, it is very promising that the amount of diseased thistle foliage has grown so much in one year. My efforts are based on research experiments being conducted at several universities, including a study completed in October of 2007 by the University of Minnesota. The pdf file available at the following link should be of great interest to prairie restorationists battling Canada Thistles.
The University of Wisconsin at Madison has some earlier studies and recommendations available at this link:
University of Wisconsin at Madison Study
You can find more to read on this subject by typing "pst + thistles" into Google.
The reading I have done indicates that the surfactant used has an effect on how infectious the liquid will be. Having no access to the surfactants used in the university experiments, I am using Dawn dishwashing detergent, which is the surfactant I have. Apparently the strains of pst vary in their lethality as well, so it might pay to take a drive and go looking through roadside ditches for an exceptionally sick looking colony of thistles. Be sure to ask the landowners if you may have a few of their thistles! Also be sure to ask if the thistles were treated with a herbicide, which might be the real cause of their sick appearance.
After collecting 5 or 6 thistles, cut them into manageable pieces, put them in a blender, add water and grind them up thoroughly. Strain the liquid through cheesecloth or a nylon stocking into a bucket and add enough water for your sprayer. Mine holds about a gallon. Add 5 or 6 drops of Dawn dishwashing detergent and stir. I filter the liquid a second time, using a nylon stocking before filling the sprayer tank. Apparently the amount of spray per thistle is not a great concern, so spray the liquid onto as many Canada Thistle shoots as you can find over the infested area. In my experience, the first year's results were very weak, but quite noticeable on the following year.
Those interested in trying these techniques can contact me by email for more information. I am not an expert at biological warfare on thistles, but I will be glad to tell you more about what techniques I have used and the results of my efforts. The best time to apply the spray is mid-July according to the latest source I've read. It is also best to apply the bacteria under wet conditions.