I have a small wildflower patch in which the Canada thistles were looking rather sickly over the past few years. Each year there seemed to be more and more thistles affected. Thought perhaps that a virus was affectng them. Decided to investigate on line and found this website which might be helpful to anyone with a thistle problem. It's by the University of Wisconsin's College of Agriculture.

http://www.cals.wisc.edu/sciencereport/2006scienceOfSustainableAgricultureThistle.html