This weakens plants and minimizes flowering and seed formation. Canada thistle populations infected plants often diminish and on occasion completely disappear over a period of years. Bacterial pathogens have not been practical biocontrol agents of weeds because plants need to be wounded to initiate infection, but the University of Minnesota developed a new inoculation technique that greatly increases the infection rate. Infected plants usually fail to flower and often die. A commercial formulation of the bacterium was field tested in many states in 1995 but failed to induce the typical symptoms.
Effort continue to find ways to make the spray application of the bacterium successful in the field.
Mean while, Mother nature continues to spread the bacterium and this can only help weaken the competitive ability of Canada thistle. Persistent and timely mowing, clipping or hand weeding can greatly reduce biennial thistle infestations but will have minimal effect on Canada thistle. Biennial thistles should be cut as close to the ground as practical each time they begin to flower. Field observations indicate that bull thistles have less regrowth capability after clipping than musk or plumeless thistles. This may explain why bull thistles are not increasing in seriousness, while musk and plumeless thistles are spreading into new areas and becoming more abundant in already infested pastures.