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White Canada Thistle – what’s the story?This year is a great year for Canada thistle that appears white or bleached, a symptom of plant disease. The causal agent of this pathogen is a soil borne bacteria called Pseudomonas syringae pv. tagetis (Pst). The bacterium is commonly found in our Pennsylvania soils. It causes apical chlorosis in Canada thistle (Cirsium arvense) and certain other composite weeds due to the production of tagetitoxin, a RNA polymerase III inhibitor that blocks chloroplast biogenesis. The Canada thistle appears white or bleached-out in appearance, particularly near the top of the plant (Image 1). I suspect the wet cool spring has really helped the disease get a foothold within some fields or roadsides. You can observe bleached-out patches of Canada thistle, easily recognizable from a windshield survey while driving along highways or secondary roads. This pathogen of thistle was identified a number of years ago and was and is being examined as a potential biocontrol. Researchers at the University of Minnesota demonstrated that plants that were inoculated weekly with the pathogen were completely controlled and that seed production was reduced when plants were severely infected. Research in Wisconsin investigated the transmission of the disease from infected to noninfected plants via plant sap. The Wisconsin researchers had some success with this technique, but also raised a number of questions such as when to apply the sap, how much to apply, and whether the disease will persist and continue to infect Canada thistle. This disease is currently not available as a biocontrol product, so for now, when you see bleached-out thistle, thank the natural wonders that occur in this world and keep your fingers crossed that someday, this pathogen will contribute to the demise of this important noxious weed.
Image 1. Pseudomonas infected Canada thistle.
W. S. Curran, wcurran@psu.edu Weed Science, Crop and Soil Sciences |
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