Iowa State University

White thistles invade Iowa   
by Bob Hartzler

October 03, 2007 - In June it was readily apparent that 2006 produced a bumper crop of Canada thistle along Iowa’s roads and highways. Due to the reproductive habits of Canada thistle, these patches most likely were not new but rather were previously established infestations that were more visible than in earlier years. I suspect the dry May weather in much of the state favored growth of the deep-rooted Canada thistle over that of the shallow-rooted smooth brome planted along most Iowa roads.

Now in September many of these patches are again making themselves known by the appearance of new shoots that are bleached white or yellow. This abnormal growth is due to infection by a bacteria (Pseudomonas syringae ). The lack of chlorophyll will reduce photosynthesis in infected plants, but normally does not result in plant death. Efforts to enhance the virulence of the bacteria so that it could be used as a biological control agent have failed to yield an organism that provides effective levels of suppression of this noxious weed.

Canada thistle patch along US20 infected
with P. syringae.

Bleached stem of infected Canada thistle.

Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University