No. 12 Article 7/June 15, 2007

Request for Waterhemp Samples

Resistance to protox or PPO-inhibiting herbicides (such as Cobra, Ultra Blazer, and Flexstar) in waterhemp was confirmed in Illinois several years ago. With the extensive use of glyphosate for postemergence weed control in soybean, PPO resistance has not been an extensive problem in the state. However, for soybean farmers still using diphenylethers for postemergence control of waterhemp, PPO-resistant waterhemp is a serious problem. Through funding from the Illinois Soybean Association, researchers at the University of Illinois have elucidated the molecular mechanism responsible for waterhemp's resistance to PPO-inhibiting inhibitors. As a follow-up, we have developed a rapid assay that we would like to test as a diagnostic tool for confirming PPO resistance.

We are soliciting your help in this endeavor. Please let us know if you observe poor control of waterhemp following the application of a diphenylether herbicide and you suspect the poor control is attributable to resistance. Keep in mind that resistance to PPO inhibitors is quite different from what is typically observed with other instances of herbicide resistance. Specifically, waterhemp plants resistant to PPO inhibitors will display typical contact injury within a day or two after herbicide application, and they may appear indistinguishable from sensitive plants. However, by a week or two after treatment, resistant plants will have healthy new leaf growth.

If you suspect you have a waterhemp population that is resistant to PPO-inhibiting herbicides, please contact Aaron Hager (217-333-4424). We will then arrange for you to send us leaf samples of suspected resistant plants. We anticipate being able to inform you very soon after receiving your samples (perhaps even within 24 hours) whether the population is resistant.--Aaron Hager and Pat Tranel

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