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Survey for PPO-Resistant Waterhemp Populations

September 6, 2002
Earlier this season, we reported confirming PPO-inhibitor resistance in a waterhemp population from western Illinois (see issue 14, June 28). Since that initial report, we have received several other anecdotal reports of PPO-inhibiting herbicides (Ultra Blazer, Flexstar, Cobra/Phoenix) failing to control waterhemp. Not all of these reports have originated from western Illinois, however, and we are concerned that PPO-inhibitor resistance in Illinois waterhemp populations may be more widespread than initially perceived.

Before going any further, let's also say it is unlikely that every instance of PPO inhibitors failing to provide complete control of waterhemp is attributable to resistance. Less than complete control of waterhemp with PPO-inhibiting herbicides is not something that is unique to the 2002 growing season. For many years, we (and many others, for that matter) have observed water-hemp control range from complete to much less than satisfactory with these herbicides. Regrowth of susceptible waterhemp plants tends to happen more frequently when postemergence applications are made to plants larger than 5 inches in height and/or under adverse growing conditions (primarily extended periods of dry soils). Late-season applications of these herbicides, usually made when waterhemp are very large and nearing the reproductive stage, also can result in poor control. Please note that instances of poor waterhemp control such as these are not necessarily attributable to herbicide resistance.

What have we observed from field and greenhouse experiments on the waterhemp population that we have confirmed to be resistant? First of all, the field on which we conducted our research in 2002 had a large percentage of PPO-resistant plants (although some susceptible plants were present within the population also). With such a large percentage of PPO-resistant waterhemp plants, the level of control achieved with our applications could be described as a complete failure. In other words, by 7 days after treatment it was fairly obvious that we had a resistant population. Following a postemergence application of a PPO-inhibiting herbicide, the resistant waterhemp plants typically demonstrate the leaf burn characteristic of this herbicide family.

However, unlike susceptible plants, the leaf burning is generally much less than complete, and the resistant plants usually begin to recover within 5 to 7 days after the application. New leaves typically continued to emerge from the apical meristem of resistant plants compared with leaf emergence from lower axils on susceptible plants that recovered. Additionally, as the resistant plants begin to resume growth, the plants appear to have a more bushy appearance,

and their coloration often becomes pale green.

We would like to conduct a survey of Illinois waterhemp populations to determine how widespread resistance to PPO-inhibiting herbicides is, and we can use your assistance. If you know of a field in which waterhemp plants that were treated with a postemergence PPO-inhibiting herbicide were not controlled and demonstrated recovery as previously described, we would be grateful to receive a sample of seed from three to six female plants. Please bag seed from each female plant separately, and send seed samples to me at N321 Turner Hall, 1102 S. Goodwin Ave., Urbana, IL 61801. We will plant the seed in the greenhouse and screen the plants to determine their level of response to PPO-inhibiting herbicides. Please include your contact information and, if possible, a brief history of herbicides that have been used on the field over the past 3 to 5 years.--Aaron Hager

Author: Aaron Hager

The Pest Management and Crop Development Bulletin
Executive Editor: Kevin Steffey, Extension Entomologist

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