At the 2002 Illinois Crop Protection Technology Conference, Dallas Peterson (Kansas State University, KSU) reported on work conducted by KSU researchers on a PPO-resistant waterhemp biotype identified in Kansas. At the 2003 Illinois Crop Protection Technology Conference, we reported that the phenomena of PPO resistance is no longer confined to the Kansas waterhemp biotype. At least one (and most likely several more) waterhemp population in Illinois is now confirmed to be resistant to PPO inhibitors. This population is located in western Illinois, but we have received other anecdotal reports of PPO inhibitors (that is, diphenylether herbicides such as Ultra Blazer, Flexstar, Cobra, and Phoenix) failing to control waterhemp. Not all of these reports have emanated 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 unique to the 2002 growing season. For many years, we (and many others) have observed waterhemp control range from complete to much less than satisfactory with these herbicides. Regrowth of susceptible waterhemp plants occurs more frequently when postemergence applications are made to plants larger than 5 inches in height or under adverse growing conditions (primarily extended periods of dry soils). Late-season applications of these herbicides, usually made when waterhemp plants 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.
In 2002, we initiated field experiments on the producer's field in western Illinois to determine the resistance characteristics of the waterhemp biotype. Each experiment (soil-applied and postemergence) included several PPO-inhibiting herbicides, as well as herbicides with other sites of action. No crop was planted in the study area because of adverse weather conditions. The soil-applied experiment was evaluated 30 days after application, whereas the postemergence experiment was evaluated 7 and 21 days after application.
Results from the soil-applied experiment indicated all herbicides, other than acetolactate synthase (ALS) inhibitors, provided excellent water-hemp control 30 days after application. Soil applications of Authority, Valor, and Flexstar (all PPO inhibitors) provided from 86 to 99% water-hemp control. Soil applications of Pursuit, Classic, and FirstRate did not provide any waterhemp control, compared with a untreated check. The fact that soil-applied PPO inhibitors controlled the waterhemp biotype was not terribly surprising, given that KSU researchers also have reported good control of the Kansas PPO-resistant waterhemp biotype with soil-applied PPO inhibitors.
Results from the postemergence experiment indicated all ALS inhibitors did not provide any waterhemp control and that control with PPO inhibitors ranged from 13 to 53%. Each PPO inhibitor was applied at four rates, representing a 1/2x 1, 1.5, and 2x rate. The 2x rates of Cobra, Flexstar, Ultra Blazer, and Aim provided only 28, 46, 53, and 23% waterhemp control, respectively, 21 days after application. These results are similar to those reported by the KSU researchers.
We are currently conducting additional experiments (greenhouse and laboratory) with this waterhemp biotype. In particular, Patrick Tranel and his graduate student William Patzoldt are attempting to determine the resistance mechanism and how the resistance trait is inherited. Stay tuned in future issues for additional information on PPO-resistant waterhemp.--Aaron Hager and Christy Sprague