| Patrick J. Tranel
Assistant Professor,Molecular Weed Science
Department of Crop Sciences
(217) 333-1531; email@example.com
Waterhemp (Amaranthus tuberculatus) continues to rank among the most problematic weeds for Illinois producers. Among its many weedy attributes, waterhemp is adept at evolving resistance to herbicides. For example, farmers are aware that most waterhemp is now resistant to acetolactate synthase (ALS)-inhibiting herbicides and, therefore, that these herbicides are no longer effective waterhemp control options.
Waterhemp biotypes that are resistant to ALS inhibitors have a mutation in the ALS gene (i.e., target-site resistance). Interestingly, however, different mutations within the ALS gene have been identified among resistant biotypes. Thus, waterhemp has figured out more than one way to be resistant to ALS inhibitors.
Figure 1. Atrazine-resistant waterhemp
biotypes may possess different resistance mechanisms;
consequently, they may exhibit different levels of atrazine resistance.
Similarly, waterhemp is resistant to atrazine by multiple ways (Figure 1). Target-site resistance to atrazine, although documented in waterhemp a few years ago, has not become a very widespread problem. More recently, we have documented in waterhemp atrazine resistance that is not due to an altered target site. This second type of atrazine resistance appears to be more widespread among Illinois waterhemp. The good news, however, is that the non-target-site atrazine resistance is not expressed in young seedlings. Consequently, atrazine applied prior to waterhemp emergence is effective on biotypes with this atrazine resistance mechanism.
The latest discovery in waterhemp’s resistance arsenal is resistance to protoporphyrinogen oxidase (PPO)-inhibiting herbicides. PPO inhibitors include the commonly used diphenylethers (Cobra, Flexstar, and Ultra Blazer) as well as Valor, Resource, Authority, and Aim. Waterhemp with resistance to PPO inhibitors was first discovered in Kansas and has since been identified in Illinois.
To make matters worse, a single waterhemp plant may possess multiple resistance mechanisms, and thereby be resistant to herbicides with different sites of action. For example, we have previously characterized a waterhemp biotype that was resistant to both ALS inhibitors and triazine herbicides. Waterhemp biotypes with three-way resistance—to ALS inhibitors, PPO inhibitors, and triazines—will exist soon, if they do not already.
What about glyphosate-resistant waterhemp? There have been several anecdotal reports of glyphosate resistance in waterhemp over the past few years and, to be sure, there exists variability in glyphosate responses among waterhemp populations. Nevertheless, it is not yet certain that there exist waterhemp populations that survive labeled rates of glyphosate applied at the proper timing and under favorable environmental conditions. However, it will not be surprising if glyphosate-resistant waterhemp populations are encountered in the future. In fact, it is not hard to envision a waterhemp biotype that is resistant to the majority of our commonly used herbicides.
Finally, as if the situation were not already gloomy enough, waterhemp can share herbicide resistance with other pigweed species via interspecific hybridization (Figure 2). For example, we have confirmed that hybrids between smooth pigweed and waterhemp are formed under field conditions. Occurrence in waterhemp of resistance to a variety of herbicides, and the potential for resistance to be transferred to other pigweed species, highlights the need to utilize resistance management strategies in our weed-control efforts.
Figure 2. This waterhemp-looking
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