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Multiple Herbicide Resistance In Waterhemp: Can It Get Any Worse?

William L. Patzoldt
William L. Patzoldt
Graduate Research Assistant
Department of Crop Sciences
(217) 333-4723; patzoldt@illinois.edu
Patrick J. Tranel Patrick J. Tranel
Associate Professor of Molecular Weed Science
Department of Crop Sciences
(217) 333-1531; tranel@illinois.edu
Aaron Hager
Aaron Hager
Assistant Professor of Weed Science
Department of Crop Sciences
(217) 333-4424; hager@illinois.edu

Common and tall waterhemp (Amaranthus rudis and A. tuberculatus) continue to be problem weeds in Illinois agriculture. A major factor contributing to the problematic nature of waterhemp is its ability to evolve resistance to a wide variety of herbicides. During the 1990s, many populations of waterhemp developed resistance to acetolactate synthase (ALS)-inhibiting herbicides, but other mode-ofaction chemistries remained effective. In 1998, the first multiple-resistant waterhemp biotype was identified, with resistance to ALSinhibiting and triazine herbicides.

Figure 1
Figure 1. Individual waterhemp plants from Adams County, IL
are resistant to three herbicide mode-of-action chemistries.

Resistance to protoporphyrinogen oxidase (PPO)- inhibiting herbicides (for example, lactofen, acifluorfen, fomesafen, sulfentrazone, and flumioxazin) was first identified in an Adams County waterhemp population during the summer of 2001. Field studies conducted in 2002 confirmed resistance to not only PPO inhibitors but also ALS-inhibiting and triazine herbicides (Figure 1). Additional waterhemp populations from other areas of the state were later identified during the summer of 2002 with resistance to PPO inhibitors, ALS inhibitors, and triazines. To date, numerous waterhemp populations have been identified in Illinois that are resistant to at least one herbicide mode-of-ofaction, with many populations resistant to two or more (Figure 2).

It is important to note that the characteristics of resistance to PPO inhibitors, triazines, and ALS inhibitors in waterhemp are different, which have significant management implications. Waterhemp resistant to ALSinhibitors are not controlled when herbicides are applied either postemergence (POST) or preemergence (PRE), while waterhemp resistant to triazines and PPO inhibitors are resistant to POST, yet susceptible to labeled rates of these herbicides when applied PRE. However, while most waterhemp populations in Illinois are susceptible to a PREapplied triazine herbicide, a second mechanism of resistance was identified in other populations that also make them resistant to PRE applications. Therefore, it is important to know that differences do exist for triazine resistance in waterhemp.

More detailed studies in the greenhouse with waterhemp resistant to PPO inhibitors revealed that plants are, in fact, resistant at all stages of development, even to PRE-applied herbicides, when low rates are used (for example, 1/10X field rates) (Figure 3). These findings suggest there could be a decreased length of control with PRE-applied PPO inhibitors: As the residual activity decreases after treatment, there will be a selective advantage to waterhemp plants that are resistant.

With the identification of waterhemp resistant to ALS and PPO inhibitors, very few options remain for POST control in soybeans. Currently, a very effective option is glyphosate used in conjunction with glyphosate-resistant soybean varieties. However, several populations of waterhemp have been identified with reduced control following glyphosate treatment (Figure 2). While there are no reports of waterhemp with “resistance” to glyphosate, waterhemp may become more difficult to control as plants with increased tolerance are selected from populations that are more variable in response. The USDA estimated that 86 percent of the soybean acres in Illinois during 2002 received at least one application of glyphosate. With widespread use of glyphosate in soybeans, the potential for resistance evolution in waterhemp could increase.

We are currently conducting studies to address the issue of glyphosate resistance development in waterhemp to learn how best to maintain glyphosate as an effective POST herbicide option.

Figure 2
Figure 2. Several counties in Illinois have been identified with at least one
waterhemp population with resistance to one or more herbicide mode-of-action
chemistries. The stars represent waterhemp populations that were reported to have
reduced control following glyphosate treatment.
Figure 3
Figure 3. Although the Adams County waterhemp population exhibits reduced
sensitivity to preemergence (PRE) applications of PPO inhibitors, resistance to PRE
applications is not apparent at normal use rates.

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