Waterhemp populations that were not effectively controlled by early summer postemergence applications of PPO-inhibiting herbicides may be resistant to the widely used PPO-inhibiting soybean herbicides such as Cobra (lactofen), Flexstar (fomesafen), Marvel (fluthiacet-methyl & fomesafen) and Ultra Blazer (aciflurofen).
Assessing herbicide resistance in the field can be challenging because other factors such as weather, weed height, antagonism with another herbicide in the tank or using the wrong adjuvant could all contribute to poor control. Now one must also consider the likelihood that the waterhemp population is resistant to the PPO class of herbicides (Site of Action Group 14).
To determine if your waterhemp population has a specific genetic sequence that confers resistance to PPO-inhibiting herbicides please consider submitting via overnight delivery a plant sample, from 5 surviving plants in a field, to:
- University of Illinois Plant Clinic
S-417 Turner Hall
1102 S. Goodwin
Urbana, IL 61801
Follow the following web links for plant collection and shipping instructions:
The fee is $50 per tested field and the Plant Clinic will process as quickly as possible but priority is given to Illinois-based submissions.
Your submitted sample will be tested for two specific mechanisms that confer resistance to glyphosate and PPO inhibitors, respectively. These are the most common mechanisms of resistance in Illinois and are common to other Midwestern states as well. Your final report response will include either a positive or negative result regarding the presence of these two specific resistance-conferring mutations.
However, a negative result, especially for glyphosate, does not mean that the plants submitted are susceptible to the herbicide; it means the plants lacked the specific mutation that was tested for and the negative result could indicate the presence of a different resistance mechanism.
Currently I know of two populations of waterhemp in Jackson County, MN that were documented by the U of Illinois Plant Clinic last year as resistant to PPO herbicides and yesterday I learned of one documented population in Faribault County, MN.
It is important to have this information before next year because there are a limited number of effective postemergence herbicide sites of action available in soybean and some of the reported PPO-resistant waterhemp populations are also resistant to glyphosate (Site of Action Group 9) and ALS-inhibiting herbicides such as Raptor, Pursuit and Classic (Site of Action Group 2).
If a waterhemp population were resistant to all three groups (2, 9 and 14), the only effective postemergence soybean herbicide treatment currently available is glufosinate (Liberty 280), which can only be applied to LibertyLink branded soybeans. At this point in the growing season, without the LibertyLink option the only option is physically removing the weeds by hand.If you receive positive results, I would appreciate a brief e-mail indicating the herbicide, County of sample origin and if known, history of herbicide use. Good luck.