Common waterhemp populations that aren’t effectively controlled by early-summer postemergence applications of PPO-inhibiting herbicides may be resistant to the widely used PPO-inhibiting soybean herbicides such as:
Marvel (fluthiacet-methyl and fomesafen).
Ultra Blazer (acifluorfen).
Assessing herbicide resistance in the field can be challenging because there are other factors, such as weather, weed height, antagonism with another herbicide in the tank or using the wrong adjuvant. All could contribute to poor control.
Now you must also consider the likelihood that the waterhemp population is resistant to the PPO class of herbicides (Site of Action Group 14).
How to know if your waterhemp is resistant
The University of Illinois Plant Clinic can help you determine if your waterhemp population has a specific genetic sequence that confers resistance to PPO-inhibiting herbicides.
You can submit a plant sample from five surviving plants in a field via overnight delivery to:
University of Illinois Plant Clinic
S-417 Turner Hall
1102 S. Goodwin
Urbana, IL 61801
Cost: The fee is $50 per tested field.
Timing: The Plant Clinic will process as quickly as possible, but priority is given to Illinois-based submissions.
Test process: The clinic will test your sample 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.
A negative result, especially for glyphosate, doesn’t mean the submitted plants are susceptible to the herbicide; it means the plants lacked the specific mutation that was tested for. The negative result could indicate a different resistance mechanism is present.
PPO herbicide-resistant waterhemp in Minnesota
As of December 2016, 35 cases of PPO resistance were documented with a subset of 7 resistant to both PPOs and glyphosate.
It’s important to have this information because there are a limited number of effective postemergence herbicide sites of action available in soybean. Also, 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.
Without LibertyLink, physically removing the weeds by hand is the only option growers have if they’re well into the growing season.
Reviewed in 2018