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), says a University of Minnesota Extension weed specialist.

In a recent blog, Jeff Gunsolus, U-M Extension agronomist, acknowledged the challenge of assessing herbicide resistance in the field because factors, such as weather, weed height, antagonism with another herbicide in the tank or using the wrong adjuvant can all contribute to poor control.

Wonder if waterhemp is resistant to weed 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),” he wrote.

To determine if your waterhemp population has a specific genetic sequence that confers resistance to PPO-inhibiting herbicides, Gunsolus suggested that you send in plant samples for herbicide resistance sampling. To conduct the test, samples must consist of five surviving plants and shipped overnight to:

University of Illinois Plant Clinic

S-417 Turner Hall

1102 S. Goodwin

Urbana, IL 61801

The following web links provide instruction for plant collection and shipping.

Waterhemp Herbicide Resistance Testing
http://web.extension.illinois.edu/plantclinic/downloads/WaterhempFlyer.pdf

Herbicide Resistance Submission Form
https://uofi.app.box.com/s/q4hrowvhtmgjmqll8v6f3ygw38jlz2g3

The fee is $50 per tested field. Gunsolus said the Plant Clinic will process samples as quickly as possible. However, priority is given to Illinois-based submissions.

Gunsolus said each 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,” he said. “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.”

Gunsolus said two populations of waterhemp coming from Jackson County were documented by the U of Illinois Plant Clinic last year as resistant to PPO herbicides. And recently he learned of one documented population in Faribault County.

“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,” Gunsolus said. “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, he added.

“At this point in the growing season, without the LibertyLink option, the only option is physically removing the weeds by hand,” he said.

Gunsolus would appreciate hearing from any farmers who send in samples and receive positive results. In a brief e-mail, please provide the name of the herbicide, county of sample origin and history of herbicide use. Contact Gunsolus at [email protected]