The continual evolution of herbicide-resistant weeds is one of the most daunting problems faced by weed management practitioners.

According to Aaron Hager, associate professor of weed science at the University of Illinois, biotypes of 12 weed species in Illinois are known to be resistant to one or more herbicide action mechanisms.

“The occurrence of multiple herbicide resistances within individual plants and/or fields is a particular challenge,” Hager said. “Waterhemp has evolved resistance to more herbicide mechanisms of action than any other Illinois weed species.”

Frequently, soil-residual herbicides are proposed as part of an integrated weed management program. They provide several benefits, including reducing the intensity of selection for resistance to foliar-applied herbicides.

Many growers believe weeds demonstrate resistance only to herbicides applied to the foliage. In fact, weeds can also develop resistance to soil-applied herbicides.

For example, imazethapyr (the active ingredient in Pursuit and some herbicide premixes) is an acetolactate synthase (ALS)-inhibiting herbicide that can be applied to the soil or plant foliage.

Worldwide, biotypes of 128 weed species have evolved resistance to ALS-inhibiting herbicides. Many Illinois waterhemp populations contain plants that are resistant to these herbicides whether they are applied to the soil or plant foliage.

The story is similar for herbicides that share a common mechanism of action but that are usually applied to either the soil or the foliage, such as herbicides that inhibit the protoporphyrinogen oxidase (PPO) enzyme.

“Waterhemp resistance to PPO-inhibiting herbicides is becoming increasingly common across Illinois,” Hager said.

“Growers began to suspect it was present after a foliar-applied PPO inhibitor failed to control plants that were treated within label guidelines. The treated plants demonstrated injury symptoms, such as leaf necrosis, that are characteristic of this herbicide family. However, the injury was less than normal, and the resistant plants begin to recover within seven to 10 days after the application.”