Palmer Amaranth Punches Through PPO Inhibitors in Illinois

Palmer amaranth is arguably the worst weed U.S. farmers currently face. Earlier this month, Southern Illinois University (SIU) reports it has taken another casualty as researchers find confirmation of resistance to PPO inhibitors in the state. Some brands that fall under this popular herbicide category include Aim, Authority, Cadet, Canopy XL and Valor.

SIU made the confirmation in three populations in coordination with the University of Illinois. The weeds showed control failure after they were treated with fomesafe or lactofen, the active ingredients in Flexstar and Cobra.

SIU weed scientist Karla Gage says this discovery is not surprising. That’s due to a number of factors, including the common use of PPO herbicides, Palmer amaranth’s rapid ability to evolve resistance to multiple modes of action, along with its knack for long-distance dispersal through either birds or machinery.

To that point, one of the fields with confirmed resistance was likely spread by neighboring geese. It was in the Conservation Reserve Program (CRP) for more than a decade and was a popular spot or geese foraging. It was tilled and planted into soybeans, and soon enough, “a sea of Palmer amaranth emerged where none was found before,” according to Gage.

Palmer amaranth has turned into a resistant-ridden nightmare, Gage says. A single plant can grow 2” to 3” per day and produce 500,000 seeds. Waiting too late to apply herbicides puts additional herbicides at risk because of survival selection of the most tolerant plants.

Palmer amaranth that is resistant to PPO herbicides has also been discovered in Arkansas and Tennessee. To see what resistance problems have popped up in your home state, visit

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