Response to suspected PPO-inhibitor-resistance [A92 (R)] And known [A66 (S)] Waterhemp (Amaranthus tuberculatas) populations from Wisconsin, 28 days after treatment with soil-applied sulfentrazone (Spartan), fomsafen (Flexstar), and flumioxazin (Valor) at 0×, 0.125×, 0.25×, 0.5×, 0.5×, 1×, 1×, 2. NTC = untreated control. Credit: Dr. Felipe de Andrade Falico, former Ph.D. Student, University of Wisconsin-Madison
Recently published research in the journal Weed Science shows that waterhemp resistance to protoporphyrinogen oxidase inhibitor (PPO) herbicides can develop from soil-applied preemergence applications, not just subsequent herbicide applications. PPO is an important herbicide that works by blocking a key enzyme necessary for the production of chlorophyll in plants. The research took place during 2022 and 2023 at the University of Wisconsin-Madison, Madison, Wisconsin.
“Our study documents the first confirmed case of a waterhemp population in Wisconsin that can survive inhibition by PPO (group 14) herbicides.”
“Until now, most reports of PPO resistance in waterhemp have been related to postemergence applications. Sprayed after weed emergence. This finding suggests that waterhemp resistance can also develop from soil-applied, preemergence herbicides.”
Preemergence PPO herbicides, such as sulfentrazone (Spartan) and fomsafin (Flexstar), have become critical for controlling waterhemp populations by identifying whorls. He added that waterhemp is a major weed problem throughout the U.S. Midwest, and growers in that region rely heavily on preemergence products for residual weed control.
“Discovering resistance at the preemergence level is concerning because it limits one of the remaining effective chemical options for early-season waterhemp control.” “This serves as an early warning to growers to diversify their weed management programs before similar resistance spreads. For researchers, this indicates the need to better understand how this resistance develops over time so we can design more effective, long-term management strategies.”
Currently, growers can help by applying fully labeled rates, using tank-mixing PPO-inhibitor herbicides with other effective preemergence herbicide groups, and integrating nonchemical dynamics such as crop rotation, cover crops, and other cultural or mechanical control methods.
“Reducing reliance on one herbicide method is key to maintaining their performance in the field,” he asserted.
More information:
Felipe de Andrade Fallico et al., Resistance to protophorphyrenogen oxidase inhibitors applied to waterhemp (preemergence or postmajority).Amaranthus tubercle, Weed Science (2025) doi: 10.1017/wsc.2025.10044
Provided by Cambridge University Press
Reference: Research Shows Waterhemp Resistance May Be Acquired by Preemergent PPO Inhibitor Use (2025, October 23) Retrieved October 23, 2025 from https://phys.org/news/2020-waterhemp-resistance-preemergent-ppohtml.
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