A new study confirms for the first time that the tenacious agricultural weed waterhemp can evolve resistance to a critical class of herbicides applied to the soil before weeds sprout. Researchers at the University of Wisconsin–Madison found that a population of waterhemp survived pre-emergent applications of protoporphyrinogen oxidase (PPO) inhibiting herbicides, a discovery that challenges the long-held assumption that this form of resistance develops primarily from herbicides sprayed on already-grown plants. The findings, published in the journal *Weed Science*, signal a significant new threat to crop production and highlight the weed’s relentless adaptability.
This development is a serious concern for farmers, particularly in the U.S. Midwest, where waterhemp is a dominant and damaging weed. Growers depend heavily on soil-applied, pre-emergent PPO inhibitors (classified as Group 14 herbicides) as a foundational tool for early-season weed control, especially in fields where weeds have already developed resistance to other common herbicides like glyphosate. The evolution of pre-emergent resistance effectively neutralizes one of the most reliable chemical defenses available, forcing an urgent reconsideration of weed management strategies to prevent its spread and preserve the viability of remaining herbicide options.
A Novel Resistance Pathway Emerges
Until now, nearly all documented cases of PPO-inhibitor resistance in waterhemp were linked to post-emergence applications, where herbicides are sprayed directly onto emerged weeds. The landmark study from Wisconsin documents the first confirmed case in the state of a waterhemp population that withstands PPO inhibitors applied to the soil before crop and weed emergence. This indicates that the evolutionary pressure from residual herbicides in the soil is sufficient to select for resistant plants, a mechanism that was previously considered less common for this mode of action.
The research, conducted between 2022 and 2023, focused on a suspected resistant waterhemp population, identified as accession A92. This population demonstrated a clear ability to survive pre-emergent treatments of widely used PPO herbicides, including sulfentrazone and fomesafen. According to Rodrigo Werle, a University of Wisconsin-Madison associate professor and the study’s corresponding author, this finding serves as a critical early warning to the agricultural community that reliance on this single class of herbicides for early control is a finite strategy.
Unraveling the Genetic Underpinnings
In seeking to understand the biological basis for this resistance, the science team investigated the weed’s genetics. PPO inhibitors work by blocking an enzyme essential for chlorophyll production, leading to rapid plant death. Resistance often evolves through specific genetic mutations at the herbicide’s target site, primarily in the PPX2 gene. Well-documented mutations, such as the ΔG210 deletion, are known to confer resistance to PPO inhibitors applied to foliage.
Beyond Known Target-Site Mutations
The researchers hypothesized that the resistance in the A92 waterhemp population was driven by one of these known target-site mutations. However, genetic analysis revealed that none of the common mutations in the PPX1 or PPX2 genes could account for the observed resistance. This crucial finding suggests that the A92 population has developed a different, and possibly novel, defense mechanism. The study proposes that non-target-site resistance could be at play, a process where the plant uses enzymes, such as cytochrome P450 monooxygenases, to metabolize and detoxify the herbicide before it can cause harm. Further research is now needed to pinpoint the precise mechanism enabling this new form of defense.
Greenhouse Trials Confirm Resistance
The study’s conclusions are based on rigorous dose-response experiments conducted in a controlled greenhouse environment. Investigators collected seeds from the suspected resistant A92 population and two known PPO-susceptible populations, A66 and A82, for comparison. They then applied three different soil-based PPO herbicides—sulfentrazone, fomesafen, and flumioxazin—at eight different concentrations, ranging from one-eighth to eight times the standard labeled rate.
The results were definitive. Twenty-eight days after treatment, the A92 plants showed significantly higher survival rates compared to the susceptible controls across the range of herbicide doses. Quantitative analysis determined that the A92 population was 3.1 times more resistant to sulfentrazone and fomesafen in pre-emergent applications. The population also showed substantial resistance to post-emergent applications of lactofen (18.6-fold) and fomesafen (5.9-fold), confirming it was a robust, multi-faceted resistance.
A Call for Diversified Weed Management
The confirmation of pre-emergent PPO resistance has profound implications for crop protection. It signals that a key chemical tool is losing effectiveness, threatening to make early-season waterhemp control far more difficult and costly. Werle and the Weed Science Society of America are urging growers to move away from reliance on a single herbicide mode of action to slow the evolution of further resistance.
Integrating Chemical and Cultural Practices
Experts advise a multi-pronged approach that integrates both chemical and non-chemical tactics. For chemical control, farmers should always use the full labeled herbicide rates, as under-dosing can allow moderately resistant weeds to survive and reproduce. Furthermore, they should apply tank-mixes that combine PPO inhibitors with other effective pre-emergent herbicides from different groups, creating multiple modes of action that are harder for weeds to overcome simultaneously. Just as important is the adoption of non-chemical strategies, including diverse crop rotations, the use of cover crops to suppress weed growth, and mechanical controls like tillage or cultivation where appropriate. These practices reduce the overall weed seedbank and decrease the selective pressure exerted by any single herbicide technology.