Multi-state survey to identify suspected resistance to four herbicides and one plant growth regulator in Poa annua

Poa annua L. has been identified as the most troublesome weed in turfgrass systems (Van Wychen, 2020). Its unwanted presence in turfgrass can undermine economic feasibility and performance by disrupting surface uniformity and increasing management costs. Controlling P. annua can be particularly challenging as it is a highly adaptive polyploid capable of surviving diverse environmental and management conditions (Carroll et al., 2021; Molina-Montenegro et al., 2016). This adaptability lends itself to widespread evolution of herbicide resistance, with confirmed resistance to at least 12 unique modes-of-action (MOAs) including several documented instances of multiple resistance (Breeden et al., 2017; Brosnan et al., 2015; Rutland et al., 2023; Singh et al., 2021).

While at least 50 distinct cases have been reported globally (Heap, 2023), the distribution of herbicide resistance in P. annua across climatic and management gradients has not been well documented. Among the documented cases of P. annua herbicide resistance collected from turfgrass or grass seed production systems (37 total), approximately 75% of biotypes were obtained from golf courses. Little to no herbicide resistance data has been reported for sports fields, lawns, and production turfgrass systems. This makes it difficult to discern and communicate the extent of herbicide resistance across the turfgrass industry and to correspondingly develop effective research and Extension strategies to address the problem. The latter is evidenced by recent studies that have identified localized skepticism, misinformation, and confusion about this issue across the turfgrass industry (Allen et al., 2022; Ervin et al., 2022).

This brief reports on findings from a multi-state survey evaluating the response of P. annua collections from various turfgrass management systems (i.e., golf courses, sports fields, residential and commercial lawns, sod production) to four herbicides and a plant growth regulator. The purpose of this survey was two-fold: (1) to establish a novel multi-state approach for the identification and advancement of P. annua collections with putative herbicide resistance across diverse climates and turfgrass systems; and (2) to discern potential trends related to P. annua proliferation and control that can inform future research and Extension strategies.

A previous report by Rutland et al. (2023) documented preliminary screening and sequencing of target-site mutations associated with four MOAs, including inhibitors of 5-enolpyruvylshikimate-3 phosphate synthase (Herbicide Resistance Action Committee [HRAC] Group 9, acetolactate synthase (HRAC Group 2), photosystem II (HRAC Group 5), and microtubule assembly (HRAC Group 3). Herein, we report screening results for paclobutrazol, a triazole-type plant growth regulator, and four additional herbicide MOAs, including hydroxyphenyl pyruvate dioxygenase (HRAC Group 27), very long-chain fatty acid synthesis (HRAC Group 15), photosystem I (HRAC Group 22), and glutamine synthetase inhibitors (HRAC Group 10).

Between the fall of 2018 and spring of 2020, P. annua populations were collected by researchers spanning 13 academic institutions across the United States, following methodologies previously described by Rutland et al. (2023) (Figure 1). Sample collection spanned USDA plant hardiness zones 4a to 10b, with most sampling locations concentrated between zones 5a and 9b. Populations represented four major industry sectors with distinct management regimes: golf courses, athletic fields, lawn care operations, and sod farms. This research will focus on 866 populations screened for susceptibility to the five postemergence treatments outlined in Table 1.

For each collection × treatment combination, approximately 20 tillers were transplanted into flats filled with herbicide-free native soil and established under greenhouse conditions before treatment, as described by Rutland et al. (2023). An additional flat was prepared for all populations as a non-treated check and subsequently used to produce seed for advanced studies. Each participating research location screened their own state's sampled populations or coordinated with another collaborator when space or labor were not adequate to conduct screenings. Populations collected by North Carolina State University were subsequently screened by University of Tennessee and those collected by Oregon State University were screened by Purdue University. Treatments were applied 1 week after the transplant using a water carrier volume of 40 gal acre⁻¹ using a CO₂-pressurized backpack sprayer and following typical research spraying methods (Rutland et al., 2023). The flats received 0.25 inches of overhead irrigation 1 h after treatment. Rates and formulations corresponding to each treatment are summarized in Table 1. A single discriminating rate was applied at each location. Collaborators selected between a 1× or a 2× label based on preliminary research and site history to distinguish between susceptible populations and those with suspected resistance. Regional limitations in space or resources precluded testing all treatments at all sites. In total, 547, 683, 724, 655, and 713 populations were screened for their response to diquat, ethofumesate, glufosinate, mesotrione, and paclobutrazol, respectively (Table 2). Populations were visually evaluated for injury 21 days after treatment. Populations that survived the standard rate of herbicide treatment were labeled “suspected resistant.”

Herbicide resistance has been identified as a “wicked problem,” without a distinct cause or solution, which increasingly threatens to undermine the social, economic, and environmental benefits of turfgrass systems (Allen et al., 2022; Brosnan et al., 2020; Jussaume and Ervin, 2016). Thoughtful collaboration among scientists has been identified as an important strategy for addressing wicked problems that threaten agricultural and environmental sustainability (Hull et al., 2020). This survey represents the first coordinated, multi-state effort to establish an approach for the identification and advancement of a key pest evolving resistance to pesticides in an urban agricultural system such as turfgrass. To date, this approach has been instrumental in identifying populations and plant material for sequencing of target site mutations (Rutland et al., 2023), and is anticipated to inform future research evaluating non-target site resistance mechanisms and additional studies pertaining to the proliferation and management of P. annua in turfgrass.

Of the 866 P. annua collections screened in this study, 146 (16.8%) were flagged with suspected resistance to one or more MOAs tested (Figure 2). Further, 97 (11.2%) were suspected of multiple resistance. While additional testing (i.e., dose-response assays) is required to confirm and understand underlying mechanism(s) of resistance in these P. annua collections (Burgos et al., 2013), several key takeaways can be shared.

First, our screening reinforces that herbicide resistance is a growing challenge for the turfgrass industry spanning multiple MOAs, climates, and industry sectors. This emphasizes the importance of continued research and efforts to communicate best management practices (BMPs) to practitioners who may not yet see the importance of this problem.

Second, suspected resistance appeared to be concentrated in warm-season turfgrass environments (Figure 2), even for herbicides that are used sparingly in warm-season turfgrass systems. This is in alignment with trends previously observed by Rutland et al. (2023) for other herbicide MOAs. Future research to confirm and identify underlying mechanisms of resistance should be conducted along geographic gradients to elucidate plant adaptive response to edaphic, environmental, and management variables including turfgrass species selection. Further, these findings suggest that Extension messaging should vary between northern and southern climates based on the need for more reactive versus preventive resistance management strategies.

Finally, the high number of uniquely managed turfgrass sites in the United States underscores the potentially overwhelming nature of this problem. This also highlights the need for efficient strategies to confirm resistance, which would ideally be less costly and time-consuming than conventional approaches. Future research should prioritize the development of rapid diagnostic tools similar to those developed by Cross et al. (2013), Kaundun et al. (2014), and Pritchard et al. (2023) that are capable of responding to the apparent volume of suspected herbicide resistance in P. annua.

Rebecca G. Bowling: Conceptualization; data curation; formal analysis; investigation; methodology; project administration; resources; software; supervision; visualization; writing—original draft; writing—review and editing. James D. McCurdy: Conceptualization; investigation; methodology; resources; writing—original draft; writing—review and editing. Edicarlos de Castro: Investigation; resources; writing—original draft; writing—review and editing. Aaron J. Patton: Conceptualization; investigation; methodology; resources; writing—original draft; writing—review and editing. James T. Brosnan: Conceptualization; investigation; methodology; resources; writing—review and editing. Shawn D. Askew: Conceptualization; investigation; methodology; resources. Gregory K. Breeden: Investigation; writing—review and editing. Matthew T. Elmore: Conceptualization; investigation; methodology; resources; writing—review and editing. Travis W. Gannon: Conceptualization; investigation; methodology; resources; writing—review and editing. Clebson G. Gonçalves: Investigation. John E. Kaminski: Conceptualization; investigation; methodology; resources. Alec R. Kowalewski: Conceptualization; investigation; methodology; resources. Wenwen Liu: Investigation. Clint M. Mattox: Conceptualization; investigation; writing—review and editing. Lambert B. McCarty: Conceptualization; investigation; methodology; resources. Patrick E. McCullough: Conceptualization; investigation; methodology; resources. J. Scott McElroy: Conceptualization; investigation; methodology; resources. Chase McKeithen: Investigation. Andrew Osburn: Investigation. Ronald R. Rogers: Investigation; writing—review and editing. Claudia Ann Rutland: Investigation. Kaiyuan Tang: Investigation. Jacob W. Taylor: Investigation. J. Bryan Unruh: Conceptualization; investigation; methodology; resources; writing—review and editing. Jose J. Vargas: Investigation. Muthukumar V. Bagavathiannan: Conceptualization; funding acquisition; investigation; methodology; project administration; resources; visualization; writing—review and editing.

The authors declare no conflict of interest.