Introduction
Downy brome (Bromus tectorum L.) is a predominantly self-pollinated, C3 winter annual grass species that originated in the Mediterranean and southwest Asia and has been introduced to most temperate regions globally (Kao et al. Reference Kao, Brown and Hufbauer2008; USDA-ARS 2023). In the United States, B. tectorum is a problematic weed in several agricultural systems, including the dryland winter wheat (Triticum aestivum L.) production areas of the Pacific Northwest (PNW) (Rydrych Reference Rydrych1974). The dryland wheat-producing regions of the PNW, including eastern Washington and northeastern Oregon, are characterized by wet winters and dry summers, where precipitation is considered inadequate to produce a crop every year (Karimi et al. Reference Karimi, Stöckle, Higgins and Nelson2018). Therefore, a summer fallow period is commonly used to maximize water storage for the following winter wheat growing season (Karimi et al. Reference Karimi, Stöckle, Higgins, Nelson and Huggins2017).
Bromus tectorum is a highly competitive and fast-growing species capable of producing up to 1,350 seeds plant−1 (San Martín et al. Reference San Martín, Thorne, Gourlie, Lyon and Barroso2021). If left uncontrolled, B. tectorum can cause winter wheat yield losses of up to 92% (Rydrych and Muzik Reference Rydrych and Muzik1968). The most common postemergence herbicides used for B. tectorum control in winter wheat are acetolactate synthase (ALS) inhibitors. Several ALS-inhibiting herbicides are available to selectively control B. tectorum in winter wheat, including mesosulfuron, sulfosulfuron, propoxycarbazone, and pyroxsulam (Ostlie and Howatt Reference Ostlie and Howatt2013), in addition to imazamox when used with imazamox-resistant wheat varieties (Clearfield® Production System) (Nakkaa et al. Reference Nakkaa, Jugulam, Peterson and Asifa2019). Metribuzin, a photosystem II (PSII) inhibitor, can be applied postemergence to winter wheat for B. tectorum control (Swan and Whitesides Reference Swan and Whitesides1988). However, winter wheat tolerance to metribuzin varies according to cultivars; thus, careful management, including proper cultivar selection and timely application, is required to achieve good crop tolerance and B. tectorum control with metribuzin (Devlin et al. Reference Devlin, Gealy and Morrow1987a, Reference Devlin, Gealy and Morrow1987b; Swan and Whitesides Reference Swan and Whitesides1988). In 2018, a new herbicide-resistant wheat technology known as the CoAXium® Wheat Production System (Limagrain Cereal Seeds, Windsor, CO) was developed to provide growers with an additional option to control winter annual grass weeds in winter wheat (Bough et al. Reference Bough, Westra, Gaines, Westra, Haley, Erker, Shelton, Reinheimer and Dayan2021; Ostlie et al. Reference Ostlie, Haley, Anderson, Shaner, Manmathan, Beil and Westra2015). The CoAXium® Wheat Production System allows postemergence applications of the acetyl-coenzyme A carboxylase (ACCase) inhibitor quizalofop (Aggressor® AX herbicide) for selective control of winter annual grass weed species, including B. tectorum.
Long-term and repeated use of a limited number of postemergence herbicides resulted in the selection of cross- and multiple-herbicide resistance to several herbicide modes of action (MOAs), including ACCase inhibitors, ALS inhibitors, PSII inhibitors, and the 5-enolpyruvylshikimate-3-phosphate (EPSPS) inhibitor glyphosate in B. tectorum populations in winter wheat and other production systems in the PNW (Ball et al. Reference Ball, Frost and Bennett2007; Mallory-Smith et al. Reference Mallory-Smith, Hendrickson and Mueller-Warrant1999; Park et al. Reference Park, Fandrich and Mallory-Smith2004; Park and Mallory-Smith Reference Park and Mallory-Smith2004, Reference Park and Mallory-Smith2005; Ribeiro et al. Reference Ribeiro, Brunharo, Mallory-Smith, Walenta and Barroso2023; Zuger and Burke Reference Zuger and Burke2020). Therefore, to delay the evolution and spread of herbicide resistance in B. tectorum, a better understanding of current B. tectorum management practices and the distribution of herbicide-resistant populations in the dryland winter wheat production areas of Oregon is needed to develop best management practices and steward the current herbicide technologies.
Recently, wheat growers in the dryland areas of northeastern Oregon reported insufficient B. tectorum control with ALS inhibitors. Despite well-documented cases of herbicide-resistant B. tectorum populations in the PNW, a comprehensive investigation of herbicide resistance status in B. tectorum across Oregon dryland winter wheat–based cropping systems is lacking. Therefore, the objectives of this study were to (1) conduct a survey of wheat growers to understand B. tectorum management practices in wheat fields in the dryland region of northeastern Oregon and (2) screen 49 B. tectorum populations collected across northeastern Oregon for resistance to herbicide MOAs commonly used in dryland wheat production.
Materials and Methods
Survey of Wheat Growers
Surveys were conducted in 2021 and 2022 to understand B. tectorum management practices in wheat fields in northeastern Oregon. Three County Extension agents helped to identify 28 growers with fields infested with B. tectorum in Gilliam, Morrow, Sherman, Umatilla, and Wasco counties for the survey. The survey included four questions (Q) related to: (Q1) crop rotation, (Q2) tillage versus no-tillage, (Q3) irrigation versus dryland, and (Q4) herbicide programs (preemergence and/or postemergence) based on the growing seasons from 2017 to 2022. The survey was distributed via paper copies handed directly to growers or by email (Supplementary Figure S1). Extension agents assisted in distributing the survey to the growers they identified.
Bromus tectorum Sampling
Bromus tectorum seed sampling was conducted during summers of 2021 and 2022 (June and July) at B. tectorum physiological maturity. County Extension agents or growers provided GPS coordinates of fields for B. tectorum seed collection. A total of 49 B. tectorum populations were collected from the surveyed grower’s wheat fields, with some growers having multiple fields that were included in the survey (Figure 1).
Figure 1. Distribution of Bromus tectorum populations collected from wheat fields in the dryland wheat-producing region of northeastern Oregon in 2021 and 2022 (n = 49 populations).
Bromus tectorum panicles were randomly collected from 10 to 20 plants within each field and placed in paper bags, except for one population (MOR5) collected in a field with fewer than 10 plants. Paper bags were labeled with the respective fields’ geographic coordinates and unique sample names. Bromus tectorum panicles were hand threshed for seed collection, and seed was stored in paper bags at room temperature until resistance screenings were initiated. A known herbicide-susceptible B. tectorum population collected from a non-agricultural area (45.79°N, 118.64°W) near Adams, OR, was included for comparison.
Herbicide Resistance Screenings: Postemergence
Bromus tectorum seeds were germinated in acrylic square germination boxes (10-cm width by 10-cm length by 2.5-cm height; 156C container, Hoffman Manufacturing, Corvallis, OR) in a growth chamber with continuous light at 15 C. Seedlings (with an approximately 5-cm shoot length) were transplanted into four-celled trays (6-cm width by 6-cm length by 5.7-cm depth; 150/CS, Grower’s Nursery Supply, Salem, OR) filled with a commercial potting mix (SS #4 PC RSi, Sun Gro® Horticulture, Agawam, MA). The experimental unit consisted of a four-celled tray containing four plants for each population. The experiment was a randomized complete block design with three replications (12 plants total). Plants were grown in a greenhouse at Oregon State University, Corvallis, OR (44.56°N, 123.28°W) at 24/15 C day/night, supplemented with 400-W high-pressure sodium light bulbs (350 μmol m−2 s−1) to ensure a 12-h photoperiod.
Herbicides from four MOAs were tested based on their importance for the winter wheat–summer fallow rotation system (Peachey Reference Peachey2024), as well as their use in rotational crops (Table 1). Herbicide rates of 0, 1, and 2 times (×) the recommended labeled rate were used. Application rates for clethodim and quizalofop were based on the recommended rate for representative rotational crops grown in the region. The glyphosate 1× rate was based on the recommended summer fallow rate on its label. Herbicides were applied to B. tectorum plants at the 2- to 3-leaf stage using a research track sprayer delivering 140 L ha−1 spray volume through a TeeJet® TP8003E nozzle (TeeJet® Technologies, Wheaton, IL). This approach to screening for herbicide resistance in B. tectorum was based on preliminary data generated by Ribeiro et al. (Reference Ribeiro, Brunharo, Mallory-Smith, Walenta and Barroso2023). Those authors characterized the response of B. tectorum to several herbicides and found that 1× and 2× are adequate discrimination rates for classifying plants as resistant or susceptible under greenhouse conditions. Bromus tectorum plants were visually assessed as dead (completely necrotic plants; assessed value of 0) or alive (green tissue and evidence of regrowth; assessed value of 1) at 21 d after treatment (DAT).
Table 1. Herbicides, application timings, trade names, companies, addresses, modes of action, chemical families, and rates used in the resistance screenings
a ACCase, acetyl-coenzyme A carboxylase; ALS, acetolactate synthase; EPSPS; 5-enolpyruvylshikimate-3-phosphate; PSII, photosystem II; VLCFA, very-long-chain fatty acid.
b Rates: 1×, label rate; 2×, two times the label rate. Clethodim and quizalofop label rates were based on the recommendation for lentils (Lens culinaris Medik.), and the glyphosate label rate was based on the recommendation for summer fallow.
c Crop oil concentrate (1% v/v) was added to the spray solution.
d Nonionic surfactant (0.25% v/v) + ammonium sulfate (2% v/v) was added to the spray solution.
e Glyphosate rates were based on grams of acid equivalent per hectare (g ae ha−1).
Herbicide Resistance Screenings: Preemergence
The 49 B. tectorum populations were screened with pyroxasulfone at 0, 1×, and 2× rates (Table 1). The experimental units consisted of square trays (30-cm width by 30-cm length by 5.7-cm height; Grower’s Nursery Supply, Salem, OR) filled with a commercial potting mix (SS #4 PC RSi, Sun Gro® Horticulture). Ten B. tectorum seeds were placed at a 1.3-cm soil depth in four rows in each experimental unit. Two rows were seeds from a field-collected population and the other two rows were seeds from a known-susceptible population for side-by-side comparison. The trays were arranged in a randomized complete block design with three replications.
Pyroxasulfone was applied directly to the seeds, which then were covered with a layer of potting mix after application. The application procedures were the same as previously described for postemergence herbicides. The soil was watered before and immediately after herbicide application to facilitate seed germination and herbicide activation. Bromus tectorum seedling emergence was counted at 21 DAT.
Statistical Analysis
Survey of Wheat Growers
Survey data were sorted, filtered, and analyzed using the pipe, select, filter, summarize, and count functions of the tidyverse package (Wickham et al. Reference Wickham, Averick, Bryan, Chang, McGowan, François, Grolemund, Hayes, Henry, Hester, Kuhn, Pedersen, Miller, Bache and Müller2019) in R statistical software (v. 4.2.2; R Core Team 2022). For crop rotation, tillage versus no-tillage, and irrigation versus dryland questions, results are presented as percent of surveyed fields. For the questions regarding herbicide programs (preemeregence and/or postemergence), results are presented as the percentage of herbicide use based on applications across the 77 winter wheat-years and 63 summer fallow-years, which were calculated by summing the number of years listed on the 29 surveys. Results of herbicide programs were only extracted from herbicides that listed B. tectorum as controlled or suppressed on their labels.
Herbicide Resistance Screenings: Postemergence
The number of surviving B. tectorum plants in the resistance screenings was expressed as a percentage:
$$P = \left( {{X}\over{Y}} \right){\rm{*}}100{\rm{\;}}$$
where P is the percent survival of B. tectorum plants after each herbicide treatment in the resistance screenings, X is the total number of B. tectorum surviving plants at 21 DAT, and Y is the total number of B. tectorum plants treated with each herbicide tested. Bromus tectorum populations with ≥50% survival to the 1× rate of the herbicides tested were considered resistant (adapted from Owen et al. Reference Owen, Walsh, Llewellyn and Powles2007).
Herbicide Resistance Screenings: Preemergence
The number of emerged B. tectorum plants in the resistance screenings were converted into percent plant density reduction compared with the untreated control:
$$D = \left( {{{B - C}}\over{B}} \right){\rm{*}}100{\rm{\;}}$$
where D is the percent plant density reduction compared with the untreated control, B is the average plant counts of the untreated control for the respective field-collected population, and C is the plant counts of the experimental unit after pyroxasulfone treatment. Bromus tectorum populations with plant density reduction ≤90% at the 1× rate of pyroxasulfone were considered resistant (adapted from Faleco et al. Reference Faleco, Oliveira, Arneson, Renz, Stoltenberg and Werle2022).
Results and Discussion
Survey of Wheat Growers
Twenty-nine completed surveys were returned out of 49. Although not all the growers returned the survey, the responses indicated a representative sample of each county with a positive correlation between herbicide use and proportion of resistance. The responses include 29 wheat fields representing 77 winter wheat-years and 63 summer fallow-years, which were used for percent calculations of crop rotation, tillage versus no-tillage, irrigation versus dryland, and herbicide programs questions.
Winter wheat–summer fallow (WW-SF; 76%) was the predominant cropping system followed by winter wheat–spring wheat–summer fallow (WW-SW-SF; 14%) and winter wheat-spring pea (Pisum sativum L.; WW-SP; 10%) rotations. Seventy-three percent of the fields surveyed were in no-tillage systems, while 27% were in conventional tillage systems (i.e., primary tillage is with a chisel plow; secondary tillage is with a field cultivator and two to three passes through the field with a rod weeder as needed for the rest of the summer), and none were irrigated. The wheat-based cropping systems in northeastern Oregon are closely tied to annual precipitation (Douglas et al. Reference Douglas, Rickman, Klepper and Zuzel1992). For instance, most of the fields surveyed were in an area with annual precipitation ranging from 180 to 304 mm, which is too low to support continuous cropping, and WW is commonly grown every other year (Karimi et al. Reference Karimi, Stöckle, Higgins and Nelson2018). Fields where WW-SW-SF was practiced were in a transitional area with annual precipitation from 304 to 457 mm, where a fallow period before WW is critical to recharge soil moisture and maximize yields, with WW followed by SW, a shorter season crop with a lower water requirement. Fields with the WW-SP rotation were in a wetter area where annual cropping is feasible with annual precipitation >450 mm. No-tillage cropping systems have become prevalent in northeastern Oregon. No-tillage systems create a favorable environment for B. tectorum infestation, because B. tectorum seeds are left on the soil surface, resulting in greater B. tectorum emergence (Wicks Reference Wicks1997). Previous research demonstrated greater B. tectorum emergence at 2- (100%) than at 4- (93%) or 6-cm depths (14%) (Hulbert Reference Hulbert1955). Additionally, no-tillage cropping systems result in a strong dependence on herbicides for B. tectorum control because of the absence of mechanical control.
Results of the survey indicated a high reliance on postemergence herbicides (88%) for B. tectorum control in winter wheat. The ALS inhibitors imazamox (28%) and pyroxsulam (25%) were the most frequently used herbicides followed by the PSII inhibitor metribuzin (16%). The use of preemergence herbicides (12%) was less frequent for B. tectorum control in winter wheat. Pyroxasulfone (9%) was the most often used herbicide followed by flufenacet + metribuzin (3%) for preemergence control of B. tectorum. ALS-inhibiting herbicides have been the primary herbicides used for postemergence winter annual grass control in winter wheat for more than two decades. Sulfosulfuron was the first ALS-inhibiting herbicide registered primarily to control winter annual grasses in winter wheat and has been used since 1999 (USEPA 2024c). Imazamox became available for use in winter wheat with the introduction of the imazamox-resistant wheat varieties (Clearfield® Production System) in 2003 (USEPA 2024e). Propoxycarbazone and mesosulfuron were registered for use in winter wheat in 2004 (USEPA 2024a, 2024b). Pyroxsulam was the last ALS-inhibiting herbicide introduced in winter wheat production systems that has activity on B. tectorum and has been used since 2008 (USEPA 2024d). The PSII inhibitor metribuzin was first registered in the United States in 1973 and is commonly used in a variety of agricultural systems, including vegetables, field crops, and grass grown for seed (Heri et al. Reference Heri, Carroll, Parshley, Nabors, Homer, Janis and Orvin2008). The metribuzin herbicide label provides a list of tolerant and sensitive winter wheat varieties to which metribuzin can and cannot be applied, respectively. However, this list has not been updated for more than two decades. For instance, several new winter wheat varieties have been launched that are not included on the label, while several others that were commonly grown when the herbicide was introduced are no longer grown. Therefore, this uncertainty of winter wheat crop safety to metribuzin might be the reason for the greater use of ALS inhibitors for B. tectorum control. Nevertheless, metribuzin use was significant, perhaps because of ALS inhibitor–resistant B. tectorum populations. The less frequent use of preemergence herbicides for B. tectorum control in winter wheat is likely because of the limited rainfall in the region. Most of the fields surveyed were in an area that typically receives between 180 and 304 mm. Growers typically complain about the reduced efficacy of preemergence herbicides, as these herbicides require incorporation (e.g., tillage or other methods) and adequate moisture for their activation. In summer fallow fields, glyphosate (89%) was the most often used herbicide followed by paraquat (11%) for B. tectorum control. There was no report of preemergence herbicide use for B. tectorum control in summer fallow.
Bromus tectorum Response to Postemergence and Preemergence Herbicides
Forty-six of the 49 B. tectorum populations screened were resistant to at least one of the ALS inhibitors tested (Figure 2). Resistance to mesosulfuron and sulfosulfuron was the most predominant, with 41 B. tectorum populations surviving the 1× rates of these herbicides. Resistance to the 2× rates of mesosulfuron and sulfosulfuron occurred in 28 and 24 of the B. tectorum populations, respectively. For pyroxsulam, resistance was observed in 31 populations to the 1× rate and 24 populations to the 2× rate. For propoxycarbazone and imazamox, 22 and 16 of the B. tectorum populations were resistant to the 1× rates of these herbicides, respectively. Bromus tectorum plant survival was slightly reduced when treated with the 2× rates of these herbicides, with 20 and 12 of the populations surviving propoxycarbazone and imazamox applications, respectively. Two B. tectorum populations (GIL2 and MOR8) were below the resistance threshold used in this study (≥50% survival to the 1× rate) but showed potential for future resistance to mesosulfuron (≤41 survival to 1× rate). Only one B. tectorum population (MOR5) was susceptible to all herbicides tested. Thirty-one of the 46 ALS inhibitor–resistant B. tectorum populations exhibited different cross-resistance patterns. Cross-resistance to four ALS-inhibiting chemical families imidazolinone, sulfonylurea, triazolinone, and triazolopyrimidine was confirmed in 16 B. tectorum populations. Additionally, six populations exhibited cross-resistance to three ALS-inhibiting chemical families, sulfonylurea, triazolinone, and triazolopyrimidine. Cross-resistance to sulfonylurea and triazolopyrimidine herbicides was confirmed in nine populations. Resistance to ALS-inhibiting herbicides can result from a target site–based (TSR) and/or a non–target site based mechanism (NTSR) (Yu and Powles Reference Yu and Powles2014). Target-site resistance is typically more common and occurs because of mutations in the ALS gene (Tranel et al. Reference Tranel, Wright and Heap2024) or other alterations such as gene amplification (i.e., copy number variation) (Iwakami et al. Reference Iwakami, Shimono, Manabe, Endo, Shibaike, Uchino and Tominaga2017) and overexpression (Sen et al. Reference Sen, Hamouzová, Mikulka, Bharati, Košnarová, Hamouz, Roy and Soukup2021; Zhao et al. Reference Zhao, Yan, Wang, Bai, Wang, Liu and Wang2018). To date, mutations at eight codon positions associated with ALS-inhibitor TSR have been reported (Tranel et al. Reference Tranel, Wright and Heap2024). The level and spectrum of resistance to different ALS inhibitors are influenced by several factors such as weed species, mutated amino acid position, specific amino acid substitutions, and the number of resistance alleles (Tranel et al. Reference Tranel, Wright and Heap2024; Yu and Powles Reference Yu and Powles2014). Despite being less common than TSR, NTSR associated with ALS inhibitors has been documented in several weed species, in which enhanced metabolism is by far the most predominant NTSR mechanism (Jugulam and Shyam Reference Jugulam and Shyam2019; Yu and Powles Reference Yu and Powles2014). Cytochrome P450s (CYP450s) represent one of the most important gene families conferring NTSR to ALS inhibitors; however, involvement of glutathione S-transferases, glucosyltransferases, and ATP-binding cassette transporters have been reported in mediating ALS-inhibitor NTSR (Jugulam and Shyam Reference Jugulam and Shyam2019). Our results corroborated Zuger and Burke’s (Reference Zuger and Burke2020) findings, which indicated that 52% of 50 B. tectorum populations collected from wheat fields in Washington were cross-resistant to multiple chemical families of ALS inhibitors, and 20% were resistant to a single ALS-inhibiting chemical familiy. These similarities are expected because the winter wheat–producing region of eastern Washington is similar in climate and management to Oregon’s northeastern region, where the use of ALS-inhibiting herbicides is prevalent. In Montana, an ALS inhibitor–resistant B. tectorum population identified in an imazamox-resistant wheat field was resistant to imazamox and exhibited cross-resistance to pyroxsulam and propoxycarbazone (Kumar and Jha Reference Kumar and Jha2017). Similarly, different cross-resistance patterns to ALS-inhibiting herbicides have been previously reported in two B. tectorum populations from different Kentucky bluegrass (Poa pratensis L.) fields in Oregon (Park et al. Reference Park, Fandrich and Mallory-Smith2004; Park and Mallory-Smith Reference Park and Mallory-Smith2004). One population with a TSR was resistant to primisulfuron, propoxycarbazone, and sulfosulfuron, but susceptible to imazamox (Park and Mallory-Smith Reference Park and Mallory-Smith2004). The other population was resistant to primisulfuron, sulfosulfuron, propoxycarbazone, and imazamox, and resistance was mediated by enhanced metabolism, an NTSR mechanism (Park et al. Reference Park, Fandrich and Mallory-Smith2004). The widespread occurrence of ALS inhibitor–resistant B. tectorum populations in this study is likely driven by management practices, as the use of ALS-inhibiting herbicides is prevalent in the region. The proportion of resistant populations correlated with the history of use of most ALS-inhibiting herbicides reported in the survey. The different ALS-inhibitor cross-resistance patterns and levels of resistance (i.e., survival to 1× rate or both 1× and 2× rates) observed in the B. tectorum populations might have occurred due to different mutations at the eight codon positions in the ALS gene known to confer ALS-inhibitor resistance and/or NTSR mechanisms. NTSR to ALS inhibitors generally results in a low level of resistance, and effective control can be achieved with higher rates (Mallory-Smith et al. Reference Mallory-Smith, Hendrickson and Mueller-Warrant1999; Owen et al. Reference Owen, Goggin and Powles2012; Park et al. Reference Park, Fandrich and Mallory-Smith2004), which may explain the resistance in some B. tectorum populations to 1× rate and susceptibility to the 2× rate in this study. The only herbicide-susceptible B. tectorum population (MOR5) in this study came from a field where fewer than 10 plants were collected, indicating a low B. tectorum infestation that had likely not been selected for herbicide resistance yet. In the field where population MOR5 was collected, the B. tectorum that escaped herbicide treatment was hoed, which was the only difference from the other growers surveyed. The continued selection of these and other B. tectorum populations with ALS inhibitors can result in increased resistance frequency and spectrum of resistance across the ALS-inhibiting herbicides. The widespread occurrence of ALS inhibitor–resistant B. tectorum populations is concerning, as it limits effective postemergence herbicide options in winter wheat.
Figure 2. Bromus tectorum plant survival (±SE) to imazamox, mesosulfuron, propoxycarbazone, pyroxsulam, sulfosulfuron, and metribuzin at the labeled rate (1×) and two times the labeled rate (2×) of each herbicide. Populations with ≥50% survival (represented by the red dashed line) to the 1× rate of postemergence herbicides were classified as resistant. Populations were collected from Gilliam (GIL), Morrow (MOR), Sherman (SHE), Umatilla (UMA), and Wasco (WAS) counties in Oregon.
Resistance to the PSII inhibitor metribuzin was less prevalent, with two populations and one population surviving the 1× and 2× rates, respectively (Figure 2). The two PSII-resistant B. tectorum populations exhibited multiple resistance to ALS inhibitors. Similar to our findings, a previously reported ALS inhibitor–resistant B. tectorum population from a Kentucky bluegrass field in Oregon exhibited multiple resistance to several herbicide MOAs, including clethodim and fluazifop (ACCase inhibitors); atrazine, diuron, metribuzin, and terbacil (PSII inhibitors); and ethofumesate (VLCFA inhibitor) (Park and Mallory-Smith Reference Park and Mallory-Smith2005). This resistant population had two different resistance mechanisms, including herbicide metabolism via CYP450s and a Ser-264-Gly mutation in the psbA gene. The widespread occurrence of cross- and multiple-resistant populations makes B. tectorum management even more challenging.
All B. tectorum populations were susceptible to the postemergence herbicides clethodim, quizalofop, and glyphosate and the preemergence herbicide pyroxasulfone. The absence of ACCase-inhibitor resistance in B. tectorum is surprising, because these herbicides (e.g., diclofop and tralkoxydim) have been used in wheat cropping systems. The adoption of quizalofop-resistant winter wheat varieties associated with quizalofop offers a control option for ALS- and PSII-resistant B. tectorum populations. Although we did not find B. tectorum populations resistant to ACCase inhibitors, ACCase-resistant B. tectorum populations have been reported in other regions of Oregon with cross-resistance to the aryloxyphenoxypropionate herbicides fluazifop and quizalofop and the cyclohexanedione herbicides clethodim, and sethoxydim (Ball et al. Reference Ball, Frost and Bennett2007; Ribeiro et al. Reference Ribeiro, Brunharo, Mallory-Smith, Walenta and Barroso2023). Stewardship guidelines and site-specific recommendations must be considered to preserve and optimize the efficacy of this technology while preventing the selection and spread of ACCase-resistant populations. Glyphosate is still an effective option for B. tectorum control in summer fallow, but alternative herbicide MOAs or tank mixes should be considered to preserve its use. It is important to note that B. tectorum populations resistant to glyphosate have already been identified in similar wheat cropping systems in the neighboring state of Washington (Zuger and Burke Reference Zuger and Burke2020). Previous studies have shown high levels of B. tectorum control with pyroxasulfone in winter wheat (Johnson et al. Reference Johnson, Wang, Geddes, Coles, Hamman and Beres2018; Kumar et al. Reference Kumar, Jha and Jhala2017). The incorporation of preemergence herbicides, such as pyroxasulfone, into a B. tectorum management program in winter wheat in locations where low precipitation is not a constraint for herbicide activation adds more diversity in herbicide MOAs to the system, which will assist in reducing the selection pressure for resistance to currently used postemergence herbicides and controlling ALS- and PSII-resistant B. tectorum populations.
This study is the first survey of B. tectorum management in winter wheat in northeastern Oregon. It will serve as a baseline for future studies focused on understanding the evolution and spread of resistance and how to mitigate it with potential alternative management strategies. Additionally, the survey findings can help regions facing similar challenges, such as the Great Plains, where B. tectorum control in dryland winter wheat is of concern (Lindell et al. Reference Lindell, Manuchehri, Kimura, Baughman and Basinger2024; Stahlman and Miller Reference Stahlman and Miller1990). Resistance to ALS inhibitors has previously been confirmed in B. tectorum in the Great Plains (Kumar and Jha Reference Kumar and Jha2017); however, an extensive survey including B. tectorum management practices information and formal resistance testing is lacking. The survey results presented herein highlight the main wheat cropping systems and B. tectorum management practices adopted in the dryland wheat-producing region of Oregon. The WW-SF rotation is the predominant cropping system northeastern Oregon where most of the fields are managed under no-tillage (Yorgey and Kruger Reference Yorgey, Kruger, Bista, Machado, Ghimire, Yorgey and Wysocki2017). Growers rely on ALS inhibitors for B. tectorum control in winter wheat and glyphosate in the summer fallow. Resistance screenings indicate that Oregon wheat fields have B. tectorum populations with resistance to ALS- and PSII-inhibiting herbicides, which are the most common MOAs currently used to control this species in winter wheat. The widespread occurrence of cross- and multiple-resistant populations to ALS and PSII inhibitors limits effective herbicide options for selective B. tectorum control in wheat.
The adoption of quizalofop-resistant wheat technology offers wheat growers a viable control option for ALS- and PSII-resistant B. tectorum. The integration of pyroxasulfone for B. tectorum management also provides another option for growers and assists in delaying resistance to currently used herbicides and controlling the resistant populations. Further research is needed to investigate the mechanisms of resistance to ALS inhibitors to increase understanding of cross-resistance patterns and multiple resistance to the PSII inhibitor metribuzin in this species. Moreover, future studies should consider the integration of nonchemical tactics, including crop rotation, cover crops, and harvest seed weed control technologies (chaff lining, seed destructor, etc.) for controlling B. tectorum seed banks in the dryland wheat cropping systems of northeastern Oregon.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/wsc.2024.52
Acknowledgments
The authors thank the Oregon Wheat Commission for funding this research, the wheat growers for participating in the survey and for allowing the collection of B. tectorum populations, and County Extension agents for their collaboration in identifying growers. The authors express appreciation to Phaedra Hinds-Cook for her technical assistance in the resistance screening experiments.
Funding statement
Funding from the Oregon Wheat Commission supported this work.
Competing interests
The authors declare no conflicts of interest.
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