Introduction
Kochia [Bassia scoparia (L.) A.J. Scott] is a problematic summer annual broadleaf weed across cropland and noncropland areas in the U.S. Great Plains (Kumar et al. Reference Kumar, Jha, Jugulam, Yadav and Stahlman2019b). Bassia scoparia can emerge early in the spring and exhibits an extended emergence period from mid-February through mid-June (Dille et al. Reference Dille, Stahlman, Du, Geier, Riffel, Currie, Wilson, Sbatella, Westra, Kniss and Moechnig2017; Kumar et al. Reference Kumar, Jha, Dille and Stahlman2018). Small B. scoparia seedlings have leaves densely covered in short, fine, white hairs (Friesen et al. Reference Friesen, Beckie, Warwick and Van Acker2009). These leaf hairs can reduce foliar absorption of herbicides by suspending droplets above the leaf cuticle, which can reduce the effectiveness of herbicides (Friesen et al. Reference Friesen, Beckie, Warwick and Van Acker2009). Bassia scoparia is highly invasive and tolerant to various abiotic stresses (heat, cold, drought, and salinity) (Christoffoleti et al. Reference Christoffoleti, Westra and Moore1997; Friesen et al. Reference Friesen, Beckie, Warwick and Van Acker2009). Its flowers are inconspicuous (without petals) and protogynous (stigmas emerge before anther development), facilitating cross-pollination between plants. A single B. scoparia plant can produce >100,000 seeds dispersed over long distances by wind via a tumbling mechanism (Friesen et al. Reference Friesen, Beckie, Warwick and Van Acker2009; Kumar et al. Reference Kumar, Jha, Jugulam, Yadav and Stahlman2019b).
Bassia scoparia is a highly competitive weed in agronomic crops because of its early emergence and stress tolerance. It can cause yield reductions in many field crops, including corn (Zea mays L.), grain sorghum [Sorghum bicolor (L.) Moench ssp. bicolor], soybean [Glycine max (L.) Merr.], sugar beet (Beta vulgaris L.), sunflower (Helianthus annuus L.), alfalfa (Medicago sativa L.), canola (Brassica napus L.), and spring cereals (Geddes and Sharpe Reference Geddes and Sharpe2022; Kumar and Jha Reference Kumar and Jha2015; Lewis and Gulden Reference Lewis and Gulden2014; Wicks et al. Reference Wicks, Martin, Haack and Mahnken1994, Reference Wicks, Martin and Hanson1997). The amount of yield loss depends on the B. scoparia density. For instance, B. scoparia density of 184 plants m−2 caused a 95% yield reduction in grain sorghum (Wicks et al. Reference Wicks, Martin, Haack and Mahnken1994). Similarly, yield losses of 23% to 77% in soybean at 20 to 135 plants m−2 (Geddes and Sharpe Reference Geddes and Sharpe2022; Wicks et al. Reference Wicks, Martin and Hanson1997), 60% in sugar beet at 268 plants m−2 (Kumar and Jha Reference Kumar and Jha2015), and 62% to 95% in sunflower at 34 to 905 plants m−2 (Lewis and Gulden Reference Lewis and Gulden2014) have been reported.
Repeated use of herbicides with the same site of action (SOA) has resulted in the evolution of herbicide-resistant B. scoparia (Kumar et al. Reference Kumar, Jha, Jugulam, Yadav and Stahlman2019b; Sharpe et al. Reference Sharpe, Leeson, Geddes, Willenborg and Beckie2023; Westra et al. Reference Westra, Nissen, Getts, Westra and Gaines2019). Currently, B. scoparia populations have evolved resistance to five different SOAs globally, including inhibitors of acetolactate synthase (ALS) (Group 2), photosystem II (Group 5), 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) (Group 9), protoporphyrinogen oxidase (Group 14), and synthetic auxins (Group 4) (Heap Reference Heap2024). Since the first discovery of glyphosate-resistant (GR) B. scoparia in western Kansas in 2007 (Godar et al. Reference Godar, Stahlman, Jugulam and Dille2015; Wiersma et al. Reference Wiersma, Gaines, Preston, Hamilton, Giacomini, Buell, Leach and Westra2015), GR populations have been reported from 10 states in the United States and 4 provinces across the Canadian prairies (Beckie et al. Reference Beckie, Blackshaw, Low, Hall, Sauder, Martin, Brandt and Shirriff2013; Godar et al. Reference Godar, Stahlman, Jugulam and Dille2015; Kumar et al. Reference Kumar, Jha, Jugulam, Yadav and Stahlman2019b; Sharpe et al. Reference Sharpe, Leeson, Geddes, Willenborg and Beckie2023; Westra et al. Reference Westra, Nissen, Getts, Westra and Gaines2019). Multiple herbicide resistance to ALS inhibitors, glyphosate, dicamba, or fluroxypyr has been reported from the northern and central Great Plains, including Kansas, Colorado, Montana, as well as the provinces of Alberta, Manitoba, and Saskatchewan in Canada (Beckie et al. Reference Beckie, Hall, Shirriff, Martin and Leeson2019; Geddes et al. Reference Geddes, Pittman, Hall, Topinka, Sharpe, Leeson and Beckie2022b; Heap Reference Heap2024; Kumar et al. Reference Kumar, Jha, Giacomini, Westra and Westra2015; Westra et al. Reference Westra, Nissen, Getts, Westra and Gaines2019). Furthermore, a single B. scoparia population with multiple resistance to chlorsulfuron (Group 2), dicamba (Group 4), atrazine (Group 5), and glyphosate (Group 9) has previously been reported from Kansas (Heap Reference Heap2024). The status of distribution and frequency of multiple herbicide–resistant (MHR) B. scoparia in the southcentral Great Plains (SGP), especially Oklahoma and Texas, is unknown. Therefore, the objectives of this study were to (1) determine the frequency and distribution of resistance to atrazine, chlorsulfuron, dicamba, fluroxypyr, and glyphosate among B. scoparia populations from Kansas, Oklahoma, and Texas; and (2) characterize the level of resistance to glyphosate, dicamba, and/or fluroxypyr in selected populations from Kansas, Oklahoma, and Texas.
Materials and Methods
Seed Source
Mature seeds from a total of 82 different B. scoparia populations were randomly collected during the fall of 2018 and 2019 from pasture, wheat (Triticum aestivum L.) stubble, fallow, corn, cotton (Gossypium hirsutum L.), and grain sorghum production fields in western Kansas (n = 19), western Oklahoma (n = 13), and the Texas High Plains (n = 50) (Supplementary Table 1). At each site, branches with seeds from B. scoparia were collected from 15 to 20 different plants and bulked. All the collection sites for these populations were georeferenced. Seeds were manually threshed from each population and cleaned with a combination of sieves and an air-column blower and stored in plastic bags at 4 C before greenhouse experiments were conducted.
Table 1. Selected putative multiple herbicide–resistant and susceptible (SUS) Bassia scoparia populations from Kansas (KS), Oklahoma (OK), and Texas (TX) for dose–response experiment.
Single-Dose Experiment
Experiments were conducted in a greenhouse at Kansas State University Agricultural Research Center (KSU-ARC) near Hays, KS, during the fall of 2021 and repeated in spring of 2022. Seeds from each B. scoparia population were separately sown (10 seeds per cell) in 50-cell plastic trays (54 by 28 by 10 cm) containing commercial potting mixture (Miracle-Gro® Moisture Control® Potting Mix, Miracle-Gro Lawn Products, Scottslawn Road, Marysville, OH). Greenhouse conditions were maintained at 25/23 ± 3 C day/night temperature and a 16/8-h (day/night) photoperiod supplemented with metal-halide lamps (560 μmol m−2 s−1). Bassia scoparia seedlings (2- to 3-cm tall) from each population were transplanted in 50-cell plastic trays (54 by 28 by 10 cm) (1 seedling per cell) containing the same potting mixture described earlier. Seedlings were watered daily to maintain sufficient moisture for growth. Experiments were arranged in a completely randomized design with 50 replications of individual plants, with each tray with 50 seedlings from each population separately treated with either glyphosate (Roundup PowerMax®, Bayer Crop Science, St Louis, MO) at 1,260 g ae ha−1 plus 20 g L–1 ammonium sulfate (AMS), chlorsulfuron (Glean® XP, FMC, Philadelphia, PA) at 26 g ai ha−1 with 0.25% v/v nonionic surfactant (NIS), atrazine (AAtrex® 4L, Syngenta Crop Protection, Greensboro, NC) at 1,120 g ai ha−1 with 1% v/v crop oil concentrate, dicamba (Clarity®, BASF, Research Triangle Park, NC) at 560 g ae ha−1 with 0.25% v/v NIS, fluroxypyr (Starane® Ultra, Corteva Agriscience, Indianapolis, IN) at 228 g ae ha−1, and a premixture of dichlorprop/dicamba/2,4-D (Scorch® EXT, Nufarm, Alsip, IL) at 374/186/186 g ae ha−1 with 0.25% v/v NIS. A tray of seedlings from each population was treated when plants were 6- to 9-cm tall. All herbicides were applied using a stationary cabinet spray chamber (Research Track Sprayer, DeVries, Hollandale, MN), equipped with an even flat-fan nozzle (TeeJet® 8001EXR, Spraying Systems, Wheaton, IL) calibrated to deliver 132 L ha−1 of spray solution at 241 kPa. At 28 d after treatment (DAT), the survival frequency (number of live seedlings/total number of seedlings treated) was determined for each population for each herbicide. A treated plant was considered dead if the plant showed chlorosis, necrosis, epinasty, stem curling/swelling, and no new regrowth at 28 DAT. We used categories previously described by Owen et al. (Reference Owen, Walsh, Llewellyn and Powles2007) and Westra et al. (Reference Westra, Nissen, Getts, Westra and Gaines2019) to classify tested B. scoparia populations as either susceptible (<2% survival), having low resistance (2% to 19% survival), or resistant (≥20% survival) to the respective herbicide applied.
Dose–Response Experiments
Based on the results from the single-dose experiment, two putative MHR B. scoparia populations (resistant to dicamba, fluroxypyr, and glyphosate) from Kansas (KS-9 and KS-14) and two from Oklahoma (OK-10 and OK-11) were selected for further characterization (Table 1). Seeds of the MHR B. scoparia populations (with the highest survival frequency to glyphosate and dicamba) from Texas showed poor seed germination; thus, TX-1 (resistant to dicamba and glyphosate) and TX-13 (resistant to glyphosate only) populations were selected for dose–response assays. Similarly, SUS populations (susceptible to glyphosate, dicamba, and fluroxypyr) from Kansas (KS-SUS), Oklahoma (OK-SUS), and Texas (TX-SUS) were also selected (Supplementary Table 1). Greenhouse experiments were conducted as described earlier at KSU-ARC near Hays, KS, in the fall of 2023 and repeated in the spring of 2024. Field-collected seeds for MHR and SUS B. scoparia populations from each respective state were separately sown in 50-cell plastic trays (54 by 28 by 10 cm) containing the same potting mixture as described earlier. Bassia scoparia seedlings (2- to 3-cm tall) from each population were transplanted in 10 by 10 cm square plastic pots (1 seedling per pot) containing the same potting mixture as described earlier. Bassia scoparia seedlings were watered daily to maintain optimum growth. Experiments were conducted in a randomized complete block design with 12 replications (1 plant per pot indicates one replication). Dicamba, fluroxypyr, and glyphosate dose–response experiments were conducted for Kansas and Oklahoma populations and only dicamba and glyphosate dose–response experiments were conducted for Texas populations. Dicamba doses were 0, 140, 280, 560 (field use rate in fallow), 1,120, 2,240, 4,480, 8,960, and 17,920 g ha−1. The tested doses for fluroxypyr were 0, 57, 114, 228 (field use rate in wheat), 456, 912, 1,824, and 3,648 g ae ha−1, and glyphosate doses were 0, 315, 630, 1,260 (field use rate in fallow), 2,520, 5,040, 10,080, and 20,160 g ha−1. The NIS at 0.25% v/v with dicamba and AMS at 20 g L–1 with glyphosate were used. All selected herbicide doses were applied to 6- to 9-cm-tall B. scoparia for each population using a stationary cabinet spray chamber as described earlier. Percent visual control ratings were recorded at 7, 14, and 28 DAT based on injury symptoms corresponding to each herbicide, such as chlorosis, epinasty (curling, twisting, and cupping), and necrosis of B. scoparia seedlings on a scale of 0% (no control) to 100% (complete control). At 28 DAT, each plant was clipped at the soil surface, placed in a paper bag, and dried at 65 C for 4 d to obtain shoot dry biomass. Dry biomass data from each replication were converted to percent reduction of biomass using Equation 1:
$${\rm{Shoot}}\;{\rm{dry}}\;{\rm{biomass}}\;{\rm{reduction}}\;\left( \% \right) = \left[ {{{C - T} \over C}} \right] \times \;100$$
where C is the shoot dry biomass from the nontreated check treatment (average of 12 replications), and T is the shoot dry biomass from a treated pot.
Statistical Analyses
The collection sites for 82 B. scoparia populations were mapped using QGIS (v. 3.22, Open Source Geospatial Foundation Project, http://qgis.org) to visualize the spatial distribution of resistance for each herbicide and state. Data from dose–response experiments for percent visual control and reduction in shoot dry biomass were subjected to ANOVA using the PROC MIXED procedure in SAS 9.3 (SAS Institute, Cary, NC). The fixed effects were experimental run, herbicide dose, populations, and their interactions. Replications and all interactions involving replication were considered random effects. The data followed all the ANOVA assumptions as tested by PROC UNIVARIATE in SAS. The experimental run-by-herbicide interaction for each population within the state was nonsignificant (P-value >0.05); therefore, data were pooled across experimental runs for each tested population within a state. Data on percent visual control and reduction of shoot dry biomass for each B. scoparia population were regressed over herbicide doses using a three-parameter log-logistic model in R software (Ritz et al. Reference Ritz, Baty, Streibig and Gerhard2015).
$$Y = \left\{ {d/1 + \exp \left[ {b\left( {\log x - \log e} \right)} \right]} \right\}$$
where Y is percent control or percent reduction in shoot dry biomass, d is maximum percent control or biomass reduction (upper asymptote, fixed to 100%), b is slope, x is herbicide dose, and e represents herbicide dose needed for 50% control or shoot dry biomass reduction (referred to as LD50 or GR50 values, respectively). All nonlinear regression parameters and LD90 or GR90 values (herbicide dose required for 90% B. scoparia control or shoot dry biomass reduction, respectively) were estimated using the drc package in R software (Ritz et al. Reference Ritz, Baty, Streibig and Gerhard2015). The resistance index for each MHR population was calculated by dividing the LD50 or GR50 value by the LD50 or GR50 value of the respective SUS B. scoparia population for that state.
Results and Discussion
Single-Dose Experiment
The resistance frequency for atrazine ranged from 0% to 100%, with 33% of B. scoparia populations classified as resistant, 51% as low resistant, and 16% as susceptible (Figure 1; Table 2). All tested populations were classified as resistant to chlorsulfuron, with a survival frequency ranging from 30% to 100%. The proportion of dicamba resistance among tested B. scoparia populations was as follows: 48% classified as resistant, 28% classified as low resistant, and 24% susceptible, with a survival frequency ranging from 0% to 96% (Figure 1; Table 2). For fluroxypyr, 30% of the total tested populations were classified as resistant, 40% as low resistant, and 30% as susceptible, with survival frequency ranging from 0% to 100% (Figure 1; Table 2). Fluroxypyr resistance in B. scoparia populations has been reported in several states of the United States (Colorado, Kansas, Montana, North Dakota, and Nebraska) and in the provinces of Alberta and Saskatchewan in Canada (Geddes et al. Reference Geddes, Ostendorf, Owen, Leeson, Sharpe, Shirriff and Beckie2021; Heap Reference Heap2024; Howatt and Ciernia Reference Howatt and Ciernia2014; Kumar et al. Reference Kumar, Currie, Jha and Stahlman2019a; LeClere et al. Reference LeClere, Wu, Westra and Sammons2018; Sharpe et al. Reference Sharpe, Leeson, Geddes, Willenborg and Beckie2023; Todd et al. Reference Todd, Patterson, Westra, Nissen, Araujo, Kramer, Dayan and Gaines2024). Geddes et al. (Reference Geddes, Ostendorf, Owen, Leeson, Sharpe, Shirriff and Beckie2021) reported fluroxypyr resistance in 13% of tested populations from Alberta, Canada. Percent survival or resistance frequency to glyphosate in all tested B. scoparia populations ranged from 0% to 97%, with 70% of populations (57 out of 82 total) classified as resistant to glyphosate, 8% classified as low resistant, and 22% as susceptible based on the classification criteria described by Owen et al. (Reference Owen, Walsh, Llewellyn and Powles2007) and Westra et al. (Reference Westra, Nissen, Getts, Westra and Gaines2019) (Figure 1; Table 2). The premixture of dichlorprop/dicamba/2,4-D provided complete control of all populations. These results are consistent with those of Dhanda et al. (Reference Dhanda, Kumar, Geier, Currie, Dille, Obour, Yeager and Holman2023), who previously reported effective control (84% to 90% control) of MHR B. scoparia with a dichlorprop-p/dicamba/2,4-D mixture.
Figure 1. Distribution of Bassia scoparia populations resistant to atrazine (A), chlorsulfuron (B), dicamba (C), fluroxypyr (D), glyphosate (E), and dichlorprop/dicamba/2,4-D (F) in western Kansas, western Oklahoma, and Texas High Plains. Populations were classified in three categories based on survival %: <2% = susceptible, 2% to 19% = low resistant, and 20% to 100% = resistant.
Table 2. The number and percentage of a total of 82 Bassia scoparia populations categorized as susceptible, low-level resistant, and resistant to each tested herbicide. a
a Populations were classified in three categories based on survival %; <2% = susceptible, 2% to 19% = low resistant, and 20% to 100% = resistant (Owen et al. Reference Owen, Walsh, Llewellyn and Powles2007; Westra et al. Reference Westra, Nissen, Getts, Westra and Gaines2019).
Considering both resistant and low-resistant categories, 84% of the total tested populations were found to be resistant to atrazine, 100% to chlorsulfuron, 76% to dicamba, 70% to fluroxypyr, and 78% to glyphosate (Table 2). These results indicate a widespread distribution of resistance in B. scoparia populations to atrazine, chlorsulfuron, dicamba, fluroxypyr, and glyphosate in the SGP region. About 12% of the total tested populations were found to be three-way resistant (≥20% survival) to atrazine, chlorsulfuron, and glyphosate, while 15% of the populations were four-way resistant to atrazine, chlorsulfuron, dicamba, and glyphosate (Figure 2). Only two B. scoparia populations from Texas were found to be five-way resistant to atrazine, chlorsulfuron, dicamba, fluroxypyr, and glyphosate (Figure 2). Three- to four-way resistance to atrazine, chlorsulfuron, glyphosate, and/or dicamba/fluroxypyr in B. scoparia populations from Kansas has also been reported previously (Kumar et al. Reference Kumar, Jha, Jugulam, Yadav and Stahlman2019b; Varanasi et al. Reference Varanasi, Godar, Currie, Dille, Thompson, Stahlman and Jugulam2015).
Figure 2. Distribution of multiple herbicide–resistant Bassia scoparia populations (out of 82 total) in western Kansas, western Oklahoma, and Texas High Plains.
These results are consistent with Beckie et al. (Reference Beckie, Hall, Shirriff, Martin and Leeson2019), who reported ALS-inhibitor resistance in all the surveyed B. scoparia populations from Alberta, Canada. They also found that 50% of the tested populations were resistant to glyphosate, 18% were resistant to dicamba, and 10% were MHR to ALS inhibitors, dicamba, and glyphosate. Geddes et al. (Reference Geddes, Pittman, Hall, Topinka, Sharpe, Leeson and Beckie2022b) documented that 41% of tested B. scoparia populations from Alberta, Canada were resistant to both dicamba and fluroxypyr. Geddes et al. (Reference Geddes, Owen, Ostendorf, Leeson, Sharpe, Shirriff and Beckie2022a) also reported resistance to glyphosate, fluroxypyr, and dicamba in 78%, 44%, and 28% of the tested B. scoparia populations, respectively. Results from a field survey conducted in Montana from 2013 to 2016 indicated that 45 field sites had GR B. scoparia, 15 sites had dicamba-resistant B. scoparia, and 10 sites had both glyphosate- and dicamba-resistant B. scoparia (Kumar et al. Reference Kumar, Jha, Jugulam, Yadav and Stahlman2019b). Westra et al. (Reference Westra, Nissen, Getts, Westra and Gaines2019) reported glyphosate and dicamba resistance in 39% to 60% and 28% to 45% of surveyed B. scoparia populations from Colorado, respectively. In that same study, multiple resistance to dicamba and glyphosate was also reported in 14% to 20% of surveyed B. scoparia populations (Westra et al. Reference Westra, Nissen, Getts, Westra and Gaines2019). Results from the present study indicate an increasing rate of resistance in B. scoparia, as 15% of the populations were found to be four-way resistant to atrazine, chlorsulfuron, dicamba, and glyphosate.
Dose–Response Experiments
Kansas Populations
Based on the percent control, both MHR B. scoparia populations (KS-9 and KS-14) were 4- to 8-fold resistant to dicamba, 2- to 4-fold resistant to fluroxypyr, and 3- to 4-fold resistant to glyphosate compared with the SUS population (Figure 3; Table 3). Based on percent shoot dry biomass reduction, both MHR populations from KS (KS-9 and KS-14) exhibited 5- to 10-fold resistance to dicamba, 3- to 6-fold resistance to fluroxypyr, and 4- to 5-fold resistance to glyphosate compared with the SUS population (Figure 3; Table 4). Dicamba at 7,485 g ha−1 and 3,489 g ha−1 was needed to achieve 90% control for the KS-9 and KS-14 populations, respectively, compared with only 514 g ha−1 for the SUS population (Figure 3; Table 3). Similarly, about 5 to 7 times higher doses of fluroxypyr and 5 to 6 times higher doses of glyphosate were needed to obtain 90% control of both MHR populations compared with the SUS population. Consistent with percent control, dicamba at 8,677 g ha−1 and 3,436 g ha−1 was needed to achieve 90% shoot dry biomass reduction of KS-9 and KS-14 populations, respectively, compared with 544 g ha−1 for the SUS population (Figure 3; Table 4). Fluroxypyr at 1,787 g ha−1 for KS-9 and 2,757 g ha−1 for KS-14 was needed to achieve 90% reduction in shoot dry biomass, compared with 253 g ha−1 for the SUS population (Figure 3; Table 4). Similarly, glyphosate at 3,635 g ha−1 for KS-9 and 4,603 g ha−1 KS-14 was needed to achieve 90% shoot dry biomass compared with 646 g ha−1 of glyphosate for the SUS population (Figure 3; Table 4). These results are consistent with those of Kumar et al. (Reference Kumar, Currie, Jha and Stahlman2019a), who previously reported 3- to 15-fold resistance to dicamba and 3- to 9-fold resistance to fluroxypyr from Kansas. Similarly, Godar et al. (Reference Godar, Stahlman, Jugulam and Dille2015) also reported 4- to 11-fold resistance to glyphosate in B. scoparia populations from Kansas.
Figure 3. Percent control (A) and shoot dry biomass reduction (B) response of Bassia scoparia populations to various doses of dicamba, fluroxypyr, and glyphosate. KS-9 and KS-14 were multiple herbicide–resistant B. scoparia populations and KS-SUS was the susceptible population collected from western Kansas.
Table 3. Regression parameter estimates (Equation 1) for percent control of multiple herbicide–resistant (MHR) and susceptible (SUS) Bassia scoparia populations from western Kansas, western Oklahoma, and Texas High Plains to dicamba, fluroxypyr, and glyphosate under separate dose–response experiments, and calculated resistance index. a
a Abbreviations: KS-9 and KS-14, MHR populations from Kansas; OK-10 and OK-11, MHR populations from Oklahoma; TX-1 and TX-13, MHR populations from Texas; CI, confidence interval; LD50, effective dose (g ha−1) required for 50% B. scoparia control.
b Resistance index is the ratio of LD50 of the resistant population to LD50 of the susceptible B. scoparia population from each respective state.
Table 4. Regression parameter estimates (Equation 1) for percent shoot dry biomass reduction of multiple herbicide–resistant (MHR) and susceptible (SUS) Bassia scoparia populations from western Kansas (KS), western Oklahoma (OK), and Texas High Plains (TX) to dicamba, fluroxypyr, and glyphosate under separate dose–response experiments and calculated resistance index. a
a Abbreviations: KS-9 and KS-14, MHR populations from Kansas; OK-10 and OK-11, MHR populations from Oklahoma; TX-1 and TX-13, MHR populations from Texas; CI, confidence interval; GR50, effective dose (g ha−1) required for 50% shoot dry biomass reduction.
b Resistance index is the ratio of GR50 of the resistant population to GR50 of the susceptible B. scoparia population from each respective state.
Oklahoma Populations
Based on percent control, the selected MHR populations (OK-10 and OK-11) exhibited 8- to 12-fold resistance to dicamba, 3- to 4-fold resistance to fluroxypyr, and 3- to 4-fold resistance to glyphosate compared with the SUS population (Figure 4; Table 3). Consistent with percent control, the MHR populations were found to be 10- to 13-fold resistant to dicamba, 3- to 4-fold resistant to fluroxypyr, and 3- to 4-fold resistant to glyphosate, based on shoot dry biomass reduction compared with the SUS population (Figure 4; Table 4). Dicamba at 5,628 g ha−1 and 5,963 g ha−1 was needed to achieve 90% control of OK-10 and OK-11 populations, respectively, compared with only 341 g ha−1 for the SUS population (Figure 4; Table 3). Similarly, about 6- to 7-fold higher doses of fluroxypyr and glyphosate were needed to obtain 90% control of both MHR populations from OK compared with the SUS population. Consistent with percent control, 7,527 g ha−1 and 9,629 g ha−1 of dicamba were needed to achieve 90% shoot dry biomass reduction of OK-10 and OK-11, respectively, compared with 427 g ha−1 for the SUS population (Figure 4; Table 4). Fluroxypyr at 1,191 g ha−1 and 891 g ha−1 was needed to achieve 90% shoot dry biomass reduction of OK-10 and OK-11, respectively, compared with 179 g ha−1 for the SUS population. Similarly, 4,934 to 6,045 g ha−1 of glyphosate were needed to achieve 90% shoot dry biomass reduction of both MHR population compared with 674 g ha−1 for the SUS population (Figure 4; Table 4). These results indicate a low-level resistance to glyphosate and fluroxypyr (up to 4-fold) and a moderate to high-level resistance to dicamba (up to 13-fold) among MHR populations from Oklahoma. To date, B. scoparia populations from Oklahoma have only been reported with resistance to ALS inhibitors (chlorsulfuron, metsulfuron-methyl, and sulfometuron-methyl) and to glyphosate (Heap Reference Heap2024). To our knowledge, the current study revealed the first case of multiple resistance to glyphosate, dicamba, and fluroxypyr in B. scoparia populations from Oklahoma. Fallow-based cropping systems in western Oklahoma, such as winter wheat–fallow and winter wheat–summer crop–fallow are prevalent, and repeated applications of glyphosate and dicamba alone or in mixtures are commonly used for weed control in the fallow phase and at postharvest time (Manuchehri et al. Reference Manuchehri, Sanders and Bushong2019). This might have resulted in strong selection pressure and, ultimately, the evolution of herbicide resistance in B. scoparia populations. Previous studies have reported 3- to 13-fold glyphosate resistance in B. scoparia populations collected from Kansas, Colorado, North Dakota, and South Dakota (Godar et al. Reference Godar, Stahlman, Jugulam and Dille2015; Wiersma et al. Reference Wiersma, Gaines, Preston, Hamilton, Giacomini, Buell, Leach and Westra2015). Jha et al. (Reference Jha, Kumar and Lim2015) reported 1.5- to 6.8-fold resistance to dicamba and 1.6- to 4.0-fold resistance to fluroxypyr in B. scoparia populations from Montana.
Figure 4. Percent control (A) and shoot dry biomass reduction (B) response of Bassia scoparia populations to various doses of dicamba, fluroxypyr, and glyphosate. OK-10 and OK-11 were multiple herbicide–resistant B. scoparia populations and OK-SUS was the susceptible population collected from western Oklahoma.
Texas Populations
Based on the percent control and shoot dry biomass reduction, two MHR populations (TX-1 and TX-13) were found 2- to 4-fold resistant to dicamba, and one population (TX-1) was 5-fold resistant to glyphosate compared with the SUS population (Figure 5; Tables 3 and 4). Dose–response studies further revealed that dicamba at 2,272 g ha−1 for TX-1 and 1,064 g ha−1 for TX-13 was required to achieve 90% control compared with 557 g ha−1 of dicamba for the SUS population. Similarly, a 4-fold higher dose of glyphosate was needed to obtain 90% control of the MHR population from Texas compared with the SUS population (Table 3). Consistent with percent control, dicamba at 2,542 g ha−1 for TX-1 and 1,838 g ha−1 for TX-13 was needed to achieve 90% shoot dry biomass reduction compared with 604 g ha−1 of dicamba for the SUS population (Table 4). For glyphosate, 1,655 g ha−1 was required to achieve 90% shoot dry biomass reduction of the TX-1 population compared with the 347 g ha−1 rate for the SUS population. These results revealed a low-level resistance to glyphosate and dicamba in selected MHR populations from Texas. Previous studies also reported GR B. scoparia from the Great Plains, including Colorado (Wiersma et al. Reference Wiersma, Gaines, Preston, Hamilton, Giacomini, Buell, Leach and Westra2015), Kansas (Godar et al. Reference Godar, Stahlman, Jugulam and Dille2015), Montana (Kumar et al. Reference Kumar, Jha and Reichard2014), Wyoming (Gaines et al. Reference Gaines, Barker, Patterson, Westra, Westra, Wilson, Jha, Kumar and Kniss2016), Nebraska (Rana and Jhala Reference Rana and Jhala2016), North Dakota (Heap Reference Heap2024), Oklahoma (Heap Reference Heap2024), and South Dakota (Heap Reference Heap2024), as well as the Canadian provinces of Alberta, Manitoba, and Saskatchewan (Beckie et al. Reference Beckie, Gulden, Shaikh, Johnson, Willenborg, Brenzil, Shirriff, Lozinski and Ford2015; Hall et al. Reference Hall, Beckie, Low, Shirriff, Blackshaw, Kimmel and Neeser2014; Heap Reference Heap2024). LeClere et al. (Reference LeClere, Wu, Westra and Sammons2018) reported B. scoparia populations with multiple resistance to dicamba (8-fold) and fluroxypyr (13-fold) from western Nebraska. Westra et al. (Reference Westra, Nissen, Getts, Westra and Gaines2019) reported multiple resistance to dicamba and glyphosate in 14% to 20% of surveyed B. scoparia populations from Colorado. Glyphosate resistance has been reported from 10 U.S. states, including Colorado, Idaho, Kansas, Montana, Nebraska, North Dakota, Oklahoma, Oregon, South Dakota, and Wyoming, whereas dicamba resistance has been reported from 6 U.S. states, including Colorado, Idaho, Kansas, Montana, Nebraska, and North Dakota (Heap Reference Heap2024). To date, herbicide resistance in B. scoparia populations from Texas has only been reported for metsulfuron-methyl (Heap Reference Heap2024). The present study confirms the first report of multiple resistance to glyphosate and dicamba in B. scoparia populations from the High Plains of Texas.
Figure 5. Percent control (A) and shoot dry biomass reduction (B) response of Bassia scoparia populations to various doses of dicamba and glyphosate. TX-1 and TX-13 were multiple herbicide–resistant B. scoparia populations and TX-SUS was the susceptible population collected from Texas High Plains.
Results from this survey revealed widespread occurrence of multiple resistance to atrazine, chlorsulfuron, dicamba, and glyphosate among B. scoparia populations from the SGP region. Resistance to fluroxypyr was also evident among one-third of the tested populations. Evolution of GR and dicamba-resistant kochia across Kansas, Oklahoma, and Texas warrants significant challenge to dicamba-tolerant crops (cotton, soybeans, etc.). These results suggest that sole reliance on these herbicides for B. scoparia control should be avoided to prevent further evolution and spread of MHR B. scoparia in the region. This study also highlights the critical need for proactive stewardship and adoption of diversified weed control strategies to preserve the long-term effectiveness of these herbicides. Adoption of effective fall- or spring-applied preemergence herbicides with multiple modes of action in conjunction with integrated weed management approaches becomes critical to mitigate the further evolution and spread of multiple resistance among B. scoparia populations (Kumar et al. Reference Kumar, Currie, Jha and Stahlman2019a).
In the present study, some populations showed low levels of resistance (2- to 3-fold); therefore, it is important to further quantify and identify the underlying mechanism(s) of resistance. However, several previous studies have shown that higher EPSPS gene copy number conferred glyphosate resistance in B. scoparia populations (Gaines et al. Reference Gaines, Barker, Patterson, Westra, Westra, Wilson, Jha, Kumar and Kniss2016; Godar et al. Reference Godar, Stahlman, Jugulam and Dille2015; Kumar et al. Reference Kumar, Currie, Jha and Stahlman2019a; Wiersma et al. Reference Wiersma, Gaines, Preston, Hamilton, Giacomini, Buell, Leach and Westra2015). Pettinga et al. (Reference Pettinga, Ou, Patterson, Jugulam, Westra and Gaines2018) reported that a greater synthesis of flavanols would compete with the intercellular transport of dicamba molecules, thus impairing dicamba translocation and resulting in dicamba resistance in B. scoparia. LeClere et al. (Reference LeClere, Wu, Westra and Sammons2018) reported a point mutation within a highly conserved region of an AUX/IAA protein that conferred cross-resistance to dicamba, 2,4-D, and fluroxypyr in B. scoparia. Future studies will investigate the underlying mechanism(s) of resistance and fitness penalty in these MHR B. scoparia populations from the SGP region. Declining effective postemergence herbicide options with the widespread evolution of MHR B. scoparia populations warrant the development of integrated weed management strategies that may include diversified competitive crop rotations, occasional or strategic tillage, fall- or spring-planted cover crops, use of effective preemergence herbicides, herbicide rotations, and multiple herbicide SOAs to delay the further evolution and spread of multiple herbicide resistance among B. scoparia populations in the SGP region.
Supplementary material
To view supplementary material for this article, please visit https://doi.org/10.1017/wsc.2024.88
Acknowledgments
The authors thank Rui Liu, Taylor Lambert, and Mathew Vredenburg for their assistance in conducting greenhouse studies.
Funding statement
The authors thank Nufarm US and Texas State Support Committee–Cotton Incorporated for providing partial financial support to conduct this work.
Competing interests
The authors declare no conflicts of interest.
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