Introduction
Palmer amaranth, a member of the Amaranthaceae, is native to northwestern Mexico, southern California, New Mexico, and Texas (Sauer Reference Sauer1957). It has been ranked the most troublesome and difficult-to-control weed in several agricultural and horticultural crops in the United States (Van Wychen Reference Van Wychen2017). Palmer amaranth interference and resultant yield losses have been documented in several crops, such as bell pepper (Capsicum annuum L.), cotton (Gossypium hirsutum L.), corn (Zea mays L.), grain sorghum (Sorghum bicolor L. Moench), muskmelon (Cucumis melo L.), peanut (Arachis hypogaea L.), pecan [Carya illinoinensis (Wangenh) K. Koch], soybean (Glycine max L.), sweet potato (Ipomoea batatas L. Lam.), tomato (Solanum lycopersicum L.), and watermelon [Citrullus lanatus (Thunb.) Matsum & Nakai] (Aulakh et al. Reference Aulakh, Price and Balkcom2011, Reference Aulakh, Price, Enloe, van Santen, Wehtje and Patterson2012, Reference Aulakh2013; Bertucci et al. Reference Bertucci, Jennings, Monks, Schultheis, Louws, Jordan and Brownie2019; Burke et al. Reference Burke, Schroeder, Thomas and Wilcut2007; Chahal et al. Reference Chahal, Varanasi, Jugulum and Jhala2017; Garvey et al. Reference Garvey, Meyers, Monks and Coble2013; Grichar Reference Grichar1997; Mayers et al. Reference Meyers, Jennings, Schultheis and Monks2010; Mohseni-Moghadam et al. Reference Mohseni-Moghadam, Schroeder, Heerema and Ashigh2013; Moore et al. Reference Moore, Murray and Westerman2004; Nerson Reference Nerson1989; Norsworthy et al. Reference Norsworthy, Griffith, Scott, Smith and Oliver2008; Price et al. Reference Price, Korres, Norsworthy and Li2018).
Palmer amaranth is extremely competitive and invasive because of C4 photosynthesis, high water use efficiency, rapid growth (0.10–0.21 cm growing degree-day−1), tall stature (≥2 m), and prolific seed production (>600,000 seeds plant−1) (Bensch et al. Reference Bensch, Horak and Peterson2003; Horak and Loughin Reference Horak and Loughin2000; Keeley et al. Reference Keeley, Carter and Thullen1987; Massinga et al. Reference Massinga, Currie, Horak and Boyer2001; Smith et al. Reference Smith, Baker and Steele2000; Sosnoskie et al. Reference Sosnoskie, Webster, Grey and Culpepper2014; Steckel Reference Steckel2007; Ward et al. Reference Ward, Webster and Steckel2013). Compared to waterhemp [Amaranthus rudis (L.) Sauer], redroot pigweed (A. retroflexus L.), and tumble pigweed (A. albus L.), Palmer amaranth produced the greatest plant dry weight, leaf area, and height, and had the fastest growth rate (Horak and Loughin Reference Horak and Loughin2000). Palmer amaranth and waterhemp are dioecious, whereas redroot pigweed, Powell amaranth, spiny amaranth (A. spinosis L.), and smooth pigweed (A. hybridus L.) are monoecious (Bryson and DeFelice Reference Bryson and DeFelice2010).
The most concerning characteristic of Palmer amaranth is its tendency to rapidly evolve herbicide resistance. The repeated use of a single herbicide with a specific site of action has led to the evolution of herbicide resistance in many weeds, including Palmer amaranth (Heap Reference Heap2020). Currently in the United States, Palmer amaranth is resistant to eight herbicide sites of action, including acetolactate-synthase inhibitors, enolpyruvyl shikimate-3-phosphate synthase (EPSPS) inhibitors, 4-hydroxyphenylpyruvate dioxygenase inhibitors, long-chain fatty acid inhibitors, microtubule inhibitors, photosystem II (PSII) inhibitors, protoporphyrinogen oxidase inhibitors, and synthetic auxins (Chahal et al. Reference Chahal, Varanasi, Jugulum and Jhala2017; Culpepper et al. Reference Culpepper, Grey, Vencill, Kichler, Webster, Brown, York, Davis and Hanna2006; Gossett et al. Reference Gossett, Murdock and Toler1992; Jhala et al. Reference Jhala, Sandell, Rana, Kruger and Knezevic2014; Salas et al. Reference Salas, Burgos, Tranel, Singh, Glasgow, Scott and Nichols2016; Sprague et al. Reference Sprague, Stoller, Wax and Horak1997; Thompson et al. Reference Thompson, Peterson and Lally2012). In addition, many Palmer amaranth populations are resistant to herbicides from more than one site of action (Heap Reference Heap2020). In the United States, Palmer amaranth resistant to acetolactate synthase (ALS)-inhibiting herbicides and glyphosate (an EPSPS inhibitor) is widespread (Heap Reference Heap2020). The first case of ALS-inhibitor resistant Palmer amaranth was reported in Kansas in 1993 (Horak and Peterson Reference Horak and Peterson1995). Burgos et al. (Reference Burgos, Kuk and Talbert2001) reported that Palmer amaranth biotypes resistant to imazaquin, an ALS-inhibiting herbicide, were cross-resistant to other ALS-inhibiting herbicides such as chlorimuron-ethyl, diclosulam, and pyrithiobac. Likewise, glyphosate-resistant Palmer amaranth was first discovered in Macon County, GA, in 2006 (Culpepper et al. Reference Culpepper, Grey, Vencill, Kichler, Webster, Brown, York, Davis and Hanna2006). As of 2020, glyphosate-resistant Palmer amaranth populations have been confirmed in 28 U.S. states (Heap Reference Heap2020). Some of these glyphosate-resistant Palmer amaranth biotypes required 115 times higher glyphosate application than the susceptible plants to achieve 50% control (Norsworthy et al. Reference Norsworthy, Griffith, Scott, Smith and Oliver2008; Steckel et al. Reference Steckel2008).
Palmer amaranth is the most recently identified pigweed species in Connecticut. It was first discovered in a pumpkin (Cucurbita pepo L.) field in Hartford County, CT, in the fall of 2019 (Aulakh Reference Aulakh2019). Palmer amaranth has also been confirmed in the neighboring northeastern states of Massachusetts and New York (USDA-APHIS 2020). Common pigweeds in the Amaranthaceae reported to occur in Connecticut are livid amaranth (A. blitum L.), Powell amaranth (A. powellii S. Watson), prostrate pigweed (A. blitoides S. Watson), redroot pigweed, smooth pigweed, spiny amaranth, waterhemp, and tumble pigweed. The Palmer amaranth biotype in Connecticut is believed to have been introduced from Massachusetts via contaminated farm equipment. As of 2019, Palmer amaranth infestation in Connecticut was reported on approximately 80 ha under pumpkin production. Because of widespread resistance in Palmer amaranth to ALS-inhibiting herbicides and glyphosate in the United States, it was necessary to understand the response of the newly discovered Palmer amaranth population in Connecticut to ALS-inhibitor and glyphosate herbicides. Therefore, Palmer amaranth seeds were collected randomly from multiple plants to conduct whole-plant dose-response bioassays with the objectives to (1) determine if the Connecticut Palmer amaranth was resistant to ALS-inhibitor and glyphosate herbicides and (2) evaluate the response of the Connecticut Palmer amaranth to POST herbicides from alternate sites of action commonly used in various cropland and noncropland areas in Connecticut.
Materials and Methods
Seed Collection and Preparation
In the fall of 2019, approximately 50 Palmer amaranth seed heads were collected randomly from a pumpkin field in Hartford County, CT (41.93°N, 72.53°W). The seed heads were manually threshed and seeds were cleaned thoroughly using a vertical air column blower and stored separately in airtight polyethylene bags at 5 C until used. The Connecticut biotype was designated “CT-Res.” In addition, seeds of a Palmer amaranth biotype from Kansas State University Agricultural Research Center near Hays, KS, with a known history of effective control with the field-use rate of glyphosate (designated KS-Sus) were included for comparison.
Single-Dose Experiments for Resistance Confirmation
Greenhouse experiments were conducted at the Connecticut Agricultural Experiment Station, Windsor, CT, to determine the response of the CT-Res biotype to field-use rates of glyphosate (840 g ae ha−1) and imazaquin (137 g ai ha−1) herbicides. Seeds of the CT-Res biotype were sown on the surface of flat trays (52 × 26 × 6 cm) containing a 2:1 composted pine bark to peat moss mixture. After emergence, Palmer amaranth seedlings were thinned to 50 seedlings tray−1. The plants were supplied with water and nutrients and kept in a greenhouse maintained at a 32/27 C day/night temperature regimen with a 16-h photoperiod supplemented by overhead sodium halide lamps. The study was conducted in a completely randomized design with two flat tray (50 seedlings tray−1) for each tested herbicide. Seedlings were treated with glyphosate (MADDOG®; Loveland Products, Inc., Loveland, CO) or imazaquin (Scepter®; BASF Corp., Research Triangle Park, NC) at the 5- to 6-leaf stage (8–10 cm tall). Each herbicide treatment was prepared in distilled water and mixed with a nonionic surfactant (Induce; Helena Chemical Co., Collierville, TN) at 0.25% vol/vol. Herbicide treatments were applied with a compressed CO2 backpack sprayer through a single flat-fan spray nozzle AIXR 8002 (TeeJet®; Spraying Systems Co., Wheaton, IL) calibrated to deliver 190 L ha−1 spray volume at 207 kPa and 3.5 kph. Palmer amaranth control was assessed visually at 7 and 14 d after treatment (DAT) on a scale ranging from 0% (no control) to 100% (complete control or death of plants). Control ratings were recorded on the basis of symptoms such as chlorosis, necrosis, and stunting of treated plants compared with nontreated control plants.
Dose-Response Bioassay
Whole-plant dose-response bioassays were conducted in a greenhouse at the Connecticut Agricultural Experiment Station, Windsor, CT, to determine the level of ALS-inhibitor and glyphosate resistance in the CT-Res biotype. Because Palmer amaranth was not known to exist in Connecticut before 2019, the known glyphosate susceptible biotype from Kansas (KS-Sus) was included for comparison. Plants from both CT-Res and KS-Sus biotypes were separately grown in plastic pots (10-cm diam) containing the same potting mixture as previously described. The study was laid out in a randomized complete block design with a 9 × 2 factorial arrangement of treatments and 21 replications (each replication was 1 plant pot−1). The two factors were (1) nine glyphosate or imazaquin rates (0, 0.125×, 0.25×, 0.5×, 1×, 2×, 4×, 8×, and 16×), where 1× is the field-use rate of glyphosate (840 g ae ha−1) or imazaquin (137 g ai ha−1) and (2) two Palmer amaranth biotypes (CT-Res and KS-Sus). Greenhouse conditions were maintained as described previously and the experiment was conducted five times under similar growing conditions.
Palmer amaranth seedlings were separately treated with different glyphosate or imazaquin rates at the 5- to 6-leaf stage (8–10 cm tall) as described in the single-dose experiments. Palmer amaranth control was assessed visually at 7 and 14 DAT using the same injury scale as described for the single-dose experiments. Control ratings were recorded on the basis of symptoms such as chlorosis, necrosis, and stunting of treated plants compared with nontreated control plants. Plants were harvested at 14 DAT, oven dried for 4 d at 65 C, and aboveground dry weight was determined. The biomass data were converted into percent biomass reduction compared to the nontreated control (Wortman Reference Wortman2014) as shown in Equation 1:
$$\;{\rm{Biomass\;reduction}}\;\left( \% \right) = \frac{{{\left( {\overline C - \left. B \right)} \right.}} \over {{\overline C}}} \times 100$$
where
$$\overline C$$
is the mean biomass of the nontreated control and B is the biomass of an individual treated plant.
Response to POST Herbicides
The responses of the CT-Res Palmer amaranth biotype to the selected POST herbicides were evaluated (Table 1). Plants were grown in greenhouse conditions at the Windsor Valley Laboratory of the Connecticut Agricultural Experiment Station using the same procedures as described previously. Two experimental runs were conducted in a completely randomized design with 21 replications. Palmer amaranth seedlings were treated with different herbicides, as described for the single-dose experiments. Visual control estimates of Palmer amaranth were recorded at 7 and 21 DAT on a scale of 0% to 100% as described for the dose-response study. Plants were cut at the soil surface at 21 DAT and oven dried for 4 d at 65 C, and dry biomass was recorded. Percent biomass reduction of treated plants was calculated using Equation 1.
Table 1. Details of POST herbicides used in the greenhouse study conducted at the Valley Laboratory in Windsor, CT, to determine response of glyphosate-resistant Palmer amaranth
Statistical Analyses
Palmer amaranth visual control and biomass reduction data collected at 14 DAT only were used for estimating the regression parameters. A three-parameter log-logistic model (Equation 2) was used to estimate the effective dose of glyphosate or imazaquin needed to control each Palmer amaranth biotype by 50% (ED50) and 90% (ED90) using the drc package (drc 2.3 in R 3.1.0 (R statistical software; R Foundation for Statistical Computing, Vienna, Austria) (Knezevic et al. Reference Knezevic, Streibig and Ritz2007):
$$Y{\rm{\;}} = {\rm{\;}}\frac{{d} \over {{1 + \exp \left[ b \right.{\rm{\;}}\left( {logx - \left. {loge} \right)}} \right.]}}$$
where Y is the percent visual control or percent aboveground biomass reduction, x is the herbicide rate, d is the upper limit, e is the ED50 or ED90 values, and b represents the relative slope around the parameter e. The level of resistance was calculated by dividing the ED90 value of the resistant biotype (CT-Res) by that of the susceptible biotype (KS-Sus).
Data from POST herbicides study were subjected to ANOVA using the PROC GLIMMIX procedure in SAS, version 9.3 (SAS Institute Inc., Cary, NC). Percent control and biomass reduction data were analyzed excluding the nontreated control as well as the treatments containing ALS-inhibitor herbicides. Herbicide treatments, experimental run, and their interactions were considered fixed effects, whereas replication was considered a random effect in the model. Before analysis, data were tested for normality using PROC UNIVARIATE and homogeneity of variance with the modified Levene test. The ANOVA requirements of normality and homogeneity of variance assumptions were met; therefore, no data transformation was needed. When the F test was significant (P ≤ 0.05), percent control and biomass reduction means were separated using the Fisher LSD test at P = 0.05.
Results and Discussion
Single-Dose Experiments to Determine Resistance Concern
The preliminary single-dose bioassay revealed complete control failure for the CT-Res biotype with glyphosate applied at 840 g ae ha−1 and imazaquin applied at 137 g ai ha−1 (data not shown). The treated plants exhibited no chlorotic, necrotic, or stunting injury compared to the nontreated control plants. Response of the CT-Res biotype to glyphosate and imazaquin dose-response bioassays are discussed separately in the following paragraphs.
Glyphosate Dose-Response Bioassay
Experiment run-by-treatment interactions for Palmer amaranth control (P = 0.121) and biomass reduction (P = 0.193) were not significant; therefore, data were pooled over experimental runs. Glyphosate applied at the labeled field-use rate (840 g ae ha−1) controlled the KS-Sus biotype by 96%, whereas the CT-Res biotype was controlled only 10%. To achieve 50% and 90% control, the CT-Res biotype required respective glyphosate rates of 1,593 g ae ha−1 and 4,204 g ae ha−1, almost 2- and 5-fold higher than the labelled field-use rate (Table 2). Almost similar ED50 (1,778 g ae ha−1) and ED90 (4,713 g ae ha−-1) values were observed for reduction in aboveground biomass of the CT-Res biotype. In contrast, the KS-Sus biotype required 69 g ae ha−1 and 460 g ae ha−1 for 50% and 90% reduction in biomass, respectively (Figure 1A; Table 2). On the basis of percent visual control or biomass reduction (ED90 values), the CT-Res biotype manifested a 10-fold glyphosate resistance compared with the KS-Sus biotype. Similar levels of glyphosate resistance have been reported with respect to Palmer amaranth biomass reduction from Kansas, Mississippi, and Nebraska (Chahal et al. Reference Chahal, Varanasi, Jugulum and Jhala2017; Kumar et al. Reference Kumar, Liu, Boyer and Stahlman2019; Kumar et al. Reference Kumar, Liu and Stahlman2020; Nandula et al. Reference Nandula, Reddy, Kroger, Poston, Rimando, Duke, Bond and Ribeiro2012). In contrast, Mohseni-Moghadam et al. (Reference Mohseni-Moghadam, Schroeder, Heerema and Ashigh2013) reported a lower resistance level (7-fold) in glyphosate-resistant Palmer amaranth biotype from New Mexico. On the basis of ED90 values for biomass reduction, the level of glyphosate resistance in CT-Res biotype is 2.6- and 3.6-fold lower than reported in the Arkansas (ED90 =12,500 g ae ha-1) and Nebraska (ED90 =16,797 g ae ha−1) biotypes, respectively (Chahal et al. Reference Chahal, Varanasi, Jugulum and Jhala2017; Norsworthy et al. Reference Norsworthy, Griffith, Scott, Smith and Oliver2008).
Table 2. Estimates of Regression Parameters and Herbicide Dose Required for 50% and 90% Visual Control and Biomass Reduction of Palmer Amaranth Biotypes at 14 d After Treatment in a Greenhouse Whole-Plant Dose Response Study Conducted at the Valley Laboratory, Windsor, CT
a Abbreviations: DAT, days after treatment; CT-Res, resistant Palmer amaranth biotype found in Hartfield Co., Connecticut; ED50, effective herbicide dose required for 50% biomass reduction or visual control at 14 DAT; ED90, effective herbicide dose required for 90% biomass reduction or visual control at 14 DAT; KS-Sus, Palmer amaranth biotype collected from Kansas State University Agricultural Research Center near Hays, KS; NA, not applicable (ED50, ED90, or resistance level could not be determined).
b Regression parameters b and d of a three-parameter log-logistic model were obtained using the nonlinear least-square function of R statistical software.
c Resistance level was calculated by dividing the ED90 value of the CT-Res biotype by that of the KS-Sus biotype.
Figure 1. Dose-response curves for the CT-Res and KS-Sus biotypes. (A) Percent biomass reduction at 14 d after glyphosate application, and (B) percent biomass reduction at 14 d after imazaquin application, in greenhouse whole-plant dose-response studies conducted at the Windsor Valley Laboratory. Percent biomass reduction was calculated using Equation 1 in the text. CT-Res, resistant Palmer amaranth biotype found in Hartfield Co., Connecticut; KS-Sus, Palmer amaranth biotype collected from Kansas State University Agricultural Research Center near Hays, KS.
Imazaquin Dose-Response Bioassay
Experiment run-by-treatment interactions for Palmer amaranth control (P = 0.096) and biomass reduction (P = 0.133) were not significant; therefore, data were pooled over the experimental runs. A known ALS-inhibitor susceptible biotype was not available for comparison. This lack of a susceptible biotype is not surprising, given that ALS-inhibitor–resistant Palmer amaranth has become widespread in the United States due to the continuous use of ALS-inhibiting herbicides in corn, cotton, and soybean. Therefore, the KS-Sus biotype, which was suspected to be ALS-inhibitor resistant, was used for comparison in this study.
The CT-Res biotype demonstrated a very high level of resistance to imazaquin. Imazaquin applied at 16× (i.e., 2,196 g ai ha−1) the effective field-use rate controlled the CT-Res biotype only 18%. Comparatively, the KS-Sus biotype was controlled 50% and 90% with imazaquin rates of 194 g ai ha−1 and 727 g ai ha−1, respectively (Table 2). Biomass reduction data indicated similar levels of imazquin resistance in the CT-Res and KS-Sus biotypes as observed with visual control estimates (Figure 1B; Table 2). Previously, Gossett and Toler (Reference Gossett and Toler1999) reported 80% control of the ALS-susceptible Palmer amaranth with imazaquin applied at 140 g ai ha−1. The imazaquin herbicide label also indicates control of 15-cm tall ALS-susceptible Palmer amaranth at 137 g ai ha−1. This confirms that both the CT-Res and KS-Sus biotypes were ALS-inhibitor herbicide resistant. Furthermore, Burgos et al. (Reference Burgos, Kuk and Talbert2001) observed that Palmer amaranth biotypes resistant to imazaquin were also cross-resistant to chlorimuron-ethyl, dichosulam, and pyrithiobac herbicides.
Our results from the response to POST herbicides (Table 3) also indicated that the CT-Res biotype was cross-resistant to other ALS-inhibiting herbicides. Chlorimuron-ethyl (13.1 g ai ha−1), halosulfuron-methyl (70 g ai ha−1), and sulfometuron-methyl (392 g ai ha−1) had no effect on visual control and biomass reduction of the CT-Res biotype compared to the nontreated control. Currently, multiple resistance to glyphosate and ALS-inhibitors has been confirmed in several Palmer amaranth biotypes in the United States (Heap Reference Heap2020; Kumar et al. Reference Kumar, Liu, Boyer and Stahlman2019, Reference Kumar, Liu and Stahlman2020).
Table 3. Glyphosate-resistant Palmer amaranth control with POST herbicides at 7 and 21 DAT and biomass reduction at 21 DAT
a Nonionic surfactant (Induce; Helena Chemical Co., Collierville, TN) at 0.25% vol/vol was added to 2,4-D, dicamba, chlorimuron-ethyl, clopyralid, glufosinate, halosulfuron-methyl, lactofen, mesotrione, oxyfluorfen, and sulfometuron-methyl. Crop oil concentrate (Agridex; Helena Chemical Co.) at 1% vol/vol was added to atrazine and carfentrazone-ethyl herbicides.
b Means within columns with no common letter(s) are significantly different according to Fisher protected LSD test where P ≤ 0.05.
c Percent visual control and biomass reduction data from the nontreated control, and treatments with zero visual control or biomass reduction were not included in analysis. Biomass reduction was calculated on the basis of comparison of the average biomass of the nontreated control, using Equation 1 in the text.
Response to POST Herbicides
Experimental run-by-treatment interactions for Palmer amaranth control (P = 0.271) and biomass reduction (P = 0.183) were not significant; therefore, data from both experiment runs were combined. Because the DAT main effect was significant, data were analyzed by DAT. The glutamine synthetase–inhibitor (glufosinate) and protoporphyrinogen oxidase (PPO)-inhibitors (carfentrazone-ethyl, lactofen, and oxyfluorfen) provided the quickest control of the CT-Res biotype compared with all other POST herbicides tested in this study (Table 3). At 7 DAT, the CT-Res biotype was controlled 90% or more with carfentrazone-ethyl (34 g ai ha−1), glufosinate (595 g ai ha−1), lactofen (220 g ai ha−1), and oxyfluorfen (1,121 g ai ha−1) herbicides. By 21 DAT, the CT-Res biotype was completely controlled (100%) with glufosinate and all PPO-inhibitors tested in this study. A similar trend was observed in biomass reduction of the CT-Res biotype at 21 DAT (Table 3).
These results are consistent with the findings of Kumar et al. (Reference Kumar, Liu, Boyer and Stahlman2019), who reported complete control (100% at 21 DAT) of a five-way resistant (2,4-D, atrazine, chlorsulfuron, glyphosate, and mesotrione) Palmer amaranth from Kansas with glufosinate in a greenhouse study. Similarly, glufosinate applied at a concentration of 409 g ai ha−1 or greater provided 93% or better control of Palmer amaranth up to 10 cm tall in studies conducted by Aulakh et al. (Reference Aulakh, Price and Balkcom2011) and Corbett et al. (Reference Corbett, Askew, Thomas and Wilcut2004). Mohseni-Moghadam et al. (Reference Mohseni-Moghadam, Schroeder, Heerema and Ashigh2013) in New Mexico obtained more than 94% biomass reduction of two Palmer amaranth biotypes with carfentrazone-ethyl, glufosinate, or oxyfluorfen at rates similar to those used in the present study. Likewise, Palmer amaranth was controlled greater than 85% with lactofen applied at 220 g ai ha−1 (Jhala et al. Reference Jhala, Sandell, Rana, Kruger and Knezevic2014; Sweat et al. Reference Sweat, Horak, Peterson, Lloyd and Boyer1998). Aulakh et al. (Reference Aulakh2016) observed complete control of waterhemp with lactofen applied at 220 g ai ha−1.
Auxinic herbicides such as 2,4-D and dicamba provided greater control than clopyralid at both 7 and 21 DAT. In this study, control of the CT-Res biotype ranged from 60% to 75% with clopyralid, 73% to 88% with 2,4-D, and 78% to 92% with dicamba at 7 and 21 DAT, respectively. Similar visual control and biomass reduction with 2,4-D and dicamba have been reported in Palmer amaranth biotypes from Nebraska and New Mexico (Chahal et al. Reference Chahal, Varanasi, Jugulum and Jhala2017; Jhala et al. Reference Jhala, Sandell, Rana, Kruger and Knezevic2014; Mohseni-Moghadam et al. Reference Mohseni-Moghadam, Schroeder, Heerema and Ashigh2013). Furthermore, mesotrione, a 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor, controlled the CT-Res biotype 70% and 82% at 7 and 21 DAT, respectively. Biomass reduction at 21 DAT was similar to the corresponding percent visual control. Mesotrione is widely used in seed corn and sweet corn because of high tolerance of corn varieties to these herbicides, a wide weed-control spectrum, flexibility for application timings, and compatibility for tank mixes with other herbicides (Bollman et al. Reference Bollman, Boerboom, Becker and Fritz2008; McMullan and Green Reference McMullan and Green2011). Atrazine, a PSII-inhibitor, controlled the CT-Res biotype less than 55% in this study. Previous studies have shown high variation in Palmer amaranth control with atrazine. Chahal et al. (Reference Chahal, Varanasi, Jugulum and Jhala2017) reported less than 25% control with atrazine applied at the same rate tested in this study (2,240 g ai ha−1). In contrast, Jhala et al. (Reference Jhala, Sandell, Rana, Kruger and Knezevic2014) observed 45% to 73% visual control and 44% to 68% biomass reduction 21 DAT in two Palmer amaranth biotypes with atrazine applied POST at 560 g ai ha−1. However, Palmer amaranth was completely controlled with atrazine at concentrations of at least 2,150 g ai ha−1 in studies conducted by Mohseni-Moghadam et al. (Reference Mohseni-Moghadam, Schroeder, Heerema and Ashigh2013), Norsworthy et al. (Reference Norsworthy, Griffith, Scott, Smith and Oliver2008), and Salas et al. (Reference Salas, Burgos, Tranel, Singh, Glasgow, Scott and Nichols2016).
ALS inhibitors (i.e., chlorimuron-ethyl, halosulfuron-methyl, imazaquin, and sulfometuron-methyl) did not affect visual control and biomass reduction in the CT-Res biotype compared to the nontreated control. Burgos et al. (Reference Burgos, Kuk and Talbert2001) reported Palmer amaranth biotypes resistant to imazaquin were also cross-resistant to chlorimuron-ethyl, dichosulam, and pyrithiobac herbicides. Chahal et al. (Reference Chahal, Varanasi, Jugulum and Jhala2017) observed less than 40% control with chlorimuron-ethyl and halosulfuron-methyl. Alternatively, some researchers reported 69% or higher control Palmer amaranth with similar rates of chlorimuron-ethyl, halosulfuron-methyl, imazaquin, and sulfometuron-methyl (Gossett and Toler Reference Gossett and Toler1999; Jhala et al. Reference Jhala, Sandell, Rana, Kruger and Knezevic2014).
Practical Implications
The newly reported Palmer amaranth biotype from Hartford County, CT, is resistant to both ALS-inhibitor herbicides and glyphosate. The CT-Res biotype was controlled only 18% with imazaquin applied at 2,196 g ai ha−1, which is 16-fold higher than the labelled imazaquin rate (137 g ai ha−1) for Palmer amaranth control. Furthermore, the CT-Res biotype required five times more glyphosate (4,204 g ae ha−1) for 90% visual control compared to the KS-Sus biotype. Presence of ALS-and glyphosate-resistant Palmer amaranth in Connecticut is a serious management concern for specialty crops producers because of limited POST herbicide options. The response of the CT-Res biotype to POST herbicides suggests the array of control options will vary with the crop. For instance, the CT-Res biotype was controlled 75% or better with an HPPD inhibitor (mesotrione), PPO inhibitors (carfentrazone-ethyl, lactofen, oxyfluorfen), glutamine synthetase inhibitor (glufosinate), and synthetic auxins (2,4-D, clopyralid, and dicamba) in this study. This suggests that adequate effective POST herbicides exist for use in corn grown for forage, seed, popcorn (Z. mays L var. Everta), and sweet corn (Z. mays L. var. rugosa). However, in most vegetables, such as bell peppers, cucurbits (Cucurbita spp.), and tomatoes (Solanum lycopersicum L.), and small fruits, such as blackberry (Rubus fructicosus L.), blueberry (Vaccinium corymbosum L.), and raspberry (R. idaeus L.), cultivation, hand weeding, or directed applications of POST herbicides such as carfentrazone-ethyl, glufosinate, or paraquat are the only viable alternatives for control of Palmer amaranth resistant to ALS inhibitors and glyphosate. Moreover, mesotrione is labelled for use as a prebloom POST directed spray in blackberry, blueberry, and raspberry and can still be used to control herbicide-resistant Palmer amaranth biotypes. The CT-Res biotype was not completely controlled with recommended field-use rates of some of the POST herbicides tested in this study. Therefore, additional control tactics, including cultural and physical methods, should be integrated to improve Palmer amaranth control and prevent replenishment of the soil seedbank.
The CT-Res biotype also demonstrated reduced sensitivity to a PSII-inhibitor herbicide (atrazine). Furthermore, Palmer amaranth resistant to long-chain fatty acid–inhibitor and microtubule inhibitor herbicides is also present in the United States (Heap Reference Heap2020). These concerns warrant the need for additional dose-response bioassays to determine the response of the CT-Res biotype to herbicides from these sites-of-action groups.
Acknowledgements
The authors acknowledge Emily Syme, Skip Hobbs, and Jim Preste for their help in this project. Partial funding was received from the Experiment Station Associates and is greatly appreciated. No conflicts of interest on this project have been declared.
