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Glyphosate-resistant Palmer amaranth (Amaranthus palmeri) confirmed in Georgia

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A glyphosate-resistant Palmer amaranth biotype was confirmed in central Georgia. In the field, glyphosate applied to 5- to 13-cm-tall Palmer amaranth at three times the normal use rate of 0.84 kg ae ha-1 controlled this biotype only 17%. The biotype was controlled 82% by glyphosate at 12 times the normal use rate. In the greenhouse, /50 values (rate necessary for 50% inhibition) for visual control and shoot fresh weight, expressed as percentage of the nontreated, were 8 and 6.2 times greater, respectively, with the resistant biotype compared with a known glyphosate-susceptible biotype. Glyphosate absorption and translocation and the number of chromosomes did not differ between biotypes. Shikimate was detected in leaf tissue of the susceptible biotype treated with glyphosate but not in the resistant biotype.
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... It is a dioecious species, therefore its reproductive biology favors the dispersion and recombination of alleles, speeding up herbicide resistance evolution along withother traits. For instance, the transfer of glyphosate resistance from resistant males to susceptible female plants by pollen has been demonstrated up to a distance of 300 m (Culpepper et al., 2006). In addition, A. palmeri can hybridize with other species of the genus (Franssen et al., 2001;Gaines et al., 2012;Trucco et al., 2007). ...
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Abstract: Background: Amaranthus palmeri S. Watson is a serious problem in soybean crops from Argentina. This weed has evolved high level of resistance to (ALS)-inhibiting herbicides mainly due to a target-site mechanism by an altered ALS enzyme. In an A. palmeri population from Totoras with cross-resistance to (ALS)-inhibiting herbicides, six allelic versions of the ALS enzyme were identified. Objective: The aim of this study was to evaluate plants from that resistant population carrying the ALS substitutions A122S, D376E or A205V, which had not been characterized before for this species. Methods: Subpopulations with each substitution were produced by a vegetative cloning procedure or by cross-pollination and dose response assays and herbicide single-dose tests were performed to evaluate in vivo resistance levels to (ALS)-inhibiting herbicides. Results: Dose-response experiments showed that all the resistant subpopulations survived at the highest doses tested (32 X) for imidazolinones, triazolopyrimidines and sulfonylureas, while the susceptible population was completely controlled at considerably lower doses. Furthermore, an analysis of the novel A122S substitution showed that it provides cross-resistance to five classes of (ALS)-inhibiting herbicides, excluding the entire ALS herbicide group as an effective control tool in weed populations carrying this substitution. Conclusions: The results indicated that D376E, A205V and A122S substitutions found for the first time in A. palmeri confer cross-resistance to the most used chemical families from herbicide group 2. Interestingly, it was confirmed that the A205V substitution conferred resistance to herbicides in the triazolopyrimidines family. Data generated should be considered in management strategies for delaying the spread of resistance.
... Ten years after the introduction of GR crops in the USA, the first case of GR in an AP population was reported in Georgia [Culpepper et al., 2006;Molin et al., 2017]. Various molecular mechanisms conferring GR in AP were reported [Gaines et al., 2011]. ...
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In agriculture, various chemicals are used to control the weeds. Out of which, glyphosate is an important herbicide invariably used in the cultivation of glyphosate-resistant crops to control weeds. Overuse of glyphosate results in the evolution of glyphosate-resistant weeds. Evolution of glyphosate resistance (GR) in Amaranthus palmeri (AP) is a serious concern in the USA. Investigation of the mechanism of GR in AP identified different resistance mechanisms of which 5-enolpyruvylshikimate-3-phosphate synthase ( EPSPS ) gene amplification is predominant. Molecular analysis of GR AP identified the presence of a 5- to >160-fold increase in copies of the EPSPS gene than in a glyphosate-susceptible (GS) population. This increased copy number of the EPSPS gene increased the genome size ranging from 3.5 to 11.8%, depending on the copy number compared to the genome size of GS AP. FISH analysis using a 399-kb EPSPS cassette derived from bacterial artificial chromosomes (BACs) as probes identified that amplified EPSPS copies in GR AP exist in extrachromosomal circular DNA (eccDNA) in addition to the native copy in the chromosome. The EPSPS gene-containing eccDNA having a size of ∼400 kb is termed EPSPS -eccDNA and showed somatic mosacism in size and copy number. EPSPS -eccDNA has a genetic mechanism to tether randomly to mitotic or meiotic chromosomes during cell division or gamete formation and is inherited to daughter cells or progeny generating copy number variation. These eccDNAs are stable genetic elements that can replicate and exist independently. The genomic characterization of the EPSPS locus, along with the flanking regions, identified the presence of a complex array of repeats and mobile genetic elements. The cytogenomics approach in understanding the biology of EPSPS -eccDNA sheds light on various characteristics of EPSPS -eccDNA that favor GR in AP.
... Thus, in Mexico, the deep tillage along with manual removal of early weeds, in-row cultivation, and crop rotation have apparently delayed the appearance of glyphosate-resistant weeds despite the fact that GM cotton technology has been adopted for more than 15 years (CIBIOGEM, 2018). In contrast, in the United States the first case of Palmer amaranth A. palmeri resistant to glyphosate was reported in 2005 (Culpepper et al., 2006), only 8 years after this technology was adopted (Norswhorty et al., 2016). ...
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The present study aimed to determine whether glyphosate-induced oxidative stress is directly related to the action mechanism of this herbicide (5-enolpyruvylshikimate-3-phosphate synthase or EPSPS inhibition) and analyse the role of oxidative stress in glyphosate toxicity of the weed Amaranthus palmeri S. Wats. Two kinds of populations were studied using EPSPS amplification: glyphosate-sensitive and glyphosate-resistant (by gene amplification). Plants were grown hydroponically and treated with different glyphosate doses, after which several oxidative stress markers were measured in the leaves. Untreated, sensitive and resistant plants showed similar values for the analysed parameters. Treated glyphosate-sensitive plants showed an increase in shikimate, superoxide and H2O2 contents and dose-dependent lipid peroxidation and antioxidant responses; however, none of these effects were observed in resistant plants, indicating that glyphosate-induced oxidative stress is related to EPSPS inhibition. Oxidative stress is associated with an increase in the activity of peroxidases due to EPSPS inhibition, although the link between both processes remains elusive. The fact that some glyphosate doses were lethal but did not induce major oxidative damage provides evidence that glyphosate toxicity is independent of oxidative stress.
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Background: Glyphosate-resistant invasive plants, including Amaranthus palmeri S. Watson, have greatly challenged management of new invasions. Elucidating their glyphosate resistance levels rapidly and accurately will better inform management strategies. Quantitative real-time PCR (qPCR) has been used to identify glyphosate resistance in A. palmeri by detecting gene copy numbers of 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), an enzyme inhibited by glyphosate. However, qPCR can only indirectly determine copy numbers because it requires a calibrator sample; it also lacks standardization, thus limiting its usefulness. Droplet digital PCR (ddPCR) is a new method to detect copy number directly and precisely. We evaluated ddPCR as a tool to determine glyphosate-resistance level while using qPCR and glyphosate dose response (GDR) assays as reference technologies to compare performance and efficiency between methods. Results: We identified seven susceptible and seven resistant populations of A. palmeri using the GDR assay. Resistant levels detected by qPCR and ddPCR were generally consistent with the GDR results. Although detected values obtained by qPCR and ddPCR were highly correlated (R² =0.94), ddPCR results had a lower proportion of non-ideal values (36%) with better accuracy (100%) and specificity (100%) than those of qPCR results. Conclusions: Our findings demonstrate that ddPCR offers improved accuracy and specificity in detecting EPSPS gene copy numbers and is a robust and rapid method for glyphosate-resistance identification in A. palmeri. Our research is the first to measure glyphosate resistance in A. palmeri by ddPCR assay and will shed light on future applications of ddPCR in identifying herbicide resistance in other invasive weeds.
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Palmer amaranth ( Amaranthus palmeri S. Watson) is not native to Africa. Based on the presence and persistence of A. palmeri populations, its invasive status in southern Africa is classified as “naturalized”. Globally, A. palmeri is one of the most troublesome weed species in several crops including soybean [ Glycine max (L.) Merr.], maize ( Zea mays L.), and cotton ( Gossypium hirsutum L.). Certain populations of A. palmeri in various countries were reported to be resistant to herbicides with different sites-of-action (SOAs). Two biotypes of A. palmeri in the United States of America reportedly each have resistance to herbicides representing five different SOAs, and between them a total of eight different SOAs are involved. Resistance mechanisms in these biotypes involve target site and/or non-target site resistance. Here we characterize a specific A. palmeri population that was found in Douglas district in South Africa and showed resistance to various herbicide SOAs. Initially, this A. palmeri population was discovered in a glyphosate-tolerant cotton field, where it survived glyphosate treatment. Subsequently, greenhouse experiments were conducted to characterize this A. palmeri population for potential resistance to herbicides of additional SOAs, and molecular analyses were conducted to reveal the mechanisms of herbicide resistance. Results indicated resistance to chlorimuron ethyl and glyphosate in this population, while <90% control (decreased sensitivity) was observed at the label rate for mesotrione, atrazine, saflufenacil, and s -metolachlor. However, glufosinate, tembotrione, acifluorfen, dicamba, 2,4-D, metribuzin, acetochlor, isoxaflutole, diflufenican and pyroxasulfone were effective at controlling this population. This profiling of herbicide sensitivity has allowed development of programs to control and potentially minimize the spread of this weed. In addition, molecular analysis of EPSPS revealed the role of higher copy number as a mechanism for glyphosate resistance in this population, and a Ser-653-Asn target site mutation likely conferring resistance to the ALS-inhibitor chlorimuron-ethyl. No known target site mutations were identified for the PPO-inhibitor group.
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Amaranthus palmeri S. Watson (Palmer amaranth) is an invasive agricultural weed that has quickly risen from a state of relative obscurity to now being globally regarded as one of the most economically destructive and difficult to manage weed species. It is now found in more than 45 countries where it poses a serious threat to agricultural production systems. Amaranthus palmeri is known to aggressively compete against crop plants for resources such as light, space, nutrients and soil moisture, all of which can result in significant crop yield reduction or even lead to crop failure. It has also been reported that A. palmeri is highly prone to evolve herbicide resistance; this makes management exceedingly challenging. Whilst there have been several control approaches introduced to manage the spread and impact of A. palmeri, many of them require more specific and focused research for their successful local and widespread application. In this regard, this global review explores the species’ biology and global distribution patterns, together with previous and current management strategies. It also explores and identifies promising areas of research that still require further investigation to more confidently assist in the control and containment of this globally concerning weed.
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Gene copy number variation (CNV) has been increasingly associated with organismal responses to environmental stress, but we know little about the quantitative relation between CNV and phenotypic variation. In this study we quantify the relation between variation in EPSPS (5‐enolpyruvylshikimate‐3‐phosphate synthase) copy number using digital drop PCR and variation in phenotypic glyphosate resistance in 22 populations of Amaranthus palmeri (Palmer Amaranth), a range‐expanding agricultural weed. Overall, we detected a significant positive relation between population mean copy number and resistance. The majority of populations exhibited high glyphosate resistance yet maintained low‐resistance individuals, resulting in bimodality in many populations. We also investigated threshold models for the relation between copy number and resistance, and found evidence for a threshold of ~15 EPSPS copies: there was a steep increase in resistance below the threshold, followed by a much shallower increase. Across 924 individuals, as copy number increases the range of variation in resistance decreases, yielding an increasing frequency of high phenotypic resistance individuals. Among populations we detected a decline in variation (s.d.) as mean phenotypic resistance increased from moderate to high, consistent with the prediction that as phenotypic resistance increases in populations, stabilizing selection decreases variation in the trait. Our study demonstrates that populations of A. palmeri can harbour wide variation in EPSPS copy number and phenotypic glyphosate resistance, reflecting the history of, and template for future, resistance evolution.
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A naturally occurring prickly lettuce biotype resistant to a 5:1 formulated mixture of chlorsulfuron:metsulfuron (DPX-G8311) was identified in a no-till winter wheat field near Lewiston, ID, in April, 1987. Field and greenhouse studies were established to evaluate its resistance to other sulfonylureas, imidazolinones, and herbicides with alternate sites of action. The resistant biotype resisted eight sulfonylurea herbicides; resisted the imidazolinone herbicides, imazapyr and imazethypyr, but not imazaquin; and resisted no other herbicides included in the studies. The resistant biotype was identified in seven of nine fields on the farm where it was discovered.
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The impact of Palmer amaranth on mechanical harvesting, ginning, and fiber quality in dryland cotton was documented. Only the highest Palmer amaranth density (3,260 weeds/ha) reduced lint and seed yields. However, all weed densities increased harvesting time 2- to 3.5-fold. Two factors increased the time required for stripper harvesting: slower ground speeds due to large weeds and work stoppages that required hand removal of weed stems lodged in the harvester. Ninety-eight percent of the weedy plant material was discarded in the field by the harvester, and the remaining 2% was successfully removed in ginning and lint-cleaning processes. Weed infestations did not result in any differences in moisture content of seed cotton, ginning time, fiber quality, or the percentage of cleaned lint.
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Four field experiments were conducted in Oklahoma to measure full-season Palmer amaranth interference on cotton lint yield and fiber properties. Density of the weed ranged from 0 to 12 plants 10 m-1 of row. Cotton lint yield vs. weed density fit a linear model for densities ≤ 8 weeds row-1 at Perkins and Chickasha in 1996 and at Altus in 1997. At Perkins in 1997, all densities fit a linear model. For each increase of 1 weed row-1, lint yield reductions were 62 kg ha-1 (or 10.7%) and 58 kg ha-1 (or 11.5%) at Perkins and at Chickasha in 1996, respectively. At Perkins and Altus in 1997, for each 1 weed row-1, lint yield was reduced 71 kg ha-1 (or 5.9%) and 112 kg ha-1 (or 8.7%), respectively. Lint yield vs. end-of-season weed volume fit a linear model except at Altus in 1997. For each increase of 1 m3 of weed plot-1, cotton lint yield in 1996 was reduced by 1.6 and 1.5% at Perkins and Chickasha, respectively. In 1997 at Perkins and Altus (≤ 6 weeds), each increase of 1 m3 of weed plot-1 reduced lint yield 1.6 and 2.3%, respectively. Lint yield vs. end-of-season weed biomass fit a linear model in all four experiments. Lint yield was reduced 5.2 to 9.3% for each increase of 1 kg of weed biomass plot-1. Fiber analyses revealed significant differences for micronaire (fiber fineness) among weed densities in two experiments, marginal significance in a third, and none in a fourth. An intermediate number of weeds often resulted in improved fiber micronaires in these environments. No other fiber properties were influenced by weed density.
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Palmer amaranth ( Amaranthus palmeri S. Wats. # AMAPA) planted in a field at monthly intervals from March through October at Shafter, CA, began to emerge in March when soil temperatures at a depth of 5 cm reached 18 C. With the exception of March and April plantings, at least 50% of the seed of later plantings produced seedlings within 2 weeks after planting. Although growth of plants was initially slower for early plantings, plantings from March to July reached 2 m or greater in height by fall. Due to longer growing times, plantings from March to June eventually produced more dry matter and a greater number of inflorescences than later plantings. Plants began flowering 5 to 9 weeks after planting in March through June and 3 to 4 weeks after planting in July through October. Some viable seed was produced as early as 2 to 3 weeks after flowering began. Total seed production in the fall ranged from 200 000 to 600 000 seed/plant for the March through June plantings, and 115 to 80 000 seed/plant for the July through September plantings. Killing frosts in November prevented Palmer amaranth planted in October from producing seed.
Article
Herbicides are important components of weed management programs for most Kansas farmers. Monocropping systems and repeated use of the same or similar herbicides in some areas of the state have resulted in the development of herbicide-resistant weeds. The development of herbicide-resistant weed populations can have an immediate and a long-term effect on the cost, implementation, and effectiveness of weed control programs. In Kansas, resistance to triazine herbicides has been confirmed in kochia (Kochia scoparia), redroot pigweed, common waterhemp (Amaranthus rudis), Palmer amaranth (Amaranthus palmeri), and downy brome (Bromus tectorum) populations, and resistance to acetolactate synthase (ALS)-inhibiting herbicides has been confirmed in kochia, Russian thistle (Salsola kali), common waterhemp, Palmer amaranth, common cocklebur (Xanthium strumarium), shattercane (Sorghum bicolor), and common sunflower (Helianthus annuus). The frequency and distribution of herbicide resistance varies among species. Producers who experience herbicide resistance problems adjust their weed control program accordingly. Producers that have not encountered an herbicide resistance problem tend to continue with a successful herbicide program until it fails. The recommended management strategies for herbicide-resistant weed populations include an integrated system of crop rotation, rotation of herbicide modes of action, tank-mixes of herbicides with different modes of action, and cultivation. The greatest direct cost to the producer occurs during the first year of poor weed control. The first response to an herbicide failure often is to reapply the same herbicide that has worked well previously. By the time the producer realizes that the treatment is not going to work, it usually is too late for any other remedial action. Consequently, the farmer experiences reduced crop production from weed competition, high herbicide costs, and a tremendous increase in the seed bank. The increase in seed bank may cost the farmer the most in the long run because the increased weed pressure often requires an intensified control program for several years.
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A 2-yr field study was conducted at Fayetteville, AR, to determine the effect of Palmer amaranth interference on soybean growth and yield. Palmer amaranth density had little effect on soybean height, but soybean canopy width ranged from 77 cm in the weed-free check to 35 cm in plots with 10 plants m –1 of row 12 wk after emergence. Soybean yield reduction was highly correlated to Palmer amaranth biomass at 8 wk after emergence and maturity, soybean biomass at 8 wk after emergence, and Palmer amaranth density. Soybean yield reduction was 17, 27, 32, 48, 64, and 68%, respectively, for Palmer amaranth densities of 033, 0.66, 1, 2, 333, and 10 plants m –1 of row. Soybean yield reduction and Palmer amaranth biomass were linear to approximately 2 Palmer amaranth m –1 of row, suggesting intraspecific interference between adjacent Palmer amaranth is initiated at Palmer amaranth densities between 2 and 3.33 plants m –1 of row.
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An experiment conducted at three locations in North Carolina during 1996 and 1997 compared weed control and cotton (Gossypium hirsutum L.) yield, fiber quality, and net returns from glyphosate [N-(phosphonomethyl)glycine]-tolerant cotton treated with various glyphosate and traditional herbicide systems. The standard system of trifluralin [2,6-dinitro-N,N-dipropyl-4-(trifluoromethyl) benzenamine] preplant incorporated and fluometuron {N,N - dimethyl - N' - [3 - (trifluoromethyl)phenyl]urea} preemergence followed by fluometuron plus MSMA (monosodium methanearsonate) postemergence directed 3 to 4 weeks after planting and cyanazine {2-[[4-chloro-6-(ethylamino -1,3,5-triazin-2-yl]amino]-2-methylpropanenitrile} plus MSMA postemergence directed 6 to 7 weeks after planting controlled large crabgrass [Digitaria sanguinalis (L.) Scop.], common cocklebur (Xanthium strumarium L.), common lambsquarters (Chenopodium album L.), common ragweed (Ambrosia artemisiifolia L.), Amaranthus species, Ipomoea species, prickly sida (Sida spinosa L.), and sicklepod [Senna obtusifolia (L.) Irwin and Barneby] at least 98% at late season. Weed control, cotton yield, and net returns were similar when pyrithiobac {2-chloro-6-[(4,6-dimethoxy-2-pyrimidinyl) thio]benzoic acid, sodium salt} applied postemergence over-the-top was substituted for fluometuron plus MSMA postemergence directed. Glyphosate applied once did not adequately control most species, and cotton yield and net returns were less than with the standard system. However, weed control, cotton yield, and net returns in systems with glyphosate applied postemergence over-the-top 3 to 4 weeks after planting followed by glyphosate or cyanazine plus MSMA postemergence directed 6 to 7 weeks after planting were similar to those with the standard system. Three applications of glyphosate were no more effective than two. Trifluralin and fluometuron were of no benefit in systems with glyphosate applied twice or glyphosate followed by cyanazine plus MSMA. No treatment affected fiber quality.