Curtis R. Thompson

Differential Kochia (Kochia scoparia) Populations Response to Glyphosate

Abstract Kochia is a troublesome weed throughout the western United States. Although glyphosate effectively controls kochia, poor control was observed in several no-till fields in Kansas. The objectives of this research were to evaluate kochia populations response to glyphosate and examine the mechanism that causes differential response to glyphosate. Glyphosate was applied at 0, 54, 109, 218, 435, 870, 1305, 1740, 3480, and 5220 g ae ha−1 on 10 kochia populations. In general, kochia populations differed in their response to glyphosate. At 21 d after treatment, injury from glyphosate applied at 870 g ha−1 range from 4 to 91%. In addition, glyphosate rate required to cause 50% visible injury (GR50) ranged from 470 to 2149 g ha−1. Differences in glyphosate absorption and translocation and kochia mineral content were not sufficient to explain differential kochia response to glyphosate. Nomenclature: Glyphosate; kochia, Kochia scoparia (L.) Schrad.

Physiological and Molecular Mechanisms of Differential Sensitivity of Palmer Amaranth (Amaranthus palmeri) to Mesotrione at Varying Growth Temperatures

Herbicide efficacy is known to be influenced by temperature, however, underlying mechanism(s) are poorly understood. A marked alteration in mesotrione [a 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibitor] efficacy on Palmer amaranth (Amaranthus palmeri S. Watson) was observed when grown under low- (LT, 25/15°C, day/night temperatures) and high (HT, 40/30°C) temperature compared to optimum (OT, 32.5/22.5°C) temperature. Based on plant height, injury, and mortality, Palmer amaranth was more sensitive to mesotrione at LT and less sensitive at HT compared to OT (ED50 for mortality; 18.5, 52.3, and 63.7 g ai ha-1, respectively). Similar responses were observed for leaf chlorophyll index and photochemical efficiency of PSII (Fv/Fm). Furthermore, mesotrione translocation and metabolism, and HPPD expression data strongly supported such variation. Relatively more mesotrione was translocated to meristematic regions at LT or OT than at HT. Based on T50 values (time required to metabolize 50% of the 14C mesotrione), plants at HT metabolized mesotrione faster than those at LT or OT (T50; 13, 21, and 16.5 h, respectively). The relative HPPD:CPS (carbamoyl phosphate synthetase) or HPPD:β-tubulin expression in mesotrione-treated plants increased over time in all temperature regimes; however, at 48 HAT, the HPPD:β-tubulin expression was exceedingly higher at HT compared to LT or OT (18.4-, 3.1-, and 3.5-fold relative to untreated plants, respectively). These findings together with an integrated understanding of other interacting key environmental factors will have important implications for a predictable approach for effective weed management.

Field‐evolved resistance to four modes of action of herbicides in a single kochia (Kochia scoparia L. Schrad.) population

BACKGROUND Evolution of multiple herbicide resistance in weeds is a serious threat to weed management in crop production. Kochia is an economically important broadleaf weed in the U.S. Great Plains. This study aimed to confirm resistance to four sites of action of herbicides in a single kochia (Kochia scoparia L. Schrad.) population from a crop field near Garden City (GC), Kansas, and further determine the underlying mechanisms of resistance. RESULTS One-fourth of the GC plants survived the labeled rate or higher of atrazine [photosystem II (PSII) inhibitor], and the surviving plants had the Ser-264 to Gly mutation in the psbA gene, the target site of atrazine. Results showed that 90% of GC plants survived the labeled rate of dicamba, a synthetic auxin. At least 87% of the plants survived up to 72 g a.i. ha(-1) of chlorsulfuron [acetolactate synthase (ALS) inhibitor], and analysis of the ALS gene revealed the presence of Pro-197 to Thr and/or Trp-574 to Lue mutation(s). Most GC plants also survived the labeled rate of glyphosate [5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) inhibitor), and the resistant plants had 5-9 EPSPS gene copies (relative to the ALS gene). CONCLUSION We confirm the first case of evolution of resistance to four herbicide sites of action (PSII, ALS and EPSPS inhibitors and synthetic auxins) in a single kochia population, and target-site-based mechanisms confer resistance to atrazine, glyphosate and chlorsulfuron.

Pyroxasulfone with and without Sulfentrazone in Sunflower (Helianthus annuus)

Abstract Pyroxasulfone (KIH-485) is a seedling growth-inhibiting herbicide developed by Kumiai America that has the potential to control weeds in sunflower. However, little is known about how this herbicide will interact with various soil types and environments when combined with sulfentrazone. The objective of this research was to evaluate sunflower injury and weed control with pyroxasulfone applied with and without sulfentrazone across the Great Plains sunflower production area. A multisite study was initiated in spring 2007 to evaluate sunflower response to pyroxasulfone applied PRE at 0, 167, 208, or 333 g ai ha−1. In 2008, pyroxasulfone was applied alone and in tank mixture with sulfentrazone. In 2007, no sunflower injury was observed with any rate of pyroxasulfone at any location except Highmore, SD, where sunflower injury was 17%, 4 wk after treatment (WAT) with 333 g ha−1. In 2008, sunflower injury ranged from 0 to 4% for all treatments. Adding sulfentrazone did not increase injury. Sunflower yield was only reduced in treatments in which weeds were not effectively controlled. These treatments included the untreated control and pyroxasulfone at 167 g ha−1. Sunflower yield did not differ among the other treatments of pyroxasulfone or sulfentrazone applied alone or in combination. The addition of sulfentrazone to pyroxasulfone improved control of foxtail barley, prostrate pigweed, wild buckwheat, Palmer amaranth, and marshelder, but not large crabgrass or green foxtail. The combination of pyroxasulfone and sulfentrazone did not reduce control of any of the weeds evaluated. Nomenclature: Pyroxasulfone (KIH-485); sulfentrazone; foxtail barley, Hordeum jubatum L. HORJU; green foxtail, Setaria viridis (L.) Beauv. SETVI; large crabgrass, Digitaria sanguinalis (L.) Scop. DIGSA; marshelder, Iva xanthifolia Nutt. IVAXA; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; prostrate pigweed, Amaranthus blitoides S. Wats AMABL; wild buckwheat, Polygonum convolvulus L. POLCO; sunflower, Helianthus annuus L

Physiological and Molecular Characterization of Hydroxyphenylpyruvate Dioxygenase (HPPD)-inhibitor Resistance in Palmer Amaranth (Amaranthus palmeri S. Wats.)

Herbicides that inhibit hydroxyphenylpyruvate dioxygenase (HPPD) such as mesotrione are widely used to control a broad spectrum of weeds in agriculture. Amaranthus palmeri is an economically troublesome weed throughout the United States. The first case of evolution of resistance to HPPD-inhibiting herbicides in A. palmeri was documented in Kansas (KS) and later in Nebraska (NE). The objective of this study was to investigate the mechansim of HPPD-inhibitor (mesotrione) resistance in A. palmeri. Dose response analysis revealed that this population (KSR) was 10–18 times more resistant than their sensitive counterparts (MSS or KSS). Absorbtion and translocation analysis of [14C] mesotrione suggested that these mechanisms were not involved in the resistance in A. palmeri. Importantly, mesotrione (>90%) was detoxified markedly faster in the resistant populations (KSR and NER), within 24 hours after treatment (HAT) compared to sensitive plants (MSS, KSS, or NER). However, at 48 HAT all populations metabolized the mesotrione, suggesting additional factors may contribute to this resistance. Further evaluation of mesotrione-resistant A. palmeri did not reveal any specific resistance-conferring mutations nor amplification of HPPD gene, the molecular target of mesotrione. However, the resistant populations showed 4- to 12-fold increase in HPPD gene expression. This increase in HPPD transcript levels was accompanied by increased HPPD protein expression. The significant aspects of this research include: the mesotrione resistance in A. palmeri is conferred primarily by rapid detoxification (non-target-site based) of mesotrione; additionally, increased HPPD gene expression (target-site based) also contributes to the resistance mechanism in the evolution of herbicide resistance in this naturally occurring weed species.

Glyphosate Weed Control Enhancement with Ammonium Sulfate and Commercial Water Conditioning Agents

Glyphosate herbicide labels generally recommend addition of ammonium sulfate (AMS) to the spray solution, which often improves weed control, especially when mixed with hard water. AMS in solution disassociates, and the sulfate binds with cations in the spray solution, preventing the development of glyphosate-cation complexes that tend to have lower absorption into plant leaves. In addition, the ammonium ion can associate with the glyphosate molecule, which helps facilitate glyphosate absorption into the leaf. The recommended AMS rate is 1–2 % w/w, and AMS is available in both dry and liquid formulations. Applicators generally find AMS inconvenient to use because of the high use rate and handling issues. Several companies are marketing low-rate water conditioner products to be used as an alternative to AMS with glyphosate. Pesticide applicators like the convenience of low-rate water conditioners, but performance of these products has been inconsistent. Field experiments were conducted near Manhattan, KS, from 2005 to 2008 to compare the efficacy of glyphosate with AMS and various commercial water conditioners on velvetleaf, sorghum, corn, and sunflower. Each experiment consisted of a sublethal (0.31 or 0.43 kg ae/ha) dose of glyphosate applied in combination with the recommended application rate of each adjuvant. Water hardness, environmental conditions, and plant growth stages varied by experiment. Control of all assay species with glyphosate was enhanced by the addition of AMS, unless control was near complete in the absence of AMS. Commercial water conditioner products that included an AMS component at the equivalent rate of 1 % w/w gave equal or slightly better control than glyphosate plus 1 % w/w AMS. Commercial water conditioners that did not provide an equivalent amount of AMS gave less control than glyphosate with 1 % or 2 % w/w AMS and were often no better than glyphosate alone for the low-rate products.

Weed Control and Crop Safety with Premixed Pyrasulfotole and Bromoxynil in Grain Sorghum

Abstract Field experiments were conducted in grain sorghum at five locations in Kansas in 2009 and 2010, to evaluate the efficacy and crop safety of early- to mid-POST (EMPOST) and late-POST (LPOST) applications of premixed pyrasulfotole and bromoxynil (PYRA&BROM) in tank mix combinations with atrazine or atrazine plus 2,4-D ester or dicamba compared to bromoxynil plus atrazine. PYRA&BROM at 244 or 300 g ai ha−1 plus atrazine at 560 g ai ha−1 applied EMPOST controlled pigweed species (Palmer amaranth, tumble pigweed, and redroot pigweed), kochia, velvetleaf, common sunflower, ivyleaf morningglory, and common lambsquarters 93% or greater. Puncturevine control among three locations ranged from 85 to 99%. Control of most weed species was not improved by increasing PYRA&BROM rate from 244 to 300 g ha−1 or by tank mixing 2,4-D or dicamba with PYRA&BROM plus atrazine. However, ivyleaf morningglory control was improved at the LPOST timing by adding 2,4-D or dicamba at 140 g ae ha−1. In no instance did any PYRA&BROM treatment provide greater weed control than bromoxynil plus atrazine at 281 + 560 g ha−1 when applied EMPOST, but in most instances PYRA&BROM treatments were more effective than bromoxynil plus atrazine when applied LPOST. Generally, PYRA&BROM treatments were more effective when applied EMPOST than LPOST, especially when 2,4-D or dicamba was added. PYRA&BROM plus atrazine treatments caused foliar bleaching in sorghum within 7 ± 3 d after treatment, but recovery was complete within 3 to 4 wk and grain yields were not reduced. Tank mixing dicamba with PYRA&BROM and atrazine occasionally reduced visible crop response compared to PYRA&BROM plus atrazine. Our results indicate that PYRA&BROM plus atrazine with or without 2,4-D or dicamba selectively controls several troublesome broadleaf weeds in grain sorghum. Foliar bleaching of sorghum leaves can occur but the symptoms are transient, and grain yields are not likely to be reduced. Nomenclature: Atrazine; bromoxynil; dicamba; pyrasulfotole; 2,4-D; common lambsquarters; Chenopodium album L.; common sunflower; Helianthus annuus L.; ivyleaf morningglory; Ipomoea hederacea Jacq.; kochia; Kochia scoparia (L.) Schrad.; Palmer amaranth; Amaranthus palmeri S. Wats.; puncturevine; Tribulus terrestris L.; redroot pigweed; Amaranthus retroflexus L.; tumble pigweed; Amaranthus albus L.; velvetleaf; Abutilon theophrasti Medik.; grain sorghum; Sorghum bicolor (L.) Moench. Resumen Se realizaron experimentos de campo con sorgo para grano, en cinco localidades en Kansas en 2009 y 2010, para evaluar la eficacia y la seguridad en el cultivo de aplicaciones tempranas a intermedias POST (EMPOST) y tardías POST (LPOST) de pre-mezclas de pyrasulfotole y bromoxynil (PYRA&BROM) en combinaciones en mezclas en tanque con atrazine o atrazine más 2,4-D ester o dicamba comparadas a bromoxynil más atrazine. PYRA&BROM a 244 ó 300 g ai ha−1 más atrazine a 560 g ai ha−1 aplicado EMPOST controló especies de amaranto (Amaranthus palmeri, Amaranthus albus, y Amaranthus retroflexus), Kochia scoparia, Abutilon theophrasti, Helianthus annuus, Ipomoea hederacea y Chenopodium album 93% o más. El control de Tribulus terrestris en tres localidades varió entre 85 y 99%. El control de la mayoría de las especies de malezas no mejoró al incrementar la dosis PYRA&BROM de 244 a 300 g ai ha−1 o al mezclar en tanque 2,4-D o dicamba con PYRA&BROM más atrazine. Sin embargo, el control de I. hederacea fue mejorado en LPOST al agregar 2,4-D o dicamba a 140 g ai ha−1. En ninguna instancia, ninguno de los tratamientos PYRA&BROM brindaron un control de malezas mayor al brindado por bromoxynil más atrazine 281 + 560 g ha−1 cuando se aplicó EMPOST, pero en la mayoría de las instancias los tratamientos PYRA&BROM fueron más efectivos que bromoxynil más atrazine aplicados LPOST. Generalmente, los tratamientos PYRA&BROM fueron más efectivos cuando se aplicaron EMPOST que LPOST, especialmente cuando se agregó 2,4-D o dicamba. Los tratamientos PYRA&BROM más atrazine causaron blanqueamiento foliar en el sorgo a 7 ± 3 días después del tratamiento, pero este se recuperó completamente en 2 a 4 semanas y los rendimientos de grano no se redujeron. El mezclar en tanque dicamba con PYRA&BROM y atrazine ocasionalmente redujo la respuesta visible del cultivo en comparación con PYRA&BROM más atrazine. Nuestros resultados indican que PYRA&BROM más atrazine con o sin 2,4-D o dicamba controla selectivamente malezas de hoja ancha problemáticas en el sorgo para grano. El blanqueamiento foliar de hojas de sorgo puede ocurrir, pero los síntomas son transitorios, y las reducciones en rendimientos de grano son poco probables.

Rapid detoxification via glutathione S‐transferase (GST) conjugation confers a high level of atrazine resistance in Palmer amaranth (Amaranthus palmeri)

BACKGROUND Palmer amaranth (Amaranthus palmeri) is an economically troublesome, aggressive and damaging weed that has evolved resistance to six herbicide modes of action including photosystem II (PS II) inhibitors such as atrazine. The objective of this study was to investigate the mechanism and inheritance of atrazine resistance in Palmer amaranth. RESULTS A population of Palmer amaranth from Kansas (KSR) had a high level (160 - 198-fold more; SE ±21 - 26) of resistance to atrazine compared to the two known susceptible populations MSS and KSS, from Mississippi and Kansas, respectively. Sequence analysis of the chloroplastic psbA gene did not reveal any known mutations conferring resistance to PS II inhibitors, including the most common Ser264Gly substitution for triazine resistance. However, the KSR plants rapidly conjugated atrazine at least 24 times faster than MSS via glutathione S-transferase (GST) activity. Furthermore, genetic analyses of progeny generated from reciprocal crosses of KSR and MSS demonstrate that atrazine resistance in Palmer amaranth is a nuclear trait. CONCLUSION Although triazine resistance in Palmer amaranth was reported more than 20 years ago in the USA, this is the first report elucidating the underlying mechanism of resistance to atrazine. The non-target-site based metabolic resistance to atrazine mediated by GST activity may predispose the Palmer amaranth populations to have resistance to other herbicide families, and the nuclear inheritance of the trait in this dioecious species further exacerbates the propensity for its rapid spread.

Effects of Herbicides and Application Timing on Woollyleaf Bursage(Ambrosia grayi)1

Abstract: Woollyleaf bursage (Ambrosia grayi) is a noxious, rhizomatous perennial with an extensive creeping root system. It is found in the central and southern Great Plains of the U.S. Clopyralid alone or fluroxypyr, picloram, or glyphosate with either 2,4-D or dicamba were applied to woollyleaf bursage at anthesis and 30 d later in three field experiments. With the exception of treatments containing picloram, the effect of application timing was inconsistent. All treatments containing picloram consistently controlled woollyleaf bursage 93% or greater for 9 mo and 74% or greater for 11 mo. Control was poor or inconsistent with all other treatments. Although a rate response was seen with clopyralid, a level higher than 0.28 kg/ha may be necessary to control woollyleaf bursage. After 11 mo, control was less than 60% with treatments containing 1.7 kg/ha of glyphosate in 10 of 12 herbicide treatments and timing combinations over 3 yr. Nomenclature: Clopyralid, 3,6-dichloro-2-pyridinecarboxylic acid; dicamba, 3,6-dichloro-2-methoxybenzoic acid; fluroxypyr, [(4-amino-3,5-dichloro-6-fluoro-2-pyridinyl)oxy]acetic acid; glyphosate, N-(phosphonomethyl)glycine; picloram, 4-amino-3,5,6-trichloro-2-pyridinecarboxylic acid; 2,4-D, (2,4-dichlorophenoxy)acetic acid; woollyleaf bursage, Ambrosia grayi (A. Nels.) shinners #3AMBGR. Additional index words: Woollyleaf franseria, bur ragweed, perennial weed control.

Target Site—Based and Non—Target Site Based Resistance to ALS Inhibitors in Palmer Amaranth (Amaranthus palmeri)

Resistance to acetolactate synthase (ALS)-inhibitor herbicides due to continuous and repeated selection is widespread in many troublesome weed species, including Palmer amaranth, throughout the United States. The objective of this research was to investigate the physiological and molecular basis of resistance to ALS inhibitors in a chlorsulfuron-resistant Palmer amaranth population (KSR). Our results indicate that the KSR population exhibits a high level of resistance to chlorsulfuron compared with two known susceptible populations, MSS and KSS from Mississippi and Kansas, respectively. MSS is highly susceptible to chlorsulfuron, whereas KSS is moderately sensitive. Dose—response analysis revealed that KSR was more than 275-fold more resistant compared with KSS. Nucleotide sequence analysis of the ALS gene from the plants that survived chlorsulfuron treatment revealed the possibility of evolution of both target site—based and non—target site based resistance to ALS inhibitors in the KSR population. The most common mutation (Pro-197-Ser) in the ALS gene associated with resistance to the sulfonylureas in many weed species was found only in 30% of the KSR population. A preliminary malathion study showed that the remaining 70% of resistant plants might have cytochrome P450—mediated non—target site resistance. This is the first report elucidating the mechanism of resistance to ALS inhibitors in Palmer amaranth from Kansas. Presence of both target site— and non—target site based mechanisms of resistance limits the herbicide options to manage Palmer amaranth in cropping systems. Nomenclature: Chlorsulfuron; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA