Jose J. Vargas

A new amino acid substitution (Ala-205-Phe) in acetolactate synthase (ALS) confers broad spectrum resistance to ALS-inhibiting herbicides

AbstractMain ConclusionThis is a first report of an Ala-205-Phe substitution in acetolactate synthase conferring resistance to imidazolinone, sulfonylurea, triazolopyrimidines, sulfonylamino-carbonyl-triazolinones, and pyrimidinyl (thio) benzoate herbicides. Resistance to acetolactate synthase (ALS) and photosystem II inhibiting herbicides was confirmed in a population of allotetraploid annual bluegrass (Poa annua L.; POAAN-R3) selected from golf course turf in Tennessee. Genetic sequencing revealed that seven of eight POAAN-R3 plants had a point mutation in the psbA gene resulting in a known Ser-264-Gly substitution on the D1 protein. Whole plant testing confirmed that this substitution conferred resistance to simazine in POAAN-R3. Two homeologous forms of the ALS gene (ALSa and ALSb) were detected and expressed in all POAAN-R3 plants sequenced. The seven plants possessing the Ser-264-Gly mutation conferring resistance to simazine also had a homozygous Ala-205-Phe substitution on ALSb, caused by two nucleic acid substitutions in one codon. In vitro ALS activity assays with recombinant protein and whole plant testing confirmed that this Ala-205-Phe substitution conferred resistance to imidazolinone, sulfonylurea, triazolopyrimidines, sulfonylamino-carbonyl- triazolinones, and pyrimidinyl (thio) benzoate herbicides. This is the first report of Ala-205-Phe mutation conferring wide spectrum resistance to ALS inhibiting herbicides.

Control of Silk Tree (Albizia julibrissin) with Aminocyclopyrachlor and Other Herbicides

Abstract Research was conducted to determine the efficacy of aminocyclopyrachlor in comparison to glyphosate, clopyralid, fluroxypyr, and triclopyr for silk tree (commonly known as mimosa) control. In the greenhouse, aminocyclopyrachlor was applied at 8.75, 17.5, 35, and 70 g ha−1 with and without methylated seed oil (MSO) at 0.5% v/v. Efficacy of these treatments was compared to glyphosate and triclopyr at 1,350 g ha−1, fluroxypyr at 103 g ha−1, and clopyralid at 100 g ha−1. Few differences in silk tree control were detected by 28 d after treatment (DAT), as aminocyclopyrachlor with MSO controlled silk tree 87 to 100% compared to 53 to 100% for aminocyclopyrachlor without MSO. Aminocyclopyrachlor at 35 g ha−1 provided silk tree control similar to glyphosate, triclopyr, clopyralid, and fluroxypyr regardless of adjuvant. Inclusion of MSO enhanced initial activity of aminocyclopyrachlor after application. At 7 DAT, 8.75 g ha−1 of aminocyclopyrachlor plus MSO controlled silk tree similar to aminocyclopyrachlor alone at 70 g ha−1. In laboratory studies, absorption of 14C-aminocyclopyrachlor 2 h after treatment (HAT) with MSO measured 93% compared to only 62% for 14C-aminocyclopyrachlor without MSO. By 24 HAT, absorption of 14C-aminocyclopyrachlor measured 99 and 71% for applications with and without MSO, respectively. Increased foliar absorption with MSO may explain enhanced activity observed 7 DAT in greenhouse studies, as no effects in 14C-aminocyclopyrachlor translocation due to adjuvant were observed. Responses suggest MSO increased the speed of silk tree control with aminocyclopyrachlor and may also improve rainfastness of aminocyclopyrachlor applications for control of silk tree and other woody species. Nomenclature: Aminocyclopyrachlor, fluroxypyr, glyphosate, triclopyr, clopyralid, silk tree, Albizia julibrissin Durazz

Influence of Herbicide Safeners on Creeping Bentgrass (Agrostis stolonifera) Tolerance to Herbicides

Abstract Glasshouse research was conducted to investigate the efficacy of herbicide safeners for improving creeping bentgrass (CBG) tolerance to various herbicides. CBG injury from amicarbazone (150 g ha−1), bispyribac-sodium (110 g ha−1), fenoxaprop-p-ethyl (35 g ha−1), imazapic (45 g ha−1), quinclorac (1,050 g ha−1), or topramezone (37 g ha−1) applied in combination with the herbicide safeners naphthalic anhydride or isoxadifen-ethyl was evaluated. These safeners reduced CBG injury from topramezone only. Topramezone was then applied in combination with naphthalic anhydride, isoxadifen-ethyl, cloquintocet-mexyl (cloquintocet), fenchlorazole-ethyl, mefenpyr-diethyl, and benoxacor. These experiments determined that CBG injury was lowest from topramezone in combination with cloquintocet. Additional experiments evaluated topramezone (37 g ha−1) with several rates of cloquintocet and determined that applications at ≥ 28 g ha−1 reduced CBG injury similarly. Cloquintocet (28 g ha−1) increased topramezone I50 values against CBG, but not large crabgrass or goosegrass. The cytochrome P450 (cP450) inhibitor malathion (1000 g ha−1) reduced topramezone I50 values against CBG in one experimental run. Topramezone–cloquintocet combinations warrant further research in field settings. Nomenclature: Amicarbazone; benoxacor; bispyribac-sodium; cloquintocet-mexyl; fenchlorazole-ethyl; fenoxaprop-p-ethyl; imazapic; isoxadifen-ethyl; malathion; mefenpy-diethyl; naphthalic anhydride; quinclorac; topramezone; creeping bentgrass, Agrostis stolonifera L.; goosegrass, Eleusine indica (L.) Gaertn.; large crabgrass, Digitaria sanguinalis (L.) Scop. Resumen En un estudio en invernadero, se investigó la eficacia de antídotos de herbicidas para mejorar la tolerancia del césped Agrostis stolonifera (CBG) a varios herbicidas. Se evaluó el daño en CBG causado por amicarbazone (150 g ai ha−1), bispyribac-sodium (110 g ai ha−1), fenoxaprop-p-ethyl (35 g ai ha−1), imazapic (45 g ai ha−1), quinclorac (1,050 g ai ha−1), o topramezone (37 g ai ha−1) aplicados en combinación con los antídotos de herbicidas naphthalic anhydride o isoxadifen-ethyl. Estos antídotos solamente redujeron el daño causado por topramezone. Posteriormente, se aplicó topramezone en combinación con naphthalic anhydride, isoxadifen-ethyl, cloquintocet-mexyl (cloquintocet), fenchlorazole-ethyl, mefenpy-diethyl, y benoxacor. Estos experimentos determinaron que el menor daño en CBG se dio con topramezone combinado con cloquintocet. Experimentos adicionales evaluaron topramezone (37 g ha−1) con varias dosis de cloquintocet y se determinó que las aplicaciones a ≥ 28 g ha−1 redujeron el daño en CBG en forma similar. Cloquintocet (28 g ai ha−1) incrementó los valores de I50 de topramezone en CBG, pero no en Digitaria sanguinalis o Eleusine indica. El inhibidor del cytochrome P450 (cP450) malathion (1000 g ai ha−1) redujo los valores de I50 de topramezone en una de las corridas experimentales. El uso de combinaciones de topramezone con cloquintocet debe ser investigado en condiciones de campo.

A Biotype of Annual Bluegrass (Poa annua) in Tennessee Is Resistant to Inhibitors of ALS and Photosystem II

Abstract Annual bluegrass resistance to inhibitors of acetolactate synthase (ALS) and photosystem II (PSII) in managed turf has been confirmed in the southeastern United States. A biotype of annual bluegrass that had developed resistance (R) to the PSII inhibitor simazine was not controlled by POST applications of foramsulfuron or trifloxysulfuron in 2011 or 2012. In whole plant dose-response experiments, trifloxysulfuron, simazine, and indaziflam controlled a susceptible (S) population of annual bluegrass > 91% when applied POST to nontillering plants. However, trifloxysulfuron applications at 3.5 to 223 g ai ha−1 only controlled R annual bluegrass ≤ 40%. Similarly, simazine at 140 to 9,000 g ai ha−1 only controlled R annual bluegrass ≤ 20%. R annual bluegrass plants were more tolerant to indaziflam applied POST to leaf stage plants prior to tillering, as rates > 100 g ai ha−1 were needed to control R annual bluegrass ≥ 96%. No differences in the activity of ALS in R and S plants exposed to increasing foramsulfuron concentrations from 0 to 100 µM were detected suggesting that nontarget mechanisms could explain reduced efficacy of POST herbicide applications in whole plant dose-response experiments. Applications of indaziflam (35 to 70 g ha−1) and oxadiazon (2,240 to 4,500 g ai ha−1) effectively controlled R annual bluegrass when applied PRE. This biotype of R annual bluegrass is the first reported instance of a weed developing resistance to multiple modes of action in managed turf. Education is needed among turf managers regarding the consequences of exclusive use of the same herbicides for annual bluegrass control leading to the onset of herbicide resistance. Nomenclature: Annual bluegrass (Poa annua L.); foramsulfuron; indaziflam; oxadiazon; simazine; trifloxysulfuron.

A Putative Prodiamine-Resistant Annual Bluegrass (Poa annua) Population is Controlled by Indaziflam

Abstract Prodiamine is a mitotic inhibiting herbicide regularly used to control annual bluegrass PRE. A population of annual bluegrass not controlled by prodiamine at 1,120 g a.i. ha−1 was identified on a golf course in Alcoa, TN, in 2012. A whole-plant hydroponics bioassay was used to screen this biotype for prodiamine resistance (PR) compared with a known susceptible population (SS). Multitiller (i.e., > 4 tillers) PR and SS annual bluegrass plants were established in hydroponic culture and exposed to 0, 0.001, 0.01, 0.10, 1.0, and 10.0 mM prodiamine. Exposure to prodiamine at 0.001 mM reduced root growth of the SS biotype to 26% of the nontreated check (i.e., 0 mM prodiamine) but had no effect on the PR biotype. When exposed to 10 mM prodiamine, root growth of the PR biotype was reduced to 24% of the nontreated check compared with 9% for the SS biotype. I50 values for the PR and SS biotypes were 0.04 and 2.8 × 10−6 mM prodiamine, respectively. The PR biotype measured lower in plant height and leaf width than the SS population. In field trials, prodiamine at 560, 840, 1,120, and 1,400 g ha−1 only controlled the PR biotype 0 to 22%. PRE applications of the cellulose biosynthesis inhibitor indaziflam at 35, 52.5, and 70 g a.i. ha−1 controlled this PR biotype 70 to 97%. This marks the second instance of annual bluegrass developing resistance to prodiamine in Tennessee during the past 5 yr. Future research should evaluate indaziflam efficacy for control of other prodiamine-resistant biotypes of annual bluegrass as well as annual bluegrass biotypes resistant to herbicidal inhibitors of 5-enolpyruvylshikimic acid-3-phosphate synthase, acetolactate synthase, and photosystem II. Nomenclature: Indaziflam; prodiamine; annual bluegrass, Poa annua L. var. annua; bermudagrass, Cynodon dactylon L. Pers.

Common Bermudagrass Seedhead Suppression and Growth Regulation with Fenoxaprop

Abstract Options for suppressing bermudagrass seedheads in managed turfgrass systems are limited. Experiments were conducted in 2009 and 2010 evaluating the use of fenoxaprop (25, 50, 75, and 100 g ha−1) for ‘Riviera’ bermudagrass seedhead suppression and growth regulation compared to imazapic (52 g ha−1), trinexapac-ethyl (91 g ha−1) and mefluidide (561 g ha−1). In field experiments, seedhead suppression ranged from 77 to 100% for fenoxaprop and imazapic at 35 d after treatment (DAT). Comparatively, seedhead suppression was < 25% for either trinexapac-ethyl or mefluidide at 35 DAT. Seedhead suppression was > 90% from 7 to 35 DAT for fenoxaprop applied at ≥ 50 g ha−1. Injury, determined visually, from fenoxaprop and imazapic in both the field and greenhouse measured < 25% on all rating dates, with no significant injury present after 21 DAT. In greenhouse experiments, fenoxaprop and trinexapac-ethyl showed similar reductions of bermudagrass growth; no differences in aboveground biomass were detected between these treatments at 42 DAT. Results of the current study illustrate that fenoxaprop and imazapic can be applied for bermudagrass seedhead suppression and growth regulation if moderate (< 25%) injury can be tolerated up to 21 DAT. Additional research is needed to evaluate the use of fenoxaprop and imazapic for seedhead suppression on other common and hybrid bermudagrasses. Nomenclature: Fenoxaprop; imazapic; mefluidide; trinexapac-ethyl; bermudagrass, Cynodon dactylon (L.) Pers. ‘Riviera’; hybrid bermudagrass C. dactylon × C. transvaalensis Burtt Davy.

Controlling Herbicide-Resistant Annual Bluegrass (Poa annua) Phenotypes with Methiozolin

Methiozolin is an isoxazoline herbicide being investigated for selective POST annual bluegrass control in managed turfgrass. Research was conducted to evaluate methiozolin efficacy for controlling two annual bluegrass phenotypes with target-site resistance to photosystem II (PSII) or enolpyruvylshikimate-3-phosphate synthase (EPSPS)-inhibiting herbicides (i.e., glyphosate), as well as phenotypes with multiple resistance to microtubule and EPSPS or PSII and acetolactate synthase (ALS)-inhibiting herbicides. All resistant phenotypes were established in glasshouse culture along with a known herbicide-susceptible control and treated with methiozolin at 0, 125, 250, 500, 1000, 2000, 4000, or 8000 g ai ha-1. Methiozolin effectively controlled annual bluegrass with target-site resistance to inhibitors of EPSPS, PSII, as well as multiple resistance to EPSPS and microtubule inhibitors. Methiozolin rates required to reduce aboveground biomass of these resistant phenotypes 50% (GR50 values) were not significantly different from the susceptible control, ranging from 159 to 421 g ha-1. A phenotype with target-site resistance to PSII and ALS inhibitors was less sensitive to methiozolin (GR50 = 862 g ha-1) than a susceptible phenotype (GR50 = 423 g ha-1). Our findings indicate that methiozolin is an effective option for controlling select annual bluegrass phenotypes with target-site resistance to several herbicides. Nomenclature: Methiozolin, annual bluegrass, Poa annua L.

Design, Synthesis, and Evaluation of Novel Auxin Mimic Herbicides

Due to the key roles of auxins as master regulators of plant growth, there is considerable interest in the development of compounds with auxin-like properties for growth management and weed control applications. Herein, we describe the design and multistep synthesis of ten compounds bearing combinations of functional groups commonly associated with auxin-type properties. Following synthesis, these compounds were tested against multiple weed species as well as sweet corn. In general, while these structures were not quite as active as commercial auxin mimic herbicides, multiple compounds exhibited broadleaf weed activity with concurrent selectivity in sweet corn (Zea mays L. var. saccharum). In addition, differential results were observed upon subtle changes to structure, providing insights into the structural properties required for activity.

Biokinetics and Efficacy of Aminocyclopyrachlor-Methyl Ester as Influenced by Diflufenzopyr

Abstract Research studies evaluated effects of the auxin transport inhibitor, diflufenzopyr, on the biokinetics and efficacy of aminocyclopyrachlor-methyl ester (AMCP-ME) applications to black nightshade and large crabgrass. Absorption, translocation, and metabolism of 14C-AMCP-ME was quantified with and without diflufenzopyr (35 g ai ha−1). Diflufenzopyr had minimal effects on translocation of radioactivity in either species. Accumulation of radioactivity in aboveground plant sections of black nightshade was greater than or equal to that in large crabgrass by 72 h after treatment (HAT). In both species, metabolism of 14C-AMCP-ME was rapid, as 60 to 78% of the extracted radioactivity was the free acid metabolite 8 HAT. In the greenhouse, black nightshade and large crabgrass were treated with AMCP-ME (9, 18, and 35 g ai ha−1) alone and in combination with diflufenzopyr (35 g ha−1). Mixtures of AMCP-ME plus diflufenzopyr did not increase large crabgrass control compared with AMCP-ME alone at any time. Diflufenzopyr (35 g ha−1) increased black nightshade control with AMCP-ME (18 and 35 g ha−1) 7 d after treatment (DAT). However, this increase in control was not observed 14 or 28 DAT. All treatments containing AMCP-ME controlled large crabgrass 70 to 79% 28 DAT compared with > 93% for black nightshade at the same time point. Nomenclature: Aminocyclopyrachlor; aminocyclopyrachlor-methyl ester; diflufenzopyr; black nightshade, Solanum nigrum L.; large crabgrass, Digitaria sanguinalis (L.) Scop.

A Diagnostic Assay to Detect Herbicide Resistance in Annual Bluegrass (Poa annua)

Turfgrass managers currently have few readily available means of evaluating herbicide resistance in annual bluegrass during the growing season. Research was conducted to determine if agar-based diagnostic tests developed for agronomic weeds could be used to reliably confirm herbicide resistance in annual bluegrass harvested from golf course turf. Annual bluegrass phenotypes with target-site resistance to acetolactate synthase (ALS; R3, R7), enolpyruvylshikimate-3-phosphate synthase (EPSPS; R5), and photosystem II (PSII; R3, R4) inhibiting herbicides were included in experiments along with an herbicidal susceptible phenotype (S). Single tiller plants were washed free of soil and transplanted into autoclavable polycarbonate plant culture boxes filled with plant tissue culture agar amended with a murashigee-skoog medium and trifloxysulfuron (6.25, 12.5, 25, 50, 75, 100, or 150 μM), glyphosate (0, 6, 12, 25, 50, 100, 200, or 400 μM), or simazine (0, 6, 12, 25, 50, 100, 200, or 400 μM). Mortality in agar was assessed 7 to 10 days after treatment (depending on herbicide) and compared to responses observed after treating individual plants of each phenotype with trifloxysulfuron (28 g ai ha-1), glyphosate (1120 g ae ha-1), or simazine (1120 g ai ha-1) in an enclosed spray chamber. Fishers exact test (α = 0.05) determined that mortality in agar with 12.5 μM trifloxysulfuron and 100 μM glyphosate was not significantly different than treating whole plants via traditional spray application. Mortality with all concentrations of simazine in agar was significantly different than that observed after treating resistant and susceptible phenotypes via traditional spray application. Our findings indicate that an agar-based diagnostic assay can be used to detect annual bluegrass resistance to ALS- or EPSPS-inhibiting herbicides in less than 10 days; however, additional research is needed to refine this assay for use with PSII-inhibiting herbicides. Nomenclature: Glyphosate; simazine; trifloxysulfuron; annual bluegrass, Poa annua L.