Timothy L. Grey

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

Abstract 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, I50 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. Nomenclature: Glyphosate; Palmer amaranth, Amaranthus palmeri S. Wats; AMAPA.

Establishing the Geographical Distribution and Level of Acetolactate Synthase Resistance of Palmer Amaranth (Amaranthus palmeri) Accessions in Georgia

Abstract Palmer amaranth resistance to acetolactate synthase (ALS)–inhibiting herbicides was first identified in Georgia in 2000. Since then, complaints from peanut producers have increased concerning failure of ALS herbicides in controlling Palmer amaranth. Because efficacy of ALS herbicides can be compromised under adverse conditions, seeds from Palmer amaranth plants that escaped weed control were collected across the peanut-growing region in Georgia to investigate the cause of these reported failures. Greenhouse and growth-chamber studies were conducted using these seeds to evaluate whether weed escapes were a result of Palmer amaranth resistance to ALS herbicides. Each of the 61 accessions collected across Georgia exhibited varying levels of resistance to imazapic applied POST (< 55% control, relative to ALS-susceptible Palmer amaranth). Subsamples of the accessions were evaluated for their response to imazapic rates, which indicated variable levels of resistance across Palmer amaranth accessions. The rate of imazapic that provided 50% reduction in Palmer amaranth plant biomass (I50) for the known susceptible biotype was 0.9 g/ha of imazapic. Of the 10 accessions evaluated, 8 of them had I50 values that ranged from 3 to 297 g/ha of imazapic. The other two accessions could not be fit to the log-logistic dose–response curve and had undeterminable I50 values because of high levels of ALS resistance (> 1,400 g/ha of imazapic). Herbicide cross-resistance experiments indicated that 30 accessions were resistant to the ALS herbicides imazapic, chlorimuron, pyrithiobac, and diclosulam at the recommended field-use rates. However, each of these 30 accessions was susceptible to glyphosate. These data demonstrate that ALS-resistant Palmer amaranth occurs throughout the peanut-growing region of Georgia. Growers in Georgia will need to alter their weed-control programs in peanut to include herbicides with multiple modes of action that do not rely on ALS herbicides for effective Palmer amaranth control. Nomenclature: Chlorimuron; diclosulam; imazapic; pyrithiobac; Palmer amaranth, Amaranthus palmeri L; peanut, Arachis hypogea L.

Pollen-Mediated Dispersal of Glyphosate-Resistance in Palmer Amaranth under Field Conditions

Abstract In addition to being a strong competitor with cotton and other row crops, Palmer amaranth has developed resistance to numerous important agricultural herbicides, including glyphosate. The objective of this study was to determine if the glyphosate-resistance trait can be transferred via pollen movement from a glyphosate-resistant Palmer amaranth source to a glyphosate-susceptible sink. In 2006 and 2007 glyphosate-resistant Palmer amaranth plants were transplanted in the center of a 30-ha cotton field. Susceptible Palmer amaranth plants were transplanted into plots located at distances up to 300 m from the edge of the resistant pollen source in each of the four cardinal and ordinal directions. Except for the study plots, the interior of the field and surrounding acreage were kept free of Palmer amaranth by chemical and physical means. Seed was harvested from 249 and 292 mature females in October 2006 and 2007, respectively. Offspring, 14,037 in 2006 and 13,685 in 2007, from glyphosate-susceptible mother plants were treated with glyphosate when the plants were 5 to 7 cm tall. The proportion of glyphosate-resistant progeny decreased with increased distance from the pollen source; approximately 50 to 60% of the offspring at the 1- and 5-m distances were resistant to glyphosate, whereas 20 to 40% of the offspring were resistant at the furthest distances. The development of resistance was not affected by direction; winds were variable with respect to both speed and direction during the peak pollination hours throughout the growing season. Results from this study indicate that the glyphosate-resistance trait can be transferred via pollen movement in Palmer amaranth. Nomenclature: Glyphosate; Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; cotton, Gossypium hirsutum L.

Behavior of sulfentrazone in ionic exchange resins, electrophoresis gels, and cation-saturated soils.

Abstract Sulfentrazone persistence in soil requires many crop rotational restrictions. The sorption and mobility of sulfentrazone play an important role in its soil persistence. Thus, a series of laboratory experiments were conducted to mimic the soil properties of cation and anion exchange with different intermediates. The molecular characterization and ionization shift of sulfentrazone from a neutral molecule to an anion were determined using a three-dimensional graphing technique and titration curve, respectively. Sorption and mobility of 2.6 × 10−5 M 14C-sulfentrazone were evaluated using a soil solution technique with ion exchange resins and polyacrylamide gel electrophoresis, respectively. Solution pH ranged from 4.0 to 7.4. As pH increased, sulfentrazone sorption to an anion resin increased and its sorption to a cation resin decreased. Percent sulfentrazone in solution was pH-dependent and ranged between 0 to 18% and 54 to 88% for the anion and cation resins, respectively. Mobility of sulfentrazone on a 20% polyacryalmide gel resulted in Rf values of +0.02 and +0.39 for pH of 4.0 and 7.4, respectively. A double peak for sulfentrazone was detected in the polyacrylamide gel when the pH (6.0 and 6.8) was near the reported pKa of 6.56. There was no clear interaction for the sorption of sulfentrazone at 1.0 mg kg−1 to Congaree loamy sand or Decatur silty clay loam saturated with either calcium or potassium. Sulfentrazone behavior with the polyacrylamide electrophoresis gels and ion resins indicate the potential for this herbicide to occur as a polar or Zwitter ion. Sulfentrazone was adsorbed by potassium, calcium, and sodium saturated resins and subsequently desorbed using variable pH solutions. The level of sulfentrazone adsorption will vary among soil types and the amount of desorption into solution may be soil cation-dependent. Nomenclature: Sulfentrazone.

Pollen Grain Size, Density, and Settling Velocity for Palmer Amaranth (Amaranthus palmeri)

Abstract Palmer amaranth is resistant to several herbicides, including glyphosate, and there is concern that the resistance traits can be transferred between spatially segregated populations via pollen movement. The objective of this study was to describe the physical properties of Palmer amaranth pollen, specifically size, density, and settling velocity (Vs), that influence pollen flight. The mean diameter for Palmer amaranth pollen, as determined by light microscopy, was 31 µm (range of 21 to 38 µm); mean pollen diameter as measured with the use of an electronic particle sizer was 27 µm (range of 21 to 35 µm). The mean density of the solid portion of the pollen grain was 1,435 kg m−3. Accounting for the density of the aqueous fraction, the mean density of a fully hydrated pollen grain was 1,218 kg m−3. By Stokess law, the estimated mean theoretical Vs for individual Palmer amaranth pollen grains was 3.4 cm s−1 for the range of pollen diameters with a mean of 31 µm and 2.6 cm s−1 for the range of pollen diameters with a mean of 27 µm. Results from laboratory studies indicated the majority of single pollen grains settled at a rate of 5.0 cm s−1. The difference between the theoretical and empirical estimates of Vs was likely due to changes in pollen density and shape postanthesis, which are not accounted for using Stokess law, as well as the presence pollen clusters. Nomenclature: Glyphosate; Palmer amaranth, Amaranthus palmeri S.Wats.

Glyphosate Hinders Purple Nutsedge (Cyperus rotundus) and Yellow Nutsedge (Cyperus esculentus) Tuber Production

Abstract The phase-out of methyl bromide requires alternative nutsedge management options in vegetable systems. Options that target tuber production, the primary means of reproduction, will be most beneficial. A study was conducted to evaluate the response of purple nutsedge and yellow nutsedge foliar growth and tuber production to a range of glyphosate rates. Glyphosate was applied at six rates between 0.41 and 2.57 kg ae ha−1 to 5-wk-old nutsedge plants with multiple shoots. The rate of glyphosate needed to reduce growth 50% (I50) was similar for purple nutsedge foliar growth (0.58 kg ha−1) and tuber biomass (0.55 kg ha−1). In contrast, I50 for yellow nutsedge foliar growth was 0.73 kg ha−1, which was greater than the I50 for tuber biomass (0.41 kg ha−1). First-order tubers, those directly attached to the initial tuber, had an I50 of 0.70 and 0.44 kg ha−1 of glyphosate for purple nutsedge and yellow nutsedge tuber biomass, respectively. For all higher-order tubers, I50 values ranged from 0.29 to 0.60 and 0.14 to 0.30 kg ha−1 of glyphosate for purple nutsedge and yellow nutsedge tuber biomass, respectively. Glyphosate at 0.74 kg ha−1 prevented fourth-order purple nutsedge and third-order yellow nutsedge tuber production (terminal tubers for yellow nutsedge). Fifth- and sixth-order purple nutsedge tuber production was eliminated by the lowest tested rate of glyphosate (0.41 kg ha−1). Effective nutsedge management options will require consistent control between spring and autumn crops. Glyphosate is economical, poses no herbicide carryover issues to vegetables, and minimizes nutsedge tuber production; therefore, it is a suitable candidate to manage nutsedges. Nomenclature: Glyphosate; purple nutsedge, Cyperus rotundus L. CYPRO; yellow nutsedge, Cyperus esculentus L. CYPES.

Growth and Reproduction of Benghal Dayflower (Commelina benghalensis) in Response to Drought Stress

Abstract Greenhouse experiments were conducted to evaluate growth and reproduction of Benghal dayflower in response to daily (nondrought stress) and weekly (drought stress) irrigation. With daily irrigation, Benghal dayflower plants added one leaf per plant each week during the initial 6 wk of growth and then increased leaf number eightfold between the intervals of 6 and 10 wk after planting (WAP) and 10 and 15 WAP. By 15 WAP each plant had in excess of 400 leaves. Benghal dayflower plant height increased 2.4 cm wk−1 between 5 and 14 WAP, increasing eightfold during this interval, while plant width increased 20-fold. Aerial spathe formation began between 7 and 8 WAP, with 26 spathes maturing (containing seeds ready for dispersal) each week beginning at 11 WAP. In another study, the influence of duration of drought stress at intervals between 7 and 56 d on early growth and development of cotton and Benghal dayflower was evaluated. Benghal dayflower aboveground biomass was 3.5 times greater than cotton. There was an inverse linear relationship between aboveground biomass and duration of drought stress for cotton and Benghal dayflower, though there was a more rapid decline for Benghal dayflower. A final study evaluated Benghal dayflower response to weekly moisture regimes that approximated 13, 25, 50, and 100% of soil field capacity. Benghal dayflower aerial spathes were 4.6 times more numerous than subterranean spathes. Rate of seed production decreased in a linear manner with decreasing water volume, however, rate of subterranean seed production was less affected by water volume than was aerial seed production. These data indicate that Benghal dayflower thrives under high soil moisture regimes, but that drought stress inhibits growth and reproduction. Cotton appears to be more drought tolerant than Benghal dayflower. Judicious water use in cotton cropping systems in the southeastern United States could be an important component of multiple-tactic Benghal dayflower management program. Nomenclature: Benghal dayflower, Commelina benghalensis L. COMBE; cotton, Gossypium hirsutum L.

Residual Herbicide Dissipation from Soil Covered with Low-Density Polyethylene Mulch or Left Bare

Abstract Field studies were conducted to examine the dissipation of three soil-applied residual herbicides for bare soil compared with soil under low-density polyethylene (LDPE) mulch in 2003 and 2004. Studies indicated that halosulfuron and S-metolachlor dissipation was more rapid for bare soil than soil under LDPE mulch. Sulfentrazone dissipation from bare soil was equal to soil under LDPE mulch in 2003. However, sulfentrazone dissipation in 2004 was more rapid for soil under LDPE mulch than for bare soil. The order for half-life, defined as time for 50% dissipation (DT50), varied by herbicide and soil exposure. Averaged across 2003 and 2004, metolachlor DT50 was 2 d, halosulfuron 7 d, and sulfentrazone 16 d for bare soil. S-metolachlor DT50 was 4 d, halosulfuron 10 d, and sulfentrazone 13 d for soil under LDPE mulch. Correlation between quantified herbicide dissipation and bioassay for bare soil compared with soil under LDPE mulch in 2004 indicated that assay species root dry weights were negatively correlated with herbicide concentration. Data indicated that S-metolachlor and sulfentrazone bioassays, with oat and cotton, respectively, could be used to assess the level of dissipation for each herbicide. Nomenclature: Halosulfuron; S-metolachlor; sulfentrazone; low-density polyethylene mulch; cotton, Gossypium hirsutum L; oat, Avena sativa L

Addition of nonionic surfactant to glyphosate plus chlorimuron

Field studies were conducted in South Carolina and Georgia to evaluate weed control and soybean tolerance and yield after nonionic surfactant addition to combinations of chlorimuron plus an adjuvant-containing glyphosate formulation. Treatments included glyphosate alone, at 420 or 840 g ae/ha, or in combination with 6 or 9 g ai/ha chlorimuron and all possible combinations with or without 0.25% (v/v) nonionic surfactant. Other treatments included a weed-free and nontreated check. Chlorimuron plus glyphosate improved entireleaf, smallflower, and tall morningglory control over glyphosate alone, but nonionic surfactant addition did not further improve the control of any species, except tall morningglory. Up to 31% early-season injury was observed with the three-way mixture. Soybean injury was greater, and yields were reduced in one of three trials when nonionic surfactant was added to chlorimuron plus glyphosate combinations. This research indicates that there would be no benefit from the nonionic surfactant addition to this adjuvant-containing glyphosate formulation when combined with chlorimuron. Nomenclature: Chlorimuron; glyphosate; entireleaf morningglory, Ipomoea hederacea var. integriuscula Gray #3 IPOHG; smallflower morningglory, Jacquemontia tamnifolia (L.) Griseb. # IAQTA; tall morningglory, Ipomoea purpurea (L.) Roth # PHBPU; soybean, Glycine max (L.) Merr. Additional index words: Adjuvant, crop injury, glyphosate-resistant soybean, herbicide mixtures. Abbreviations: ALS, acetolactate synthase.

Response of seven peanut (Arachis hypogaea) cultivars to sulfentrazone.

Abstract: Field studies were conducted to evaluate the tolerance of peanut cultivars ‘Florunner’, ‘Georgia Green’, ‘Sunoleic 95R’, ‘AgriTech GK7’, ‘NC-7’, ‘ViruGard’, and ‘Spanco’ to sulfentraone. Herbicide treatments included sulfentrazone applied as a single treatment preemergence (PRE) at 0.14, 0.21, 0.28, 0.35, or 0.42 kg ai/ha or as a PRE followed by (fb) an at cracking (AC) application (0.14 kg ai/ha PRE fb 0.14 kg ai/ha AC, 0.21 kg ai/ha PRE fb 0.14 kg ai/ha AC, 0.21 kg ai/ha PRE fb 0.21 kg ai/ha AC, 0.28 kg ai/ha PRE fb 0.07 kg ai/ha AC, or 0.28 kg ai/ha PRE fb 0.14 kg ai/ha AC). Imazapic and paraquat applied early postemergence (EPOT) were included along with a weed-free control. NC-7 exhibited higher early-season injury (ranging from 1 to 29%) than other cultivars across all sulfentrazone applications. However, this injury did not affect yield when compared with the untreated weed-free check. Overall, peanut tolerance to sulfentrazone was high across all varieties. Nomenclature: Imazapic, (±)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-methyl-3-pyridinecarboxylic acid); paraquat-dichloride, 1,1′-dimethyl-4,4′-bipyridinium dichloride; sulfentrazone, N-[2,4-dichloro-5-[4-(difluoromethyl)]-4,5-dihydro-3-methyl-5-oxo-1H-1,2,4-triazol-1-yl]phenyl]methanesulfonamide; peanut, Arachis hypogaea L., ‘AgriTech GK7’, ‘Florunner’, ‘Georgia Green’, ‘NC-7’, ‘Sunoleic 95R’, ‘Spanco’, ‘ViruGard’. Additional index words: Peanut injury, peanut cultivar, peanut yield, herbicide susceptibility. Abbreviations: AC, at cracking; ALS, acetolactate synthase; EPOT, early postemergence; fb, followed by; POST, postemergence; PPI, preplant incorporated; PRE, preemergence.