In greenhouse and field studies, fluazifop {(±)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy] propanoic acid} controlled quackgrass [Agropyron repens (Beauv.) # AGRRE] in potato (Solanum tuberosum L. ‘Russet Burbank’). Quackgrass control with fluazifop at 0.25 and 0.5 kg ai/ha in the greenhouse was greatest with application at the two-leaf stage compared with the four-and six-leaf stages. Control was not affected by rhizome length, with similar control obtained of plants grown from 2- and 10-node rhizome pieces. Removal of quackgrass shoots at 6 or 24 h after treatment resulted in decreased control at application rates of 0.25 and 0.5 kg/ha. Fluazifop at 0.5 kg/ha controlled quackgrass treated at the four-leaf stage. The active enantiomer of fluazifop at half the rate of the racemic mixture controlled the quackgrass to the same extent. Fluazifop did not affect potato yield.

Two phototoxic compounds, xanthotoxin (8-methoxysporalen) and bergapten (5-methoxypsoralen), were isolated from spring parsley [Cymopterus watsonii (Coult. & Rose) Jones] by thin-layer chromatography, and identified by comparison of Rf values, infrared spectra, and melting points, with pure samples. The concentration of xanthotoxin was two to four times that of bergapten in plant samples collected during the 2-month growing season. Seeds contained xanthotoxin and bergapten in equal concentrations.

Path analysis was used to assess the efficacy of the fiddleneck flower gall nematode as a weed biocontrol agent of coast fiddleneck in competition with wheat during 2 yr of field experiments. The path analysis revealed that the number of inflorescences/plant for fiddleneck and the number of heads/plant for wheat were the most important yield components that determine fecundity and seed yield. The density of fiddleneck had a much greater impact on the yield components of fiddleneck than did the density of wheat or the nematode rate of inoculation. The nematode had its greatest negative impact on the number of seeds/flower of fiddleneck and its greatest positive impact on the number of heads/plant of wheat. Path analysis predicts that a biocontrol agent that has a large negative direct effect on the number of inflorescences/plant for fiddleneck would be more efficacious in decreasing fecundity and seed yield than an agent that only impacts the number of flowers/inflorescence, seeds/flower, or biomass/seed.

Henbit is a facultative broadleaf winter annual in the Lamiaceae family. Acetolactate synthase (ALS) inhibitors are primarily used to control a broad spectrum of weeds, including henbit. During 2012 to 2013, field applications of ALS-inhibiting herbicides were ineffective in controlling a henbit population from Marion County, KS (MCK). To confirm field-evolved resistance to ALS inhibitors, response of MCK henbit and a known susceptible henbit population from Kansas (DPS) to varying doses of three different ALS inhibitors were examined: chlorsulfuron, imazamox, and propoxycarbazone. Results of the dose–response experiments suggest that the MCK population is highly resistant to chlorsulfuron (resistance index [R/S] > 1,000) and propoxycarbazone (R/S = 331) but is susceptible to imazamox. A full-length ALS gene sequence obtained using the 5′- and 3′- rapid amplification of complementary DNA ends approach revealed a Pro197 to Arg point mutation (a common mutation that confers resistance to sulfonylurea herbicides, e.g., chlorsulfuron) in the MCK henbit. No other known resistance-conferring mutations were found in the study. Evolved resistance to major classes of ALS inhibitors in the MCK henbit will reduce herbicide options for its control. To our knowledge, this is the first case of evolution of herbicide resistance in henbit.

Field studies were conducted at Delta Research and Extension Center, Stoneville, MS, in 1996, 1997, and 1998 to assess the effect of tillage systems (conventional tillage and subsoiling) on the environmental fate of imazaquin in a Sharkey clay soil. Imazaquin was applied preemergence at 140 g ai ha−1. Subsoiling in the fall did not affect imazaquin dissipation, total volume of runoff, imazaquin concentration in runoff, or imazaquin concentration in soil, as determined by chemical extraction. A corn root bioassay revealed no differences due to tillage systems in plant-available imazaquin in soil. Imazaquin concentration measured by chemical extraction or bioassay diminished over time, with a half-life ranging from 8 to 25 d. A field bioassay utilizing cotton and corn was conducted in 1997 and 1998 using plots that had received imazaquin the previous year. In 1997, 2 wk after planting, cotton and corn injury ranged from 3 to 15%, whereas no injury was observed in 1998. Injury symptoms declined over time, with no injury 5 wk after planting in either year. Although early-season cotton stunting and slight discoloration of corn was apparent in 1997, imazaquin residues did not affect subsequent vegetative and reproductive growing patterns of cotton or corn. In 1998, corn and cotton height were significantly greater in subsoiled plots compared to conventional tillage.

Fomesafen is a protoporphyrinogen oxidase–inhibitor herbicide with an alternative mode of action that provides PRE weed control in strawberry [Fragaria×ananassa (Weston) Duchesne ex Rozier (pro sp.) [chiloensis×virginiana]] produced in a plasticulture setting in Florida. Plasticulture mulch could decrease fomesafen dissipation and increase crop injury in rotational crops. Field experiments were conducted in Balm, FL, to investigate fomesafen persistence and movement in soil in Florida strawberry systems for the 2014/2015 and 2015/2016 production cycles. Treatments included fomesafen preplant at 0, 0.42, and 0.84 kg ai ha−1. Soil samples were taken under the plastic from plots treated with fomesafen at 0.42 kg ha−1 throughout the production cycle. Fomesafen did not injure strawberry or decrease yield. Fomesafen concentration data for the 0.0- to 0.1-m soil depth were described using a three-parameter logistic function. The fomesafen 50% dissipation times were 37 and 47 d for the 2014/2015 and 2015/2016 production cycles, respectively. At the end of the study, fomesafen was last detected in the 0.0- to 0.1-m depth soil at 167 and 194 d after treatment in the 2014/2015 and 2015/2016 production cycles, respectively. Fomesafen concentration was less than 25 ppb on any sampling date for 0.1- to 0.2-m and 0.2- to 0.3-m depths. Fomesafen concentration decreased significantly after strawberry was transplanted and likely leached during overhead and drip irrigation used during the crop establishment.

Control of glyphosate-resistant (GR) horseweed in soybean with glyphosate (900 g ai ha−1) plus saflufenacil (25 gaiha−1) has been variable. The objective of this research was to determine the effect of GR horseweed height, density, and time of day (TOD) at application on saflufenacil plus glyphosate efficacy in soybean. All experiments were completed six times during a 2 yr period (2014, 2015) in fields previously confirmed with GR horseweed. Applications from 0900 to 2100 hours provided optimal control of GR horseweed 8 WAA. Soybean yield paralleled GR horseweed control with the highest yield of 3000kgha−1 at 1500 hours, and the lowest yield of 2400kgha−1 at 0600 hours. The height and density of GR horseweed at application had minimal effect on saflufenacil efficacy. Saflufenacil provided>99% control of GR horseweed when applied to small plants and low densities; however, control decreased to 95% when>25 cm tall, and to 96% in densities>800 plants m−2 at 6 WAA due to some plant regrowth. TOD of application had a greater influence on GR horseweed control with saflufenacil than height or density. To optimize control of GR horseweed, saflufenacil should be applied during daytime hours to small plants at low densities. Optimizing GR horseweed control minimizes weed seed return and weed interference.

Root colonization by soil microorganisms has been shown to increase the activity of glyphosate in resistant and susceptible biotypes of giant ragweed and a susceptible common lambsquarters biotype, but not in horseweed biotypes. The objective of this study was to investigate the colonization of roots in glyphosate-resistant and -susceptible giant ragweed and horseweed biotypes, and glyphosate-tolerant and -susceptible biotypes of common lambsquarters after a sublethal glyphosate application. The three weed species were grown separately in sterile and unsterile field soil and treated with glyphosate at two sublethal rates. Soil microbes were isolated from the roots onto sterile media 3 d after the glyphosate treatment. The susceptible biotypes of giant ragweed and horseweed grown in unsterile soil were colonized by more soil microbes at the higher rate of glyphosate, compared to the resistant biotype grown in unsterile soil. Oomycetes were isolated separately on a selective media and they were also more prevalent in the roots of the susceptible biotypes of each weed species grown in the unsterile soil when glyphosate was applied at the highest rate. Therefore, the ability of these three weed species to tolerate a glyphosate application may involve differences in the susceptibility to soil microbial colonization, especially oomycetes.

The effect of constant or various fluctuating temperature regimes and single or multiple tubers in rhizome chains on tuber sprouting of six purple nutsedge ecotypes was determined. After 24 d at constant 20 C, budbreak of tubers detached from the rhizome chain (single tubers) ranged from 11 to 85% among ecotypes. When dormant tubers were exposed to a single 0.5- to 12-h, 35 C pulse followed by constant 20 C, budbreak increased for all ecotypes; daily 0.5-h, 35 C pulses from a 20 C base temperature for three to seven cycles did not significantly increase budbreak more than these did for a single cycle. Shoot length increased linearly for all ecotypes as the number of 0.5-h, 35 C pulse cycles increased, although the magnitude of shoot elongation varied with ecotype. At a 20 and 30 C (12:12 h) daily alternating temperature regime, 98% of single tubers from a Kamuela, HI, ecotype produced actively growing shoots (active tubers), whereas only 32 to 60% of tubers in two- to six-tuber chains were active. The rhizome chain effect on budbreak was minor because ≥ 90% of the tubers in rhizome chains had budbreak. Using a range of constant and alternating temperatures on single tubers and four-tuber chains, similar results were observed for all six ecotypes as for the Kamuela ecotype. Although alternating temperature increased active tubers for both single tubers and tubers in chains, it did not overcome apical dominance among tubers in rhizome chains in suppressing active tubers. The budbreak and shoot elongation stimulation by alternating temperatures and high-temperature pulses appear to be common physiological responses to all purple nutsedge ecotypes examined in this study.

Control of common lambsquarters with POST applications of glyphosate in glyphosate-resistant crops generally has been effective. In 2002, common lambsquarters plants from Westmoreland County, VA, were identified after not being controlled with a POST glyphosate application in glyphosate-resistant soybean. Plants from this site that survived glyphosate were collected in both 2002 and 2004. The objective of this research was to evaluate the susceptibility of F1 and F2 progeny from these common lambsquarters plants, relative to the susceptibility of common lambsquarters collected in Montgomery County, VA. F1 progeny of the Westmoreland plants from both the 2002 and 2004 collections showed reduced response to glyphosate relative to the Montgomery collection. Vigor reduction of F1 progeny from three 2004 Westmoreland source plants with 0.84 kg ae ha−1 of glyphosate ranged from 66 to 85% at 28 d after treatment (DAT), compared with 89% for the Montgomery collection. Evaluation of four Westmoreland F2 common lambsquarters lines derived from 2002 collections indicated significant differences in glyphosate sensitivity. Fifteen F2 lines were generated from 2004 collections from each of three Westmoreland source plants and from the Montgomery source. For the least sensitive Westmoreland source, vigor reduction ranged from only 24 to 36% across F2 lines in response to 1.68 kg ha−1 of glyphosate at 28 DAT relative to 55 to 100% for the Montgomery source. I50 estimates for fresh weight reduction were 0.91 and 0.32 kg ha−1, for these sources, respectively. Sequential treatments of 0.42, 1.26, and 1.68 kg ha−1 applied at 3-wk intervals to the least susceptible 2004 Westmoreland F2 line resulted in only 37% vigor reduction and no mortality among 360 treated plants.

Vapor of a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-loluidine (trifluralin) emitted from soils was collected in xylene and assayed by gas chromatography. Increasing the trifluralin application rate in soil surface treatments from 0.56 to 4.48 kg/ha resulted in substantial increases in vaporization from soils during the 2 hr immediately after treatment. Increasing the spray volume from 2.34 to 300 kl/ha with water reduced the rate of trifluralin vapor emission by approximately half. When trifluralin was applied as a spray (2.34 kl/ha) and then incorporated into soils of varying moisture contents, 4 to 10 times more trifluralin volatilized in 12 hr than when it was percolated into air-dry soils with sufficient water to yield equivalent soil moisture contents. The techniques employed were not sufficiently sensitive to detect trifluralin vapor loss occurring more than 12 hr after treatment. The amount of trifluralin lost during the first 12 hr was less than 5% of that applied to the soil, even under the most favorable conditions for volatilization.

Under greenhouse conditions, vaporization of oxyfluorfen [2-chloro-1-(3-ethoxy-4-nitrophenoxy)-4-(trifluoromethyl)benzene] formulations (2 EC, 25 WP, 1G) was studied on cabbage (Brassica oleracea L. var. capitata L. F. alba DC. ‘Market Prize’), tomato (Lycopersicon esculentum Mill. ‘Campbell 37’), cucumber (Cucumis sativus L. ‘Straight 8’), and lettuce (Lactuca sativa L. ‘Salad Bowl’). The emulsifiable concentrate and granular formulations caused vapor injury to plants exposed under glass bell jars. Vapor injury to all species increased when exposure times were increased from 8 to 72 h. Lettuce was the most susceptible test species. Exposure temperatures of 18 and 29 C did not alter the amount of vapor injury apparent on tomato foliage. Oxyfluorfen vapor injury did not reduce dry weight of any test species.

Solutions containing 24 or 120 g ae/L of amine or salt formulations of glyphosate [N-(phosphonomethyl) glycine], triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid}, and picloram (4-amino-3,5,6-trichloropicolinic acid) were applied to small shrubs with a carpeted roller. Glyphosate reduced the live canopy cover of false broomweed (Ericameria austrotexana M.C. Johnston) by 83 to 87% at one site but only by 37 to 58% at another location. The lower concentration of picloram reduced false broomweed cover by at least 92%, and the higher concentration completely controlled the weed at both study sites. Effectiveness of triclopyr was usually intermediate between glyphosate and picloram. Removing a scraper designed to abrade stems prior to herbicide deposition reduced the effectiveness of picloram but not other herbicides. The herbicides reduced canopy cover of common goldenweed [Isocoma coronopifolia (Gray) Greene ♯3 IOCCO] by 92 to 100%. Common goldenweed was also controlled by 2,4-D [(2,4-dichlorophenoxy)acetic acid] amine applied with the carpeted roller. In glasshouse experiments, 1.2 to 60 g ae/L of 2,4-D amine reduced live canopy dry weight of common goldenweed by 73 to 88% at 5 months after application with a carpeted roller.

The loss of atrazine [2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine] from metal planchets was decreased by an increase in the amount applied and linseed oil (LO) additive. LO reduced volatility to a greater extent than did other additives. Vapor loss from plant leaves was influenced by plant species, plant leaf maturity, soil moisture, concentration of atrazine applied, volume of oil additive, type of additive, and duration of exposure. Generally, atrazine uptake by plants was inversely related to volatility. Uptake and translocation of 14C-atrazine label was increased more by oil additives than surfactant (SURF) compared to no additive. LO increased 14C-label translocation more in yellow foxtail [Setaria glauca lutescens (Weigel) Hubb.], green foxtail [Setaria viridis (L.) Beauv.], and barnyardgrass [Echinchloa crus-galli (L.) Beauv.] than in corn (Zea mays L.).

Field studies were conducted to determine the effect of crop species, tillage, and rye mulch on sicklepod growth. Competition from soybean and sunflower reduced sicklepod biomass by 19 and 77%, respectively. Sicklepod height was reduced by rye mulch. Rye root residue caused 49% reduction in sicklepod biomass, while whole plant rye mulch reduced weed growth an additional 12%. Crop competition reduced sicklepod shoot biomass to a greater extent when rye residue was present. Crop seed yield was 32% lower in tilled plots compared with no-tillage due to more weed competition in the tilled plots. In the presence of rye root residue, soybean yield showed a two-fold increase over no mulch or whole plant rye mulch. Sunflower yield increased by 30% in the presence of rye residue compared with no mulch.

A greenhouse study was conducted to determine the effectiveness of adjuvants on postemergence large crabgrass control with dithiopyr. Using barriers to isolate foliage or soil, the primary site of dithiopyr uptake was found to be via foliage. Laboratory and greenhouse studies elucidated the factors involved in enhancement of crabgrass control by dithiopyr with nine superior adjuvants. Two adjuvants increased dithiopyr absorption by 3.7 to 4.3% over the herbicide alone. However, none of the adjuvants increased dithiopyr translocation out of the treated leaf. Treatment of a single large crabgrass leaf indicated that enhancement of dithiopyr activity by adjuvants involved an effect on translocation, on spray retention, and on the external movement of the spray solution to the apical meristem, with the relative importance of any 1 factor depending on the particular adjuvant used. Because of the large losses of dithiopyr during absorption/translocation studies, a separate volatilization experiment determined that 70% of the applied dithiopyr volatilized within 24 h at 24 C. Dithiopyr absorption decreased and volatilization increased with each 10 C increase in temperature. Nonrecoverable dithiopyr ranged from 46 to 55% at 5 C to 92 to 95% at 35 C. Volatility losses of this magnitude may mitigate adjuvant enhancement of postemergence grass activity of dithiopyr.

In field plots, Japanese millet [Echinochloa crus-galli (L.) Beauv. var. frumentacea (Link) W. F. Wight] competed strongly with yellow nutsedge (Cyperus esculentus L.), reducing its dry weight and the number of plants and tubers with no loss of dry weight to the millet. Because Japanese millet did not shade the nutsedge for more than half the day, the main competition was thought to be ‘root’ competition (nutrient and water) with light competition as a minor factor. The greenhouse studies verified that roots were the primary area of competition. However, light competition between the two species was stronger in the greenhouse than in field plots. Also, when confined to limited light and soil volumes with a 1-month growing period, Japanese millet was reduced in dry weight by nutsedge competition.

Laboratory experiments were conducted to determine the effect of three phenolic acids, acetone, and riboflavin on aqueous photolysis of chlorimuron and imazaquin. The phenolic acids investigated were caffeic acid (CA), ferulic acid (FA), and p-coumaric acid (PCA). Treatment solutions were contained in quartz vessels and irradiated with 300 to 400 nm UV light in a photoreactor. The extrapolated photolysis half-life of chlorimuron in pure solution was 107 h, compared to a half-life of 0.42 h for pure aqueous imazaquin. Chlorimuron in solutions containing 10 ppmw riboflavin, acetone, CA, FA, or PCA exhibited half-lives of 9, 57, 58, 67, and 146 h, respectively. Imazaquin in solutions containing 10 ppmw riboflavin, acetone, CA, FA, or PCA had half-lives of 0.70, 0.55, 0.55, 0.48, and 0.55 h, respectively. The presence of PCA in aqueous media delayed chlorimuron photolysis, whereas all other compounds, especially riboflavin, sensitized chlorimuron photolysis. In contrast, imazaquin photolysis was delayed in the presence of the test compounds, with riboflavin having the greatest effect and causing in a 68% increase in imazaquin half-life over that of imazaquin alone. Quantum yields for sensitized photolysis of chlorimuron by riboflavin and for riboflavin by imazaquin were 0.1134 and 0.0477, respectively. These results suggest that some soluble and naturally occurring organic compounds may enhance chlorimuron photolysis yet delay imazaquin photolysis in surface waters.

The herbicide 3-tert-butyl-5-chloro-6-methyluracil (terbacil) applied in an isoparaffinic oil caused, in susceptible ivyleaf morningglory (Ipomoea hederacea (L.) Jacq.), rapid and nearly complete inhibition of photosynthesis, from which it never recovered. Photosynthesis in peppermint (Mentha peperita L.), a tolerant species, was decreased only temporarily. Terbacil was readily absorbed by leaves of both species; however, there was little or no movement out of the treated peppermint leaves. Terbacil was translocated out of the treated leaves of ivyleaf morningglory to the untreated leaves and shoot apex. It was metabolized in both species but at a higher rate in peppermint. Therefore, it appears that foliarly-applied terbacil may be bound in peppermint leaves and this, together with the higher rate of metabolism, may contribute to the tolerance of peppermint. Terbacil was readily taken up and translocated to the foliage of both species when applied to the roots. Again, it was metabolized in both plants but at a higher rate in peppermint. However, the rate of metabolism alone did not appear to be sufficient to account for peppermint tolerance to root-applied terbacil.

Seventy-three rice (Oryza sativa L.) cultivars and uniform advanced-generation dwarf lines were evaluated for their response to molinate (S-ethyl hexahydro-1H-azepine-1-carbothioate) at concentrations of 0, 1, 2, and 4 ppmw in a 7-day laboratory screening test. These rice entries varied significantly in their response to each molinate concentration when shoot height of each entry was compared to that at the 0 ppmw level. Entries were ranked according to their molinate response at the 2 ppmw concentration and categorized as being either tolerant, intermediate or susceptible. Entries categorized as susceptible at 2 ppmw were further subdivided into highly susceptible and moderately susceptible categories based on their response to the 1 ppmw concentration. Those categorized as tolerant at the 2 ppmw concentration were likewise divided into highly tolerant and moderately tolerant categories on the basis of their response to the 4 ppmw concentration. The results of this bioassay indicates the possible existence of a genetically inherited response to molinate which might be manipulated by plant breeders to develop cultivars which have a high tolerance to molinate.