bluuebunny/crossref_metadata_2025 huggingface dataset:
Physiological Responses of Downy Brome (Bromus tectorum) Roots toPseudomonas fluorescensStrain D7 Phytotoxin
Authors: Tranel Patrick J. , Gealy David R. , Irzyk Gerald P.
Published: May 2019

Effects of a crude preparation of a phytotoxin fromPseudomonas fluorescensstrain D7 (D7) on various physiological processes were evaluated in roots of downy brome seedlings. Cell division, respiration, and synthesis of protein, RNA, and DNA were not inhibited or only slightly inhibited under treatment conditions that caused substantial inhibition of root elongation. Significant inhibition of each of these processes was detected by known inhibitors included in each study to verify sensitivity of the procedures used. Disruption of lipid synthesis and membrane integrity by the crude phytotoxin preparation was significant and might account for inhibition of root elongation. Additional studies on these two possible target sites were conducted with partially purified phytotoxin. In intact roots, incorporation of [14C]acetate and [14C]malonic acid into lipophilic fractions was reduced by 50% or greater during a 1-h treatment with concentrations between 10 and 100 μg ml−1of the partially purified phytotoxin preparation. In a membrane integrity study, a similar treatment increased radioactivity efflux 250% from seedlings preloaded withscyllo-[R-3H]inositol. Additionally, disruption of lipid synthesis and membrane integrity by the partially purified phytotoxin was dose dependent. Collectively, these findings indicate that the D7 phytotoxin may inhibit downy brome root elongation through its effects on lipid synthesis and membrane integrity.
Soybean [Glycine max(L.)] Response to Lactofen
Authors: Wichert Rex A. , Talbert Ronald E.
Published: May 2019

Field studies were conducted in 1987 and 1988 to evaluate the vegetative response of soybeans to early postemergence applications of lactofen and to measure crop recovery. Soybean injury 4 d after treatment (DAT) averaged 29 and 34% with 0.22 and 0.44 kg ai ha−1of lactofen, respectively. Severity of leaf injury depended on leaf size at the time of lactofen application. Leaf area of soybean trifoliates one through three were reduced 30 to 45% at 12 DAT but recovered by 26 DAT. Later emerging trifoliates were not affected. Fresh weights of lactofen-treated soybeans were less than those of untreated plants 4, 8, and 16 DAT, and dry weights were less at 16 and 26 DAT. Lactofen reduced stomatal conductance of expanded leaves but had no effect on unexpanded leaves. Soybean yields did not differ with lactofen treatment.
Shoot and Root Interference of Common Cocklebur (Xanthium strumarium) and Soybean (Glycine max)
Authors: Bozsa Robert C. , Oliver Lawrence R.
Published: May 2019

Field experiments were conducted in 1986 and 1987 to determine the effects of shoot and root interference of common cocklebur and soybean. Plants were grown in porous membrane envelopes. Common cocklebur plants had a longer vegetative growth period than soybean and were twice as tall as soybean at maturity. Soybean root and shoot dry weight and seed yield were reduced by shoot and whole plant (root and shoot) interference of common cocklebur, with whole plant interference giving the greatest reduction. Common cocklebur growth was affected little by soybean interference. Common cocklebur shoot interference alone reduced total soybean seed weight by 48%, the amount caused by common cocklebur whole plant interference.
Influence of Tillage on Soybean (Glycine max) Herbicide Carryover to Grass and Legume Forage Crops in Missouri
Authors: Walsh J. D. , Defelice M. S. , Sims B. D.
Published: May 2019

Studies were established in 1988, 1989, and 1990 at two locations in Missouri to study the influence of fall tillage and herbicides on carryover of several residual soybean herbicides to grass and legume forage crops. Chlorimuron, clomazone, imazaquin, imazethpyr, and metribuzin plus chlorimuron were applied at their registered and 2X-registered rates in soybeans. Forage crops were planted the following fall and spring after herbicide application and evaluated for carryover effects. Fall tillage did not influence the carryover potential of these herbicides. However, herbicides injured several of the rotational crops. This injury was crop species and herbicide specific.
Late-Season Interference of Spurred Anoda in Chile Peppers
Authors: Schroeder Jill
Published: May 2019

Broadleaf weeds, including spurred anoda, emerge after direct-seeded chile peppers are thinned. Field experiments were conducted in 1989, 1990, and 1991 to determine the effect of spurred anoda density on green and red pepper yield, quality, and ease of hand harvest. Spurred anoda was established immediately after peppers were thinned at initial densities of 0, 3, 6, 12, 24, or 48 plants 9 m−1row. The 1991 experiment also evaluated the influence of delayed pepper thinning and concurrent spurred anoda establishment on the competitive effect of spurred anoda. Spurred anoda were beginning to flower at green harvest and senescing at red harvest regardless of planting date. Spurred anoda were taller and accumulated more biomass when planted at a pepper thinning stage of 10 cm compared to 20 cm. Spurred anoda that emerged after thinning peppers reduced yield and ease of harvest of green and red peppers but not the quality of green peppers. Yield reduction at the highest spurred anoda density was 31 to 49% and 12 to 27% when peppers were thinned at 10 or 20 cm, respectively. Yield reduction was smaller when peppers were thinned at 20 cm tall than 10 cm tall and appeared to be associated with reduced spurred anoda biomass. Time required to hand harvest 1 kg of green or red peppers increased as spurred anoda density increased when peppers were thinned at 10 cm.
Formation and Movement of14C-Atrazine Degradation Products in a Sandy Loam Soil Under Field Conditions
Authors: Sorenson Brent A. , Wyse Donald L. , Koskinen William C. , Buhler Douglas D. , Lueschen William E. , Jorgenson Michael D.
Published: May 2019

Formation and distribution of14C-atrazine degradation products in the top 120 cm of soil were determined over 16 mo under field conditions in an Estherville sandy loam. After 16 mo, 78% of applied14C was still present in the soil. By 2 mo after treatment (MAT),14C had moved to the 30- to 40-cm depth; however, movement to depths greater than 40 cm was not observed. Greater than 98% of the14C remaining in the soil profile after 16 mo was in the top 20 cm. Twenty-seven percent of the14C applied was atrazine 16 MAT. Atrazine was the predominant14C-compound in soil below 10 cm. Hydroxyatrazine (HA) was the major degradation product in the top 10 cm of soil. The proportion of14C as HA in the top 10 cm increased from 15% 2 MAT to 37% 16 MAT. Deethylatrazine (DEA) was the predominant degradation product at the 10- to 30-cm depth and accounted for up to 23% of the14C present in the 10- to 20-cm depth. Deisopropylatrazine (DIA) accounted for less than 6% of the radioactivity recovered at any soil depth. The proportion of DEA and DIA increased while the proportion of HA decreased as soil depth increased, indicating that DEA and DIA are more mobile in soil than HA. Detection of HA at depths greater than 10 cm appears to be due to in situ degradation of atrazine previously moved to that soil depth. The large amount of14C remaining in the soil 16 MAT suggests that a large pool of atrazine and its degradation products are present in the soil for an extended period following application and have the potential to contaminate ground water.
Maize Dwarf Mosaic Virus Severity in Corn When Infected Johnsongrass is Controlled with a Postemergence Herbicide
Authors: Vangessel Mark J. , Coble Harold D.
Published: May 2019

The impact of either nicosulfuron or primisulfuron on maize dwarf mosaic virus (MDMV-A) severity in corn and corn susceptibility to MDMV-A infection were evaluated in greenhouse and laboratory studies. Neither herbicide influenced severity of MDMV-A in corn or corn susceptibility to the virus. Field experiments at five sites examined MDMV-A severity in corn as influenced by POST johnsongrass control with either nicosulfuron or primisulfuron applied at the fifth or eighth visible collar stage, no johnsongrass control, or johnsongrass control throughout the season with hoeing. Area under the cumulative virus curve (AUCVC) was reduced when either herbicide was applied at the fifth-leaf stage compared to the eighth-leaf stage, at four sites. Also, AUCVC was reduced when johnsongrass was controlled with a POST herbicide applied at the fifth or eighth collar stage compared to no control, at two sites. Increases in AUCVC were due to a greater number of infected plants rather than more severe MDMV-A infections.
Hexazinone and its Metabolites in Boreal Forest Vegetation
Authors: Sidhu Surindar S. , Feng Joseph C.
Published: May 2019

Concentrations of hexazinone and two metabolites in vegetation were determined for 2 yr after broadcast application of a 10% granular formulation of hexazinone at 2 and 4 kg ai ha−1rates in August 1986 in a boreal forest. Prewinter concentrations of hexazinone in stems of trembling aspen, Saskatoon berry, and willow ranged from 0.02 to 0.05 μg−1dry wt at 64 d after treatment (DAT). Absorption of hexazinone accelerated during spring thaw (1987), and residues in foliage of several woody and herbaceous species peaked during May to July. Patterns of accumulation of hexazinone and its metabolites varied with the species. Foliar concentrations diminished significantly by the end of the first growing season in 1987 (372 DAT) and were undetectable or extremely low at the end of the second growing season in 1988 (707 DAT). Based on the highest residue concentrations detected in several plant species, it is estimated that wildlife would ingest a maximum of 16, 28, and 24 mg of hexazinone, metabolite A, and metabolite B, respectively, for every kg of dry matter consumed. Reported LD50values suggest that application of hexazinone at the 4 kg ai ha−1rate or less poses no toxicological threat to wildlife.
Canopy Light Reflectance and Remote Sensing of Shin Oak (Quercus havardii) and Associated Vegetation
Authors: Everitt James H. , Escobar David E. , Villarreal Ricardo , Alaniz Mario A. , Davis Michael R.
Published: May 2019

Shin oak is a deciduous shrub that forms dense stands of brush on sandy soils in rangeland areas of the Rolling and High Plains of Texas. Plant canopy reflectance measurements made on shin oak showed that it had both low visible (0.63- to 0.69-μm waveband) and nearinfrared (0.76- to 0.90-μm waveband) reflectance values, a characteristic generally not shared by associated plant species or mixtures of species. The low reflectance values of shin oak caused it to have dark-red, reddish-brown, or brown image tones on color-infrared photographic, videographic, and SPOT satellite images that made it distinguishable from associated vegetation and other land use features. The optimum time to remotely distinguish this noxious shrub is during the mature phenological stage from June to September. Computer-based image analyses of video and satellite images showed that shin oak populations could be quantified. This technique can permit “percent land area” estimates of shin oak on rangelands. The aerial imagery is useful for detecting shin oak on smaller rangeland areas, whereas the satellite imagery is applicable in mapping large areas of shin oak distribution.
Compensatory Responses of Common Cocklebur (Xanthium strumarium) and Velvetleaf (Abutilon theophrasti) to Partial Shading
Authors: Regnier Emilie E. , Harrison S. Kent
Published: May 2019

Lower leaves of greenhouse-grown common cocklebur and velvetleaf were shaded to 5% of full light over a 12-d period while upper leaves remained exposed to full light to determine weed foliar and branching responses to partial shading similar to that encountered in soybean crops. Shading increased lower leaf senescence and specific leaf area, and decreased branch length and number of second-order leaves in both species compared to unshaded controls. Common cocklebur branched more extensively along the lower portion of its stem than velvetleaf under both shaded and unshaded conditions. Upper leaves of partially shaded velvetleaf were held in a more perpendicular position to the light source beginning 3 days after treatment (DAT) compared to upper leaves of unshaded plants. Shading of lower leaves caused an increase in upper (unshaded) leaf area beginning 3 and 6 DAT in velvetleaf and common cocklebur, respectively. Petiole length of upper leaves also increased in response to shading in both species. Total plant dry weight at 12 DAT was unaffected by shading in velvetleaf but was reduced 10% by shading in common cocklebur. While common cocklebur maintained greater lower shoot growth in the presence of shade than velvetleaf, there was a greater change in upper leaf angle by velvetleaf in response to shading than by common cocklebur. These results support previous field observations of apparent greater shade tolerance of common cocklebur compared to velvetleaf and indicate that both species have the ability to compensate for shading of lower leaves by altering upper shoot growth.
Fertilization and Mowing on Persistence of Indian Mockstrawberry (Duchesnea indica) and Common Blue Violet (Viola papilionacea) in a Tall Fescue (Festuca arundinacea) Lawn
Authors: Gray Elmer , Call Neysa M.
Published: May 2019

Research was conducted to determine the influence of soil fertilization and mowing on persistence of Indian mockstrawberry and common blue violet in an old, weakened tall fescue lawn. Fertilization treatments included: F0 = none applied; F1 = spring and fall application of 0.49 kg N, 0.21 kg P, and 0.41 kg K 100 m-2; and F2 = spring and fall application of twice the F1 rate. Mowing treatments were: M0 = not mowed; and M1 and M2 = mowing heights of approximately 4 and 6 cm, respectively, at biweekly intervals. Persistence of both weeds was influenced more by mowing than by fertilization. Survival of Indian mockstrawberry was greatest when mowed at 6 cm and not fertilized and lowest when highly fertilized and not mowed. Blue violets were virtually eliminated by either mowing treatment and did not respond to fertilization. Mowing and fertilization effectively reduced survival of Indian mockstrawberry and common blue violet in the tall fescue lawn.
Downy Brome (Bromus tectorum) Density and Relative Time of Emergence Affects Interference in Winter Wheat (Triticum aestivum)
Authors: Blackshaw Robert E.
Published: May 2019

Field experiments over 3 yr at Lethbridge, Alberta, determined the effect of various downy brome densities and times of its emergence on winter wheat biomass and seed yield. Downy brome reduced wheat biomass up to 59% and seed yield up to 68%. Time of downy brome emergence relative to wheat affected the magnitude of these yield reductions more than the density of downy brome. At comparable densities, downy brome caused 2- to 5-fold greater reductions in yield when it emerged within 3 wk after winter wheat than when it emerged 6 wk after wheat or in early spring. Late-emerging downy brome caused significant wheat yield or biomass losses only at densities of 200 to 400 plants m-2. Late-emerging downy brome plants were strongly shaded (70 to 90%) by winter wheat throughout much of the growing season.
Downy Brome (Bromus tectorum) Interference in Winter Rye (Secale cereale)
Authors: Blackshaw Robert E.
Published: May 2019

Field experiments over 3 yr at Lethbridge, Alberta, determined the effect of various downy brome densities and times of its emergence on winter rye biomass and seed yield. Downy brome reduced yields most when it emerged within 3 wk of rye, but densities of more than 100 downy brome m-2were required to reduce yields by 20 to 30%. The greatest reductions in rye biomass (28%) and seed (33%) yields over the 3 yr occurred when 400 downy brome m-2emerged with the crop. Downy brome, at densities up to 400 plants m-2, emerging 6 wk after rye or in early spring, reduced rye biomass and seed yield less than 10% in all years. Winter rye effectively shaded downy brome (40 to 90%) for much of the growing season.
Effects of Temperature and Photoperiod on Growth and Development of Sicklepod (Cassia obtusifolia)
Authors: Patterson David T.
Published: May 2019

Sicklepod was grown in controlled-environment chambers in 16 day/night temperature regimes ranging from 19/11 to 34/26 C. Maximum dry weight, leaf area, plant height, node number, and leaf number after 46 d occurred at 29/26 and 34/26 C. Temperatures of 29/21 C or lower reduced dry weight by more than 50%. Leaf number, leaf weight, and leaf area were more sensitive to changes in day temperature, whereas plant height and root, stem, and total dry weight were more sensitive to night temperature. Dry matter production was more closely correlated with leaf area duration than with its other component, net assimilation rate. Leaf appearance rate and dry matter production were linearly related to average daily temperature. The low-temperature threshold for leaf production was 13 ± 1 C. Observations of plant development in photoperiods ranging from 10 to 16 h confirmed that sicklepod is a short-day plant with a critical day length of 13 to 14 h. No reproductive development occurred in photoperiods of 15 or 16 h. Seedlings that emerged in 10-h photoperiods required more than 1-wk exposure to short days to initiate and continue reproductive development. Plants from a North Carolina population flowered earlier than plants from a Florida population in photoperiods of 12, 13, or 14 h, but in an 11-h photoperiod the two populations flowered at the same time.
Micromorphology of Johnsongrass (Sorghum halepense) Leaves
Authors: Mcwhorter Chester G. , Ouzts Clark , Paul Rex N.
Published: May 2019

Adaxial and abaxial epidermal surfaces of johnsongrass leaves were studied to determine which cells contribute to leaf microroughness. Cork-silica cell (CSC) pairs, three types of prickles, macrohairs, bicellular trichomes, stomata, and ordinary short and long epidermal cells were found and described. CSC pairs made up about 22% of all cells and probably contribute more to microroughness than any other single type because each cork cell produces 11 ± 3 wax filaments that are up to 100 μm long. Bicellular trichomes represented 4 to 5% of the total cells but decreased leaf roughness by secreting a type of mucilage that covers microscopic wax crystals. Stomatal complexes comprised 15 to 18% of all cells and contributed to leaf roughness because they are slightly recessed below the leaf surface. Long prickles occur primarily over veins and represent less than 1% of all cells. Small prickles were present primarily on adaxial surfaces and represent only 3% of all cells. Macrohairs were the largest appendages, 237 ± 104 μm, but they represent far less than 1% of all cells and occur primarily over veins. Ordinary short cells comprised 6 to 13% of all cells. Long cells were most common (41%) of all cells. Short and long cells contribute to leaf roughness because the surface is often convex. A typical johnsongrass leaf may contain more than 25 million appendages on each surface that increase the roughness already caused by epicuticular wax crystals.
Density and Species Proportion Effects on Interference between Redstem Filaree (Erodium cicutarium) and Round-Leaved Mallow (Malva pusilla)
Authors: Blackshaw Robert E. , Schaalje G. Bruce
Published: May 2019

Interference between redstem filaree and round-leaved mallow was studied under controlled environmental conditions. Each species was grown in monoculture at densities of 2, 4, 8, and 12 plants per 20-cm-diam pot and in mixtures at all possible combinations of these densities. Leaf area per plant was similar for both species but round-leaved mallow grew taller and produced more shoot biomass than redstem filaree when each was grown in monoculture. Mixed culture responses varied with the proportion density of each species. A reciprocal yield model was tested and modified to account for this significant density interaction. When grown in mixture, round-leaved mallow usually gained in leaf area and shoot biomass at the expense of redstem filaree indicating that it was the superior competitor. Calculated competition ratios indicate that round-leaved mallow was about twice as competitive as redstem filaree under the growing conditions of this study.
Weed Control and Peanut (Arachis hypogaea) Response to Nicosulfuron and Bentazon Alone and in Mixture
Authors: Richburg John S. , Wilcut John W. , Eastin E. Ford
Published: May 2019

Field studies in 1990 and 1991 evaluated treatment timings of nicosulfuron and bentazon alone and in mixture for weed control, peanut injury, and yield. The experimental areas were treated with a broadcast PPI application of benefin at 1.68 kg ai ha-1. Nicosulfuron was applied at 40 g ai ha-1and bentazon at 560 g ai ha-1. Nicosulfuron mixed with bentazon applied 1 wk after crop emergence (WAE) controlled yellow nutsedge 81%. Bentazon or bentazon plus nicosulfuron applied 1 or 3 WAE controlled smallflower morningglory at least 94%. Nicosulfuron controlled sicklepod at least 76% in 1990. However, control of sicklepod was 71% from 1 WAE nicosulfuron application in 1991 and less than 26% for 3, 5, or 7 WAE application. Bentazon controlledIpomoeaspp. at least 70% but did not control Florida beggarweed and sicklepod. Nicosulfuron alone controlled Florida beggarweed andIpomoeaspp. Nicosulfuron reduced early-season peanut growth, but peanut recovered by mid-August. Bentazon reduced nicosulfuron control of Florida beggarweed when applied 1, 3, or 5 WAE, but not 7 WAE. Bentazon mixed with nicosulfuron applied 3 WAE in 1990 reduced weed-free peanut yields compared to nicosulfuron alone. Nicosulfuron and bentazon did not significantly reduce peanut yields under weed-free conditions in 1991.
Metolachlor Distribution in a Sandy Soil Under Irrigated Potato Production
Authors: Burgard Daniel J. , Koskinen William C. , Dowdy Robert H. , Cheng H. H.
Published: May 2019

Irrigated sandy soils have a high potential for leaching of agricultural chemicals to ground water. Objectives of this study were to determine the movement and persistence of metolachlor applied annually to a Hubbard loamy sand soil (Udorthentic Haploboroll) under irrigated potato production. Metolachlor was applied to the plots at rates of 1.7 and 3.4 kg ai ha-1. A KBr tracer was applied to the same plots at the rate of 130 kg ha-1. Intact soil cores were collected before and after herbicide application during the cropping season. Water samples were collected weekly from suction samplers, 90 and 150 cm deep in each plot, during the growing season. Potato yields of 60 and 59 Mg ha-1in 1989 and 1990, respectively, were above Minnesota state averages. Bromide tracer data confirmed the movement of water through the soil profile to at least a 150-cm depth. No detectable quantities of metolachlor were found below 30 cm in soil during either year. Time for dissipation of 50% of metolachlor ranged from 85 to > 106 d. Dissipation in 1990 appeared slower than in 1989 and residues remaining at the end of the season, averaged across application rates, were 44 and 64% in 1989 and 1990, respectively. No water samples contained metolachlor in 1989. Metolachlor was detected in 18 of 409 water samples collected during and after the growing season in 1990. Median detectable concentration of metolachlor was 0.4 μg L-1in water samples from suction samplers at 90- and 150-cm depths in eight of 16 plots. Metolachlor Kdand Kocvalues, 3 and 220, respectively, were similar to reported values. Metolachlor sorption in the soil profile was correlated, in order of contribution, to organic carbon content, cation exchange capacity, percent sand, and percent clay.
Evaluation ofFusarium oxysporumas a Potential Bioherbicide for Sicklepod (Cassia obtusifolia), Coffee Senna (C. occidentalis), and Hemp Sesbania (Sesbania exaltata)
Authors: Boyette C. Douglas , Abbas Hamed K. , Connick William J.
Published: May 2019

A strain ofFusarium oxysporumwas isolated from infected sicklepod stems in Stoneville, MS, in 1989. When formulated as granules either in a fungus-infested rice preparation or encapsulated in a wheat-gluten matrix called ‘Pesta’, biocontrol of sicklepod, coffee senna, and hemp sesbania was achieved with preemergence or preplant-incorporated treatments. Liquid conidial applications were less effective. Plants were killed by a preemergence damping-off. Applications made postemergence were significantly less effective. Results of these tests suggest that this fungus has potential as a mycoherbicide for controlling sicklepod, coffee senna, and hemp sesbania.
Resistance to Chlorotoluron of a Slender Foxtail (Alopecurus myosuroides) Biotype
Authors: Menendez Julio , Jorrin Jesus , Romera Eva , De Prado Rafael
Published: May 2019

The effect of chlorotoluron on different slender foxtail populations that survived normal agricultural rates of this herbicide (2.5 to 3.5 kg ai ha–1) was investigated under controlled laboratory conditions. Among five populations tested, two had tolerance for the herbicide. The ED50values for these biotypes ranged from 0.33 to 2.43 kg ha–1. Assays with radiolabeled14C-chlorotoluron indicated similar absorption and translocation of the herbicide between susceptible and resistant biotypes. Chlorophyll fluorescence and Hill reaction analysis seem to support the view that the mechanism of resistance of one biotype to the herbicide is due to degradation/detoxification rather than to modification of the target site, although compartmentation processes cannot be ruled out. This resistant biotype had high tolerance to other photosynthetic inhibiting phenylureas, including linuron, chlorbromuron, diuron, monolinuron, and neburon.
Genetic Variation in Life History Traits in Yellow Nutsedge (Cyperus esculentus) from California
Authors: Holt Jodie S.
Published: May 2019

Genetic variation of morphological and phenological characters of yellow nutsedge (Cyperus esculentusL.) from California was investigated and compared with isozyme data from the same populations. The importance of collection location, individual genotype, and isozyme genotype to phenotypic characteristics was evaluated. Analyses were conducted on 20 individuals collected from each of 10 widely separated populations in California. Replicate plants were started by tubers, grown in pots buried outdoors, and measured during the period from planting through flowering. Results showed that yellow nutsedge is variable across its range in California in all measured traits, including days to sprouting and flowering, height, rachis number and length, aboveground biomass, tuber weight, number of rays, ray length, number of spikelets per ray, as well as spikelet length and width. Population (collection location) was a larger variance component than genotype (individuals over all locations). As found in isozyme analysis, relatively more variation was found among than within populations, typical of clonally reproducing species. Overall, more variation was found in quantitative traits than in isozymes. The level of variation, based on coefficients of variation for each character and population, was similar within populations and not clearly related to cropping history or climate at each site. The two most common isozyme genotypes in the collection differed in four characters: date of emergence, weight of 10 tubers, rachis number, and mean ray length, which may represent different adaptive responses to cultural practices. These results indicate that isozymes do not reflect the high level of genetic and adaptive diversity of yellow nutsedge. Factors that are likely to determine the patterns of variation in this species include breeding system (vegetative reproduction), founder effects, polyploidy, and homogeneity of the agricultural environmental.
Dissipation of Alachlor, Metolachlor, and Atrazine from Starch-Encapsulated Formulations in a Sandy Loam Soil
Authors: Buhler Douglas D. , Koskinen William C. , Schreiber Marvin M. , Gan Jianying
Published: May 2019

Research was conducted to determine the effect of starch encapsulation on soil dissipation and weed control with alachlor, metolachlor, and atrazine on an Estherville sandy loam in the field. Starch encapsulation increased persistence of alachlor in the surface 15 cm of soil compared to the emulsifiable concentrate formulation during the first 60 d after treatment More alachlor was detected 30 to 75 cm deep with emulsifiable concentrate than the starch-encapsulated formulation 30 d after treatment. Little alachlor was detected below 15 cm thereafter. Starch encapsulation also increased persistence of metolachlor in the surface 15 cm, but reduced concentrations at 15 to 30 cm compared to the emulsifiable concentrate 30 d after treatment. By 120 and 340 d after treatment, metolachlor concentrations at 15 to 30 cm were greater with starch-encapsulated than emulsifiable concentrate formulation. Starch encapsulation greatly increased atrazine persistence in the surface 15 cm compared to the dry flowable formulation. Although encapsulation increased atrazine concentration in the surface 15 cm, it reduced the concentration below 15 cm compared to the dry flowable formulation 30 and 60 d after treatment However, by 120 and 340 d after treatment, starch encapsulation often resulted in greater atrazine concentrations below 15 cm than the dry flowable formulation. Control of green foxtail and redroot pigweed with starch-encapsulated herbicides was similar or superior to the commercial formulations.
Alleviating the Antagonistic Effect of Moisture Stress on Smooth Crabgrass (Digitaria ischaemum) Control with Fenoxaprop
Authors: Rossi Frank S. , Neal Joseph C. , Senesac Andrew F.
Published: May 2019

The influence of moisture stress intensity and duration, as well as tank mixtures with pendimethalin, which mitigated the antagonistic effects of drought, on fenoxaprop efficacy were evaluated. In growth chamber tests, specific soil matric potential (Ψm) levels of −0.01 to −1 MPa were imposed and maintained using a polyethylene glycol semipermeable membrane system. Moisture stress was alleviated by irrigation 48 or 96 h after herbicide treatment. Fenoxaprop efficacy was influenced by moisture stress intensity, posttreatment stress duration, and application rate. Under conditions of mild stress (Ψ ≥ −0.2 MPa), both increased application rate and irrigation 48 h after treatment alleviated the effects of drought on fenoxaprop efficacy. Under moderate stress (Ψm = −4.4 MPa), the antagonistic effects of drought were alleviated by a combination of increased rate and irrigation 48 h after treatment However, under more severe stress (Ψm ≤ −0.8 MPa), irrigation coupled with increased rate did not increase crabgrass control above 50%. Where irrigation within 48 h of treatment is not feasible, tank mixes of fenoxaprop with pendimethalin were shown to enhance smooth crabgrass control. Field lysimeter tests indicated that under moisture stress, Ψm of −0.7 MPa, smooth crabgrass shoot dry weight was reduced 65 to 94% with tank-mix combinations of ≥ 0.27 kg ai ha−1fenoxaprop plus ≥ 2.2 kg ai ha−1pendimethalin. These rate combinations were shown to be synergistic and able to alleviate the antagonistic effect of moisture stress on fenoxaprop efficacy.
Prediction of Soil Sorption (Koc) of Herbicides Using Semiempirical Molecular Properties
Authors: Reddy Krishna N. , Locke Martin A.
Published: May 2019

Relationships between soil sorption normalized to organic carbon (Koc) and molecular properties of 71 herbicides were examined. The Koc values were obtained from the literature. Various molecular properties were calculated by quantum mechanical methods using molecular modeling software. The quantitative structure activity relationship (QSAR) models based on four molecular properties, van der Waals volume (VDWv), molecular polarizability (α), dipole moment (μ), and energy of highest occupied molecular orbital (eHOMO), together accounted for 70% of the variation in Koc. Herbicides were broadly divided into six families based on structural similarities, and separate equations were established for each group. The three descriptors, VDWv, α, and μ, along with either energy of lowest unoccupied molecular orbital (eLUMO), or electrophilic superdelocalizability (SE), or eHOMO appeared to be determinants and accounted for 82 to 99% of the variation in Koc. Applicability of these models was tested for one herbicide analogue and 10 metabolites. The QSAR models appear to be specific to structurally similar chemicals. The QSAR models could be developed to predict Koc of structurally similar compounds even before they are synthesized or for some of the metabolites of existing herbicides. Models of this type can also be developed to create priority lists for testing, so that time, money, and efforts can be focused on the potentially most hazardous chemicals.
Isolation and Characterization of Metabolites fromPseudomonas fluorescens-D7 for Control of Downy Brome (Bromus tectorum)
Authors: Gurusiddaiah S. , Gealy David R. , Kennedy Ann C. , Ogg Alex G.
Published: May 2019

Downy brome is a grassy weed that infests millions of acres of winter wheat in the Western United States and costs farmers millions of dollars in lost yields. It can not be controlled consistently using chemicals or cultural practices. Isolates of naturally occurring rhizobacteria have shown potential as biological control agents for downy brome. Centrifuged supernatant from aerobic shake cultures ofPseudomonas fluorescensstrain D7 suppressed germination of seeds and reduced root and shoot growth of downy brome in agar diffusion assays. The present article relates to the isolation and characterization of active compounds from strain D7. The active compounds were denatured and lost activity with commonly used techniques of extraction, concentration, and purification of microbial products. Two unusual techniques for isolation and partial purification of the active compounds from supernatant of strain D7 were developed. With one technique, supernatant was concentrated at 30 to 35 C under vacuum to 10% of the initial volume. Subsequently, the concentrate was chromatographed on sephadex gels to separate the active fraction. With the second technique, cell-free supernatant was frozen, with or without prior concentration, at −12 C for 12 to 24 h. The frozen supernatant was then thawed at 2 to 4 C, resulting in precipitation and crystallization of the active fraction. The active fraction isolated with either method inhibited downy brome. Inhibition was complete at concentrations as low as 1 mg total dry matter L-1agar. Also, the active fraction inhibited the plant pathogenic fungusGaeumannomyces graminisvar.tritici.Chemical analysis of the active fraction complex revealed the presence of chromopeptides and other peptides, fatty acid esters, and a lipopolysaccharide matrix. Separation of any of the components from the complex resulted in nearly complete loss of activity against downy brome.
Critical Period of Quackgrass (Elytrigia repens) Removal in Potatoes (Solanum tuberosum)
Authors: Baziramakenga R. , Leroux Gilles D.
Published: May 2019

Field trials were carried out in 1989 and 1990 at St-Augustin, near Québec City, Canada, to determine the critical periods of quackgrass control in potato submitted to three levels of infestation. Potato yield losses due to quackgrass interference increased with quackgrass infestation and length of duration of interference. Quackgrass interference influenced marketable tuber yields more than total tuber yields. Duration of the critical period varied depending on the level of quackgrass infestation and year. Based on an arbitrary 5% level of marketable yield loss, the critical period started at ca. 15 days after emergence (DAE) of potato at low level of infestation, and at ca. 3 DAE at medium level of infestation. At high level of infestation, the critical period began prior to the emergence of potato. The end of the critical period of quackgrass removal was extremely variable across quackgrass infestation level and year and ranged from 23 to 68 DAE of potato at a 5 % yield loss level. It appears that onset of interference varied less than the end of it, indicating that early quackgrass control is necessary to prevent yield loss.
Influence of Phosphorus Fertility on Intra- and Interspecific Interference between Lettuce (Lactuca sativa) and Spiny Amaranth (Amaranthus spinosus)
Authors: Shrefler James W. , Shilling Donn G. , Dusky Joan A. , Brecke Barry J.
Published: May 2019

Greenhouse studies were conducted to determine the effect of phosphorus (P) fertility on intra- and interspecific competition between lettuce and spiny amaranth for 4 wk after emergence. Total lettuce shoot biomass per pot and weight per plant increased 39 and 44% in response to increased P fertility, respectively. P fertility had no impact on growth of spiny amaranth. Total shoot biomass of spiny amaranth increased with increasing density from four to eight plants, however, lettuce did not. Total shoot biomass of both species increased as density increased from 4 to 16 plants. Spiny amaranth, but not lettuce, weight per plant decreased in response to intraspecific competition. Reciprocal yield analysis showed that spiny amaranth produced 2.4 times more biomass than lettuce when competing intraspecifically and four times more biomass under interspecific competition. Lettuce weight per plant was not affected. Relative yield analysis indicated that spiny amaranth was more competitive than lettuce regardless of P fertility. However, increased P fertility increased competitiveness of lettuce. Relative crowding coefficients indicated that spiny amaranth at the low density with low P fertility was 33 times more competitive than lettuce. Addition of P caused lettuce and spiny amaranth to be equally competitive at the lowest density; however, at the highest density, spiny amaranth was 4 tunes more competitive than lettuce regardless of additional P.
Improving Soybean (Glycine Max) Tolerance to AC 263,222 by Limiting Terminal Exposure
Authors: Newsom Larry J. , Shaw David R.
Published: May 2019

Field experiments were established to evaluate the effects of various band widths left open directly over the soybean row to improve soybean tolerance to AC 263,222 and chlorimuron applied POST, while maintaining weed control. Drop tubes with even fan tips extended below the spray boom to within 40 and 30 cm from the ground to obtain 5 and 10 cm open bands at the soybean terminals. AC 263,222 applied broadcast at 13, 18, and 36 g ai ha-1injured soybean 14, 16, and 32%, respectively, 3 DAT. AC 263,222 at 36 g ha-1applied with a 5-cm and 10-cm open band injured soybean 11 and 3%, respectively. Chlorimuron injury was 12% or less for all treatments. Metribuzin followed by AC 263,222 or chlorimuron was not more injurious to soybean than AC 263,222 or chlorimuron alone. Metribuzin PRE followed by 36 g ha-1AC 263,222 applied in a 10-cm open band controlled sicklepod and pitted morningglory 86 and 92%, respectively, 6 WAT. Sicklepod control was similar with open-band treatments compared to the same rate applied broadcast; however, 18 g ha-1AC 263,222 applied in a 10-cm open band provided less pitted morningglory control than the broadcast treatment. Soybean treated with herbicides in a band produced yields similar to or greater than soybean treated with the same herbicides broadcast. Metribuzin followed by 36 g ha-1AC 263,222 with a 10-cm open band resulted in a soybean yield of 2840 kg ha-1, which was greater than any other herbicide treatment and equivalent to the weed-free check.
Dissipation of Chlorimuron in Southern Soils
Authors: Vencill William K. , Banks Philip A.
Published: May 2019

Chlorimuron dissipation as determined by bioavailability in nine soils across the Southern United States was characterized by rapid initial loss followed by a slower dissipation rate following field application of the herbicide in soybean. The rate of bioavailable dissipation varied by year and soil The calculated time for 50% dissipation of bioavailable chlorimuron (DT50) ranged from 5 to 18 d in 1988, 5 to 20 d in 1989, and 5 to 11 d in 1990. In 2 of 3 yr, chlorimuron DT50was positively correlated with temperature 24 h after application. Chlorimuron DT50was positively correlated with precipitation in 1988 when regional rainfall was below normal and negatively correlated with precipitation 30 d after application in 1989 when regional rainfall was above normal.
Investigation into the Mechanism of Isoxaben Tolerance in Dicot Weeds
Authors: Schneegurt Mark A. , Heim Dale R. , Larrinua Ignacio M.
Published: May 2019

Isoxaben is an inhibitor of the synthesis of cellulose from glucose. Some dicot weed species are relatively insensitive to isoxaben inhibition. This study investigates mechanisms by which decreased sensitivity may occur in three dicot weed species: catchweed bedstraw, redroot pigweed, and velvetleaf. Dose response curves were generated to determine I50values for the inhibition of glucose incorporation into cellulose by isoxaben and compared to that of a sensitive species, mouse-ear cress. Metabolic detoxification and uptake rates were measured and the degree of tolerance conferred by these mechanisms was calculated. In all cases, metabolic detoxification was negligible. Lower uptake rates were significant but minor components of tolerance in all species. It is suggested that the principal cause of isoxaben tolerance in these dicot weed species is decreased sensitivity at the target site.
Perennial Weed Populations After 14 Years of Variable Tillage and Cropping Practices
Authors: Buhler Douglas D. , Stoltenberg David E. , Becker Roger L. , Gunsolus Jeffery L.
Published: May 2019

Management of perennial weeds is a major concern in reduced-tillage cropping systems. Field research was conducted at Nashua, IA, from 1977 through 1990 to evaluate the long-term impacts of tillage and cropping patterns on perennial weed populations in corn and soybean production. Continuous corn and a corn/soybean rotation were conducted utilizing moldboard plow, chisel plow, ridge tillage, and no-tillage systems. The research area was free of established perennial weed species at the initiation of the experiment in 1977. Hemp dogbane was observed by 1980, with the greatest densities in no-tillage. By 1990, continuous corn had greater hemp dogbane densities with no-tillage than other tillage system by crop rotation treatments. American germander densities were not affected by tillage systems in 1980 and 1981, but by 1990, corn/soybean rotations had greater densities in moldboard plow than other tillage systems. Field bindweed developed primarily in the corn/soybean rotations with the greatest densities occurring in no-tillage. Greater and more diverse populations of perennial weeds developed in reduced-tillage systems than in the moldboard plow system. However, practices used to control annual weeds and environmental factors interacted with tillage to regulate perennial weed populations.
Sequestration and Oxygen Radical Detoxification as Mechanisms of Paraquat Resistance
Authors: Hart Jonathan J. , Di Tomaso Joseph M.
Published: May 2019

Evidence in the literature has generally supported either of two paraquat resistance mechanisms: an increase in activity of oxygen radical-scavenging enzymes in resistant plants which affords protection from active oxygen species formed by paraquat; and sequestration of paraquat away from its site of action in the chloroplast. Evidence for the first model relies primarily on measurement of increased enzyme activity and cross-resistance to other oxygen radical-generating stresses in resistant plants. The sequestration model is supported by data showing decreased translocation of paraquat and absence of paraquat injury in plant systems that do not have increased levels of protective enzymes. An alteration in paraquat transport at one of several plant cell membranes could confer resistance by modifying movement of paraquat into the compartment bounded by that membrane. Properties of the plasmalemma, chloroplast envelope, and tonoplast that may be important to paraquat transport are discussed and data supporting or discounting specific membrane alterations in resistant plants are presented. Finally, the possibility that both mechanisms may work in concert is addressed.
Leafy Spurge (Euphorbia esula) Cell Cultures for Screening Deleterious Rhizobacteria
Authors: Souissi Thouraya , Kremer Robert J.
Published: May 2019

Bioassays using cell cultures and callus tissues of leafy spurge were devised to evaluate the potential of rhizobacteria as biocontrol agents. Rhizobacteria isolated from roots of leafy spurge seedlings were screened in suspension-cultured leafy spurge cells. Cell viability was assessed using the Evan's blue bioassay 48 h after bacterial inoculation. Among the 30 isolates tested, LS102 and LS105 consistently caused intensive cell death determined by measuring the A630of the inoculated cell cultures. Cell death was 2.5 to 3 times higher in cultures inoculated with LS105 and LS102, respectively, than in the control. Population levels of the two isolates within cell cultures and callus tissues of leafy spurge increased during the first 48 h. Leafy spurge callus tissues were inoculated with rhizobacteria either directly or by using the Host Pathogen Interaction System (HPIS). The latter exposes calli to bacteria without any physical contact. LS102 caused cellular leakage and eventually death of the callus tissue. Callus growth was reduced by about 30 to 70% when exposed to LS102 and LS105, respectively. Results suggest that these two isolates may affect leafy spurge at the cellular level by different mechanisms. A screening method based on cell cultures and callus tissues offers a good and rapid technique for detecting deleterious rhizobacteria with potential as biocontrol agents for leafy spurge.
Influence of Winter Wheat (Triticum aestivum) Cultivars on Weed Control in Sorghum (Sorghum bicolor)
Authors: Wicks Gail A. , Nordquist Paul T. , Hanson Gordon E. , Schmidt John W.
Published: May 2019

Winter wheat cultivars that are competitive with weeds help control weeds in crop rotations. Ten winter wheat cultivars were evaluated for interference with summer annual grasses in the wheat and the subsequent grain sorghum crop in a winter wheat-ecofallow sorghum-fallow rotation in which there are two 10–mo fallow periods and two crops in 3 yr during 1983 to 1987. The medium–tall (100 to 109 cm tall) and medium–statured (90 to 99 cm tall) winter wheat cultivars (‘Buckskin’, ‘Siouxland’, ‘Lancota’, ‘Centurk 78’, and ‘Brule’) were more competitive than medium-short (80 to 89 cm tall) and short (68 to 79 cm tall) cultivars (‘Eagle’, ‘Homestead’, ‘Colt’, ‘Vona’, and ‘TAM 101’). Atrazine plus paraquat was applied to all cultivars 30 d after wheat harvest. When grain sorghum was planted in areas previously seeded with medium–tall and medium-statured winter wheat, summer annual grass weed biomass in sorghum was 61% less than in grain sorghum seeded into areas previously planted with medium-short and short wheat cultivars. Use of pendimethalin plus 2,4–D in winter wheat and glyphosate plus alachlor in grain sorghum eliminated differences in summer annual grass weed density and weed biomass among wheat cultivars. Sorghum grain yields were improved 7% when herbicides were used in the winter wheat and sorghum but value of the increase was less than cost of herbicides. Substituting less costly herbicides for herbicides used in this study still would not have been enough to pay for cost of herbicides for five cultivars. Grain sorghum grown on weed–free stubble of medium–tall and medium–statured winter wheat produced more grain than grain sorghum grown after medium–short and short-statured winter wheat by 5%. Volunteer wheat density during the fallow period following grain sorghum was lower in areas originally seeded to Centurk 78 and Siouxland wheat while volunteer wheat density was higher in areas planted to Homestead and Vona.