Absorption and translocation of 14C-dicamba (3,6-dichloro-o-anisic acid) in seven salt formulations were determined 60 h after application to leaves of soybean [Glycine max (L.) Merr. ‘Williams’] grown in nutrient solution. The dimethylamine (DMA) formulation was consistently absorbed and retained in the plant in amounts equal to or greater (46% of recovered 14C) than other formulations (which averaged 19% of recovered 14C) when applied without surfactant. Absorption and subsequent retention of the DMA formulation in the plant was least affected of all formulations by the addition of a surfactant. With a surfactant, absorption of the DMA, monoethanolamine (MEA), and inorganic salt formulations was similar (>75% of recovered 14C). Addition of seven surfactants to the K-salt of dicamba increased both the amount of 14C absorbed by 35 to 56% and the amount recovered in the plant. All surfactants except one enhanced absorption of the K-salt of dicamba to a similar degree. Dicamba exhibited predominantly symplastic translocation with the majority of 14C being recovered in the new second trifoliolate leaves and nutrient solution. As much as 66% of the radioactivity absorbed through the leaves was exuded by roots into the nutrient solution 60 h after leaf treatment.

Field studies were conducted at two North Carolina locations to determine the effect of interference between pale smartweed and cotton on plant growth and productivity. Pale smartweed remained shorter than cotton until at least 70 d after cotton planting. However, pale smartweed grew over twice as tall as cotton and produced considerable dry biomass by cotton harvest. Pale smartweed biomass per plant was not affected by weed density up to 3.5 plants m−1 of row when grown with cotton. Cotton competition reduced pale smartweed dry biomass per plant at least 400%. The relationship between pale smartweed and cotton percent yield loss was described by the rectangular hyperbola model with the asymptote (coefficient a) constrained to 100% maximum yield loss. The estimated coefficient i (yield loss per unit density as density approaches zero) was 29 ± 4 and 23 ± 4 in 1998 and 2000, respectively. Pale smartweed achene production was also described by the hyperbolic function. Estimated achene production of smartweed at 1 plant m−1 cotton row was 63,000 and 25,000 achenes m−2 in 1998 and 2000, respectively.

Research on biological control of weeds in the United States began early in this century with the use of introduced insects to control the weed, lantana (Lantana camara L.), in Hawaii. The highly successful program for biological control of St. Johnswort (sometimes called “Klamath weed”) (Hypericum perforatum L.) in California by the use of introduced insects in the 1940's and 1950's caused increased interest in this approach to weed control. As a result of this increased interest, the United States Department of Agriculture (USDA) established two overseas laboratories to study arthropod natural enemies of a number of weeds that had been introduced into the United States – in Rome, Italy, in 1959; and near Buenos Aires, Argentina, in 1962. The recent and successful use of an introduced rust pathogen (Puccinia chondrillina Bubak & Syd.) to control rush skeletonweed (Chondrilla juncea L.) in Australia, and the production and utilization of pathogens for control of weeds in the United States, has spurred further interest in biological control.

The influence of nitrogen form on bentazon uptake and toxicity to soybean cell cultures and medium pH was examined. Soybean cell growth was completely inhibited at 40 μM bentazon when cells were cultured in a Murashige and Skoog medium containing 20 mM ammonium and 38 mM nitrate. However, bentazon, up to 100 μM, did not affect the growth of cells cultured in B5 medium (Gamborg B5 medium containing 2 mM ammonium and 25 mM nitrate). Twenty-minute 14C-bentazon uptake experiments on 48- and 96-h-old cells demonstrated that 14C uptake was nitrogen-type dependent, with cells cultured in AMM medium (B5 medium containing 38 mM nitrate and 12 mM ammonium) accumulating 5 to 10 times more 14C than those in B5 or NIT media (B5 containing 50 mM nitrate). Differential bentazon uptake between ammonium and nitrate-containing media appears to be attributable to the effect of nitrogen form on pH. From an initial pH of 5.5, the pH of AMM medium decreased to 4 by 48 h. The pH of NIT or B5 medium increased to approximately 7 from 5.5. Changes in pH are likely attributable to cellular maintenance of ionic balance; NH4+ and NO3− uptake results in H+ and OH- efflux, respectively. Methylamine at 12 mM did not result in a decrease in medium pH. Bentazon, a weak acid (pKa 3.3), would be expected to penetrate the plasma-membrane faster in the molecular form at low pH. Support for a nitrogen pH theory is provided by the finding that 14C-bentazon uptake was similar for ammonium and nitrate-containing media when media were buffered.

Six ecotypes of saltcedar (Tamarix pentandra Pall.), an introduced competitive phreatophyte, were collected from Wyoming to Texas and grown at Los Lunas, New Mexico. Cladophyll epicuticular alkane and fatty acid contents were analyzed by gas-liquid chromatography. Fatty acid and alkane contents varied among the six morphologically-indistinguishable ecotypes. Alkane content patterns varied on the basis or absence of C39 and other constituents. Fatty acid contents of the C16 to C22 were different among the ecotypes. Variation was not explicable on the basis of latitude, light intensity, temperature, nutrition, or water status. Thus, the chemical taxonomy hypothesis of uniform epicuticular alkane quality within a species is not substantiated in saltcedar, and a portion of the ecotypic resistance to postemergence herbicide sprays may be due to differences in epicuticular wax quantity and quality.

Single applications of several herbicides were compared after 12 to 48 months for control of chamise (Adenostoma fasciculatum H. & A.) and redshank chamise (Adenostoma sparsifolium Torr.). Glyphosate [N-(phosphonomethyl)glycine] provided more effective control and canopy reduction than any other herbicide tested. Triclopyr {[(3,5,6-trichloro-2-pyridinyl)oxy] acetic acid} and DX-1108 (ammonium ethyl carbanoylphosphonate) were also effective for control of chamise. Reduction in chamise canopy without leaf necrosis was observed with DPX-1108. Any herbicide tested could provide an alternative to currently used herbicides for chamise suppression.

Growers of glyphosate-resistant crops apply micronutrients tank-mixed with glyphosate to save time and production costs. Therefore, effect of zinc (Zn), as Zn sulfate, on absorption, translocation, and efficacy of glyphosate on yellow nutsedge was investigated. Glyphosate at 850 g ae ha−1 provided 90% yellow nutsedge control at 5 wk after treatment (WAT). Presence of Zn at 1,000 ppmw in the glyphosate spray solution reduced yellow nutsedge control to 24 and 8%, 3 and 5 WAT, respectively. Yellow nutsedge control decreased with increasing Zn level (500 to 2,000 ppmw) in the spray solution. Yellow nutsedge treated with higher rates of Zn tank-mixed with glyphosate produced more tubers and tillers per plant than untreated plants. An abrupt decrease in absorption and translocation of 14C–glyphosate occurred between 500 and 1,000 ppmw Zn. The antagonistic effect of Zn on glyphosate depended mainly on reduced absorption and translocation of 14C–glyphosate within treated tissues. Less than 10% of applied 14C–glyphosate was absorbed when glyphosate was mixed with 1,000, 2,000, or 4,000 ppmw Zn as compared with 85% absorption for glyphosate alone. These treatments inhibited > 90% of 14C–glyphosate translocation out of the treated leaf and > 50% of tuber translocation relative to glyphosate alone. Results indicate that micronutrients containing Zn are not suitable for tank-mixing with glyphosate.

There is little information published on patch expansion of perennial weeds and none for Apocynum cannabinum. Studies were conducted to measure the between-season and in-season expansion patterns of natural A. cannabinum patches over three growing seasons. Regression analysis indicated strong relations between patch area in consecutive years 1996 to 1997 (r 2 = 0.81) and 1997 to 1998 (r 2 = 0.76). Patches less than 20 m2 in 1996 increased in area by more than 100% in 1997 during a fallow season. However, patches decreased in size 6 to 51% between 1997 and 1998 when Glycine max was grown. Evidence suggested that a late-season mowing of the A. cannabinum patches in 1997 contributed more to the decline in patch area than competition from G. max during the 1998 season. The relations between patch area and growing degree units (r 2 = 0.97) indicated that greater than 89% of the terminal patch expansion occurred prior to the accumulation of 435 growing degree units (GDU) (June 19, 1997; May 31, 1998; June 9, 30-yr average), with minimal patch expansion between 435 and 1,000 GDU. Patches were at 50% of their final area on May 27, 1997, and May 14, 1998, a time when only 22% of the A. cannabinum population had emerged (r 2 = 0.99). Knowledge of patch size and expansion could help growers time weed scouting, to account for the later emergence patterns of this species, as well as assist in timing appropriate weed management efforts. This information could also be used in conjunction with aerial photographs to project potential patch size for site-specific management of this weed.

Giant sensitiveplant interference at different population densities in cassava established at 10,000 plants ha−1 was investigated on a Ferric Luvisol in a humid tropical environment. Interference for 12 mo was compared at 0, 10,000, 20,000, 30,000, and 40,000 plants ha−1 and at natural populations (averaging 630,000 plants ha−1) in four randomized complete blocks. Results showed that the order of cassava growth parameter response to giant sensitiveplant interference for 12 mo was leaf number > height > stem girth > leaf size = petiole length. The natural population density of giant sensitiveplant reduced growth faster and more than populations of 10,000 to 40,000 plants ha−1 in cassava. All giant sensitiveplant populations from 10,000 plants ha−1 and higher reduced storage root yield in cassava 12 mo after planting. Yield reduction increased as giant sensitiveplant population increased and the highest reduction of 85% occurred in the natural population of giant sensitiveplant.

Research on best management practices, including vegetative filter strips, is needed to evaluate the potential for reducing herbicides in surface runoff. Laboratory studies were conducted to determine the influence of different filter strip components on fluometuron adsorption. Samples were taken from a switchgrass filter strip (1-m wide) established on a Brooksville silty clay. Sampled components included switchgrass stems clipped to 4 cm, plant residue on the soil surface, and topsoil < 5-mm, 0.5- to 1-cm, 1- to 3-cm, and 3- to 5-cm deep. The filter strip topsoil samples contained 1.8, 2.2, 2.3, and 2.8% organic matter for the aforementioned depths, respectively, compared with 1.8% in soil collected from an adjacent cropped area. Fluometuron adsorption on each sample was compared at initial concentrations of 0.017, 0.5, 1.0, 2.0, and 4.0 mg L−1, the last of these representing four times the peak concentration in surface runoff. Fluometuron adsorption was greater in soil from filter strip areas than in soil from cropped areas. Averaged over concentration, the soil–water partition coefficient (K d) value for soil 1- to 3-cm and 3- to 5-cm deep was greater than the value for soil from the cropped area. Stems and residue had K d values 4.9- and 4.1-fold greater, respectively, than soil from the cropped area. Average adsorption coefficients normalized for organic carbon content (K oc) of soil < 5-mm and 3- to 5-cm deep from filter strip areas were greater than the values for soil from cropped areas.

Effects of root-shoot interference, soil water regimes, and soil nitrogen were evaluated to determine aggressivity for pea relative to mayweed chamomile in the greenhouse using replacement series experiments. Interference between pea and mayweed chamomile occurred mainly below ground, and soil water was more important than soil nitrogen in controlling the outcome of interference. Pea was a stronger competitor than mayweed chamomile under all conditions investigated. Leaf area, root weight, and shoot weight of mayweed chamomile were reduced 55 to 87% by full interference and 27 to 60% by root only interference from pea. Full interference from mayweed chamomile for 53 days after planting released the remaining pea from intraspecific interference and it grew 80% larger and produced 100% more pod weight compared to a pea in monoculture. Low soil water potential (–175 kPa) compared to high water potential (–33 kPa) reduced the height, leaf area, and shoot weight of pea in 1:1 mixtures, but had no effect on mayweed chamomile. Decreasing soil water increased the aggressivity of mayweed chamomile relative to pea. Nitrogen added at 20 mg wk-1 compared to no added nitrogen had no effect on peas, but more than doubled the size of mayweed chamomile in monoculture and in mixtures with pea. Added nitrogen did not increase the aggressivity of mayweed chamomile relative to pea.

Eclipta, widespread in tropical, subtropical, and temperate regions, is one of the main malignant broadleaf weeds and thrives in moist and dryland fields. Field rates of acetolactate synthase (ALS) inhibitors have failed to control eclipta in some farmlands in China. One ALS inhibitor–resistant population (R) collected from Jiangsu province in China was confirmed in the greenhouse in our preliminary work. Whole-plant assays revealed that this R population was highly resistant to four sulfonylureas (pyrazosulfuron-ethyl, 134-fold; bensulfuron-methyl, 172-fold; metsulfuron-methyl, 30-fold; and tribenuron-methyl, 195-fold), two triazolopyrimidines (pyroxsulam, 98-fold; penoxsulam, 30-fold), and one pyrimidinylthio-benzoate (bispyribac-sodium, 166-fold) and was moderately resistant to two imidazolinones (imazethapyr, 10-fold; imazapic, 19-fold). ALS enzyme-activity assays showed insensitivity of the ALS from the R population (resistance index values ranged from 12 to 293) to all of the above ALS inhibitors in vitro. Chromatograms from ALS gene sequence analysis detected a homozygous Pro-197-Ser amino acid substitution in the R population. These results confirmed that the Pro-197-Ser substitution results in broad-spectrum cross-resistance to ALS inhibitors in the eclipta R population. To our knowledge, this study is the first to report broad cross-resistance to ALS inhibitors in eclipta and to obtain the full-length ALS gene sequence.

The objective of this study was to identify some epidemiological conditions that affect the pathogenicity and weed control efficacy of the fungal pathogen Dactylaria higginsii on Cyperus rotundus. In controlled environments, the fungus required a minimum dew period of 12 h and a temperature of 25 C during the dew period to produce severe disease on four- and six-leaf-stage plants inoculated with 106 conidia ml−1. Under these conditions, 75% disease (percent leaf area damaged) and excellent weed control (nearly 100% plant mortality) were achieved. In experiments to test the interaction among dew period temperature, dew period duration, and plant growth stages, younger plants (four- to six-leaf) were more susceptible to D. higginsii than older (eight-leaf-stage) plants. At the dew period duration of 24 h and dew period temperature of 30 C, the number of days to obtain 50% disease severity on four- to six-leaf-stage plants was significantly less (10 d) compared with older plants (16 d). To achieve effective control, D. higginsii should be applied early in the growing season when C. rotundus plants are young and the temperature and dew period requirements are not limiting. The need for a long dew period (>12 h) for infection and disease development may be a limiting factor in this pathosystem. This limitation may be overcome by using inoculum amended with moisture-retaining gels.

Sethoxydim reacts spontaneously with water resulting in immediate structural changes. Simulation of field conditions of light, moisture, oxygen, pH, and soil and evaporation on siliceous surfaces duplicated this lability. Sethoxydim degradation was enhanced by alkaline conditions, ultraviolet and incandescent light, and adsorption on solid surfaces. No sethoxydim was detected immediately after application to moist soil. Less than 2% extractable sethoxydim was present in dry soil after 24 h.

The phytotoxicities of three phenylurea herbicides applied annually to cotton at rates used for selective weed control were studied in irrigated soils of the southwestern United States. Yields of seed cotton were generally not affected by annual applications of 3-(p-chlorophenyl-1)-1,1-dimethylurea (monuron) and 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron). Annual applications of 3-phenyl-1,1-dimethylurea (fenuron) often reduced seed cotton yields. Mild chlorosis of foliage of cotton seedlings was occasionally observed when monuron was used annually. Although diuron affected the growth of oats and barley 6 to 12 months after application, it caused no adverse effect on cotton seedlings. Monuron residues affected barley and oats more severely than did diuron residues. Accumulation of diuron and monuron from annual applications at rates used for selective weed control in cotton appears unlikely.

In greenhouse tests, temperature, humidity, drought stress, and concentration of surfactant influenced the foliar activity of spray mixtures containing a dodecyl ether of polyethylene glycol (Surfactant WK) and 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (diuron) or 3-(3,4-dichlorophenyl)-1-methoxy-1-methyl urea (linuron). In comparisons with other commonly used surfactants, Surfactant WK gave a superior performance when used in conjunction with diuron, linuron, and 5-bromo-3-sec-butyl-6-methyl uracil (bromacil). Increased water solubility of these herbicides in the presence of surfactant is only one factor among many involved in the enhanced activity of such combinations. Surfactant WK facilitated the deposition of high concentrations of linuron on leaf surfaces of Johnsongrass. Once a gradient was established, subsequent rapid movement of the herbicide into the leaf was noted.

Structural characteristics of surfactants and a herbicide were incorporated in the same molecule by forming long-chain alkylamine salts of 2,4-dichlorophenoxyacetic acid [2,4-D]. In water sprays, all of the 2,4-D salts showed surface activities characteristic of surfactants. The application rate of 2,4-D was held constant at 1/16 lb ae/A; salt concentration was varied by varying spray volume. Seventeen salts were herbicidally active on soybeans at a high concentration (1/16 lb ae in 10 gpa of spray) and a few were also active at a lower concentration (1/16 lb ae in 40 gpa). The remaining thirteen salts were relatively inactive in water. Structural features associated with water-active salts of 2,4-D were those which contribute to hydrophilic characteristics of surfactants. Lipophilic structural features contributed to inactivity in water.

The 2,4-D toxicity of fourteen water-active and water-inactive salts of 2,4-D was maximally and equally elicited in one or more of six water and oil spray systems. Differential activities of the salts permitted their classification into six distinct classes—three hydrophilic and three lipophilic. No one of five solution properties—surface tension, interfacial tension, turbidity, specific conductivity, or pH—could be correlated with herbicidal toxicity of a spray. However, a combination of three properties—interfacial tension, turbidity, and specific conductivity—provided some indication of the relative hydrophilic or lipophilic character of a salt and of the 2,4-D toxicity in different sprays. Data suggested that herbicides move into plants by two distinct pathways—one hydrophilic and the other lipophilic.

The discovery of the herbicidal properties of 2,4–dichlorophenoxyacetic acid (2, 4–D) created a great impetus in the search for new herbicides. As the herbicidal properties of 2,4–D became more generally known, it was obvious that many closely related chemicals and other organic compounds should be evaluated for their herbicidal properties.

Absorption and translocation characteristics of both root- and shoot-applied mefluidide N-[2,4-dimethyl-5-[[(trifluoromethyl) sulfonyl] amino] phenyl] acetamide were determined in soybean [Glycine max (L.) Merr. ‘Wells’], common cocklebur (Xanthium pensylvanicum Wallr.), and giant foxtail (Setaria faberi Herrm.). Absorption of 14C-mefluidide was greater through the foliage than the roots of all plant species. Rate of leaf absorption and translocation of 14C initially was greater in giant foxtail than in either soybean or common cocklebur, but these processes failed to increase greatly with time in giant foxtail. Absorption and translocation increased over time in soybean and common cocklebur. Soybean absorbed slightly greater quantities of 14C-mefluidide than common cocklebur, but by 8 days after labeling, common cocklebur translocated approximately 47% more radio-label out of the treated leaf than soybean. The foliar-applied 14C-label appeared to move in the phloem along with the assimilate stream in all species, mainly to areas of high metabolic activity. 14C was associated with the root systems and/or nutrient solutions of soybean and giant foxtail, but not of common cocklebur. Exuded material chromatographed as mefluidide. Movement of 14C-label in all root-treated plants appeared to be confined within the xylem with older tissue containing greater amounts of 14C than the younger tissue. Differences in selectivity between crop and weed species may be partially due to differential mefluidide absorption, translocation, and subsequent concentration in certain plant tissues.

Twenty accessions within 11 perennial Glycine species plus soybean were tested for tolerance to 2,4-D. Soybean was severely injured by 2,4-D, but 13 of the Glycine accessions had 15% or less injury 4 weeks after 2,4-D application. Greatest 2,4-D tolerance occurred with accessions of G. latifolia and G. microphylla. There was no difference among tolerant accessions of G. latifolia and G. microphylla and susceptible accessions of G. canescens in recovery or absorption of 14C-2,4-D 1, 3, 7, or 14 days after treatment (DAT). Distribution of 14C from 14C-2,4-D in various plant parts was similar among accessions. Metabolism of 2,4-D in the tolerant accessions (81 to 89% 1 DAT) was higher and more rapid than in susceptible accessions (approximately 50%, 1 DAT). The same five metabolites plus parent 2,4-D were extracted from the treated leaf of all accessions at all sampling dates. However, relative distribution between metabolites differed between tolerant and susceptible accessions. More rapid metabolism of 2,4-D in treated leaves of tolerant Glycine accessions can explain differential 2,4-D responses.