Field experiments to compare the loss of oryzalin (3,5-dinitro-N4,N4-dipropylsulfanilamide) from straw-mulched and nonmulched soils indicated that oryzalin disappeared more rapidly in soils covered by straw in 1980 and 1981 but not in 1982. It appeared that greater rainfall in 1982 was responsible for this difference. Straw mulch on the soil at the time of application reduced the amount of oryzalin reaching the soil surface after subsequent rains or irrigation. Straw levels of 2250 or 4500 kg/ha, when present at the time of treatment, reduced oryzalin concentration in the soil by approximately 15 or 43%, respectively, following 1.3 cm of water applied by sprinkle irrigation. Increasing the straw levels above 4500 kg/ha did not significantly affect the amount of oryzalin detected in the soil beneath the straw mulch.

Early-spring, broadcast, disked-in applications of nitrogen fertilizers caused an increased germination of wild oat (Avena fatua L.) seed present in the soil. With adequate moisture, there was increased wild oat infestation in the seeded crop from early-spring application, whereas with dry weather the effects did not carry over into the crop period. The apparent breaking of seed dormancy by nitrates from fertilizer applications may offer another method for combating wild oats other than by herbicides. Fertilizer treatment might be used to deplete seed reserves in the fallow years and to reduce the amount of seed returned to the soil in the crop years by combining a fertilizer treatment with delayed crop seeding.

Kudzu is an aggressive, nonnative vine that currently dominates an estimated 810,000 ha of mesic forest communities in the eastern United States. To test an integrated method of weed control, abundances of kudzu and other plant species were compared during 4 yr after six herbicide treatments (clopyralid, triclopyr, metsulfuron, picloram + 2,4-D, tebuthiuron, and a nonsprayed check), in which loblolly pines were planted at three densities (0, 1, and 4 seedlings m−2) to induce competition and potentially delay kudzu recovery. This split-plot design was replicated on each of the four kudzu-dominated sites near Aiken, SC. Relative light intensity (RLI) and soil water content (SWC) were measured periodically to identify mechanisms of interference among plant species. Two years after treatment (1999), crown coverage of kudzu averaged < 2% in herbicide plots compared with 93% in the nonsprayed check, and these differences were maintained through 2001, except in clopyralid plots where kudzu cover increased to 15%. In 2001, pine interference was associated with 33, 56, and 67% reductions in biomass of kudzu, blackberry, and herbaceous vegetation, respectively. RLI in kudzu-dominated plots (4 to 15% of full sun) generally was less than half that of herbicide-treated plots. SWC was greatest in tebuthiuron plots, where total vegetation cover averaged 26% compared with 77 to 111% in other plots. None of the treatments eradicated kudzu, but combinations of herbicides and induced pine competition delayed its recovery.

Introduction of purple loosestrife into North America and its spread into wetlands has led to the degradation of these important habitats for wildlife. Conventional control efforts are unsuccessful in providing long-term control. A classical biological control program offers the best chance for reducing the numbers of this invasive plant and improving regeneration of the native flora and fauna. European studies demonstrated that the root boring weevil Hylobius transversovittatus is highly host specific to the target weed. Attack of two test plant species (winged lythrum and swamp loosestrife) during host range screening was most likely due to artificial test conditions. An environmental assessment of the potential effects of the release of the purple loosestrife borer in North America indicated that benefits outweigh any potential negative impact Therefore its field release was approved in 1992.

The phytophagous insects associated with silverleaf nightshade (Solanum elaeagnifolium Cav.) were surveyed in southern California, Arizona, New Mexico, and southwestern Texas. In California, this introduced weed was largely free of deleterious insect injury. Its insect associates mostly were sap or foliage-feeding species that attack a wide range of plants, a few were species restricted to Solanaceae, and several were species economically important as crop pests. The insect fauna became increasingly complex in species diversity and plant parts attacked as the survey progressed eastward from California towards southwestern Texas and the presumed native habitats of this weed in adjacent Mexico. The feasibility of biological control of silverleaf nightshade and the value of faunistic surveys in ascertaining the indigenous or alien status of plants are good.

Both gibberellic acid and the substituted phthalimide AC-94377 [1-(3-chlorophthalimido)cyclohexanecarboximide] actively promoted germination of dormant seeds in five species out of nine. The species that responded to both compounds included wild oat (Avena fatua L.), wild mustard [Brassica kaber (DC.) L.C. Wheeler], curly dock (Rumex crispus L.), field pennycress (Thlapsi arvense L.) and tansy phacelia (Phacelia tanacetifolia L. Benth.). on a weight-per-weight basis, the ability of AC-94377 to stimulate germination was equal to, or greater than, GA3. The other two substituted phthalimides tested, AC-92803 [2-(3-chlorophthalimido)-2-methyl-2-isobutyl-acetamide] and AC-99524 [1-tetrahydrophthalimido-cyclohexane-carboximide], had either slight or no stimulatory activity.

Germination response of slender amaranth to temperature, solution pH, moisture stress, and depth of emergence was evaluated under controlled environmental conditions. Results indicated that 30 C was the optimum constant temperature for germination. Germination of slender amaranth seed at 21 d was similar, with 35/25, 35/20, 30/25, and 30/20 alternating temperature regimes. As temperatures in alternating regimes increased, time to onset of germination decreased and rate of germination increased. Slender amaranth germination was greater with acidic than with basic pH conditions. Germination declined with increasing water stress and was completely inhibited at water potentials below −0.6 MPa. Slender amaranth emergence was greatest at depths of 0.5 to 2 cm, but some seeds emerged from as deep as 6 cm. Information gained in this study will contribute to an integrated control program for slender amaranth.

A new technique designed for the study of various modes of weed competition has been developed and tested. It applies to almost all environmental conditions as they affect plant growth. The technique is adaptable to single factors and multiple factors in almost any combination. The only major restriction imposed by present dimensions is that only early competition can be studied.

The uniformity of seed used in closely controlled competition studies is extremely important. Heavy seeds of redroot pigweed (Amaranthus retroflexus L.) produced plants that brought about earlier and more intense shading than plants from light redroot pigweed seed. Applying an aluminized Mylar reflective surface on the inside of the aerial portion of the competition box increased the growth rate and development of species and shortened the time interval for experiments by 10 days.

As an agrestal weed, downy brome (Bromus tectorum L. # BROTE) is most troublesome in winter wheat (Triticum aestivum L. # TRIAE) and alfalfa (Medicago sativa L. # MEDSA) because of the ability of downy brome to reproduce prior to crop harvesting (70, 76). Geographically, downy brome is most common in the western half of the United States where annual rainfall totals 15 to 55 cm and autumn rainfall ranges from 5 to 12 cm (10, 11, 86, 98). In contrast, cheat (Bromus secalinus L. # BROSE) is more common in the eastern portion of the hard red winter wheat producing regions and in the more humid soft red winter wheat region. Behrendt and Hanf (12) state that cheat has been more common on acid sandy or loamy soils while downy brome commonly occurs on dry, mostly alkaline, sandy to loamy soils. Since soils are frequently alkaline in the relatively dry climate of the western regions (61), it is not clear exactly what role pH plays in affecting geographical distribution of the various Bromus spp. Recent serious problems with downy brome in the New England nursery industry (69) and orchards (57) indicate that downy brome could thrive under a relatively wide range of edaphic and climatic conditions. However, as a weed problem in wheat, the common prevalent conditions of low annual precipitation and alkaline soils should not go unnoticed in the development of chemical weed control practices.

Weed interference during crop establishment is a serious concern for Florida strawberry [Fragaria×ananassa (Weston) Duchesne ex Rozier (pro sp.) [chiloensis×virginiana]] producers. In situ remote detection for precision herbicide application reduces both the risk of crop injury and herbicide inputs. Carolina geranium (Geranium carolinianum L.) is a widespread broadleaf weed within Florida strawberry production with sensitivity to clopyralid, the only available POST broadleaf herbicide. Geranium carolinianum leaf structure is distinct from that of the strawberry plant, which makes it an ideal candidate for pattern recognition in digital images via convolutional neural networks (CNNs). The study objective was to assess the precision of three CNNs in detecting G. carolinianum. Images of G. carolinianum growing in competition with strawberry were gathered at four sites in Hillsborough County, FL. Three CNNs were compared, including object detection–based DetectNet, image classification–based VGGNet, and GoogLeNet. Two DetectNet networks were trained to detect either leaves or canopies of G. carolinianum. Image classification using GoogLeNet and VGGNet was largely unsuccessful during validation with whole images (Fscore<0.02). CNN training using cropped images increased G. carolinianum detection during validation for VGGNet (Fscore=0.77) and GoogLeNet (Fscore=0.62). The G. carolinianum leaf–trained DetectNet achieved the highest Fscore (0.94) for plant detection during validation. Leaf-based detection led to more consistent detection of G. carolinianum within the strawberry canopy and reduced recall-related errors encountered in canopy-based training. The smaller target of leaf-based DetectNet did increase false positives, but such errors can be overcome with additional training images for network desensitization training. DetectNet was the most viable CNN tested for image-based remote sensing of G. carolinianum in competition with strawberry. Future research will identify the optimal approach for in situ detection and integrate the detection technology with a precision sprayer.

When vapor losses of isopropyl N-phenylcarbamate (IPC) and isopropyl N-(3-chlorophenyl)carbamate (CIPC) were studied under controlled laboratory conditions using a direct-trap for vapors, IPC proved more volatile than CIPC. Temperature, air-flow rate, moisture, and cation exchange capacity of soil were important factors influencing volatility. Vapor loss increased with increasing air-flow rate and temperature; losses from moist soil decreased as the percent clay, organic matter, or both, and the exchange capacity increased. Vapor losses were negligible from spray applications on dry soil but were sizable from soils at field capacity which were similar to those from soils saturated with moisture. Vapor losses of IPC from granules were much higher than from a surface spray, and CIPC losses were about the same for the granules as from the spray. Covering the soil-applied herbicide with ⅛ to ¼ in of soil was effective in reducing vapor loss.

Genetic diversity within and among populations of yellow nutsedge (Cyperus esculentus L. # CYPES) was analyzed to evaluate and quantify the genetic consequences of the reported predominance of asexually-produced tubers as colonizing agents. Ten populations were examined using starch gel electrophoresis for allozyme analysis. Four populations of purple nutsedge (Cyperus rotundus L. # CYPRO) were surveyed for comparison. Twelve loci were identified in yellow nutsedge among the eight enzyme systems examined; ten of these loci were found in purple nutsedge. Yellow nutsedge showed relatively low genetic diversity. Most of the genetic diversity occurred as differences among individuals within populations (Hs), compared to differences among populations (Dst) for the four variable loci identified in this species. Thus, most genetic distances between its populations were small. Generally, only a few genotypes occurred within each population. Purple nutsedge was found to possess even lower within- and among-population gene and genotypic diversity. This study supports the view that tubers account for most of the establishment of new populations of both species.

Experiments were conducted to compare germination efficiencies and vegetative growth of soybean and the competing weed species, sicklepod and Palmer amaranth, over a range of temperatures in the root zone and aerial environments. From genetic origins we hypothesized that the weeds would have a higher temperature optimum, which would help explain competitive interactions seen in the southeastern U.S. Germination experiments indicated that germination efficiency of the weeds was much more sensitive to low temperature than soybean, being markedly inhibited below 18 C. Similarly, experiments in an automated, temperature-controlled hydroponic system revealed that the weed species were less tolerant of low root zone temperature but more tolerant of high root zone temperature than soybean. At 16 C, dry weight of soybean was 74% of the control dry weight at 24 C, whereas dry weights of sicklepod and Palmer amaranth were 5 and 20% of the control, respectively. At 32 C, soybean root dry weight was only 80% of the 24 C treatment, whereas root dry weight of the weed species was not significantly different. When plants were grown at a low aerial temperature, growth of all plants was strongly inhibited] but the negative effects were somewhat more severe in the weed species than with soybean. An increase in aerial temperature from 26/22 C to 34/30 C (day/night) had a positive influence on dry matter accumulation of the weed species, stimulating sicklepod 150 to 200% and Palmer amaranth 150 to 1,600% compared to their respective controls, whereas soybean remained at about 80 to 90% of the control. All species grew taller with increasing temperature. Leaf area of the weeds increased but leaf area of soybean did not increase. Consistent with our original hypothesis, the results clearly show that the weeds, which originate from warm geographical regions, respond more negatively than soybean to low temperatures in the growth environment but more positively to high temperatures. The temperature characteristics help to explain why the intensity of weed pressure increases as the soybean growing season progresses, even after canopy closure.

Translocation of 14C-glyphosate [N-(phosphonomethyl) glycine] in tall morningglory [Ipomoea purpurea (L.) Roth., ‘Heavenly Blue’] was compared with movement of 14C-photosynthate. The flow of photosynthate was manipulated by creating assimilate sinks in mature leaves and cotyledons using combinations of stem girdling, leaf shading, and localized cytokinin application. Comparisons of 14C distribution patterns indicated minor differences between glyphosate and assimilate sink partitioning when 14C-glyphosate or 14CO2 were applied to a single leaf. Labeled glyphosate moved upward through stem girdles in greater proportions than 14C from photosynthate, suggesting a greater capacity for glyphosate to transfer from symplast to apoplast. When 14C-glyphosate was applied to the stem, 14C still moved symplastically to natural and artificial sinks, but apoplastic movement into all transpiring tissues above the site of application greatly increased. Results support the classification of glyphosate as an ambimobile rather than a phloem-mobile herbicide.

Because adsorption is a major factor controlling herbicide persistence, activity, and mobility in soils, an extensive literature exists on the measurement of this process. The adsorption data are usually fitted to the Freundlich equation, but attempts to interpret this equation theoretically have had only limited success. A meaningful thermodynamic interpretation of the Freundlich equation can be developed using a proposed standard state for adsorbed herbicide, which assumes that soil organic matter forms a solid solution with the herbicide. When herbicide vapor pressures and aqueous solubilities are taken into account, the losses in free energy of different herbicides, on adsorption onto wetted soil surfaces, are shown to be similar to each other and are slightly less than the loss on absorption to the solid herbicides themselves. The Freundlich exponent is related in a simple way to the decreasing energy of available soil sites as the amount of herbicide adsorbed increases, but this change is relatively small over the range of concentrations used in adsorption experiments. Sample results are given for six triazine, three urea, and one uracil herbicides.

Field experiments were conducted in 1999 and 2000 in northern Greece to study interference between johnsongrass from seed or rhizomes and three corn hybrids (‘Dunia’, ‘Papea’, and ‘Costanza’). Fresh weight of each corn hybrid was lower with greater duration of johnsongrass interference, and it was lower for corn grown with johnsongrass from rhizomes than from seed. Corn-silage yield and corn-grain yield were 10 to 14% less than weed-free corn yield when grown with johnsongrass plants from rhizomes 4 wk after sowing (WAS) or from seed 6 WAS. Regression analysis indicated that a corn-grain yield loss of 10% was associated with an interference period of 2.6 to 2.7 WAS for johnsongrass from rhizomes and 4.3 to 5.1 WAS for johnsongrass from seed compared with weed-free corn. Corn-silage yield with season-long interference by johnsongrass from rhizomes or from seed was 83 and 62% lower, respectively, than was weed-free corn yield. Similarly, corn-grain yield with season-long interference by johnsongrass from rhizomes or from seed was 88 and 57% lower, respectively, than was the yield from weed-free corn. Corn-ear length of each hybrid was more affected by johnsongrass interference than was 1,000-seed weight. Johnsongrass plants from rhizomes emerged earlier, grew faster, and produced greater fresh weight than did plants from seed. However, fresh weight and stem number of johnsongrass plants from either seed or rhizomes were not affected by corn hybrids.

Perennial pepperweed is an aggressive, exotic weed invading wetland and riparian areas in California, including the San Francisco Bay/Sacramento–San Joaquin Delta Estuary. Effective management will require detailed and accurate maps of its distribution. Remote sensing technologies offer the capability to map weed species over broad areas and with rapid return intervals. As a first step in assessing the potential to map perennial pepperweed with hyperspectral remote sensing data, this study determined its spectral uniqueness relative to co-occurring species. Spectral measurements were conducted during summer drought conditions in the Sacramento–San Joaquin Delta region. Reflectance spectra of perennial pepperweed and seven co-occurring species were collected with a portable spectrometer. Nineteen physiological indexes were calculated from the reflectance data. Physiological indexes are sensitive to narrow spectral features and encapsulate reflectance information in ecologically relevant ways. Classification trees generated from these indexes were able to discriminate both flowering and fruiting perennial pepperweed from co-occurring species with high levels of cross-validated accuracy when using the original spectrometer data and also when this data set was resampled to simulate the spectral resolution of two widely used airborne hyperspectral imagers. Perennial pepperweed's characteristic white flowers are the major component of the spectral uniqueness of this species. Phenological state influenced reflectance spectra more strongly than variation in intraseasonal maturity. Field spectrometer spectra were qualitatively and quantitatively similar to perennial pepperweed spectra extracted from airborne image data. These results suggest that hyperspectral remote sensing will be a powerful tool for the mapping and monitoring of perennial pepperweed. Future work will extend these analyses to image data encompassing the San Francisco Bay/Sacramento–San Joaquin Delta region.

Added surfactant (Atlox 210) at 1% (v/v) and high relative humidity enhanced the penetration of both 4-amino-3,5,6-trichloropicolinic acid (picloram) and (2,4-dichlorophenoxy)acetic acid (2,4-D) into detached leaves of aspen poplar (Populus tremuloides Michx.). The influence of added surfactant was greater for picloram and the dimethylamine of 2,4-D than for the ethyl or butoxyethanol ester of 2,4-D. Penetration of picloram and the dimethylamine of 2,4-D occurred more readily from the abaxial than from adaxial surfaces of leaves. The ethyl ester of 2,4-D penetrated equally readily from both leaf surfaces. Penetration of picloram and the dimethylamine of 2,4-D from the adaxial surface of leaves occurred readily in young leaves in early June. There was an increase in penetration in early July followed by a decrease in August and September to a level equal to or less than that in June. Penetration from the abaxial surface of leaves was nearly equal in June and July, but there was a gradual decrease in August and September. An increase in temperature from 10 to 25.5 or to 40.5 C resulted in a sharp increase in penetration of both picloram and 2,4-D under both low and high relative humidity. Autoradiographic evidence showed that movement of picloram within the leaf also was much more extensive at the higher temperatures. Partial removal of cuticular waxes from the adaxial surface of leaves with chloroform resulted in up to four-fold increases in penetration of picloram and 2,4-D. Differences in penetration rate of picloram between leaves of aspen poplar and balsam poplar (Populus balsamifera L.) did not account for reported differences in susceptibility between these two species.

Jointed goatgrass is a serious weed in winter wheat, and presently no herbicides are available for its selective control. This study examined the effect of time of emergence and removal on jointed goatgrass interference in winter wheat, as well as its rate of development and soil water extraction. The goal of this study was to suggest cultural practices that minimize jointed goatgrass interference in winter wheat. Jointed goatgrass development was identical to ‘Vona’ winter wheat in two crop seasons, even though precipitation differed drastically between seasons. Depth of soil water extraction of both species was also similar. Jointed goatgrass at 18 plants m−2 reduced grain yield 27 and 17% when emerging 0 and 42 d after Vona, respectively. The relationship between time of jointed goatgrass emergence after winter wheat and grain yield loss was Y = 30.6–0.29X (X = days, r = 0.72), indicating that plants emerging in late fall still caused yield loss. Removing jointed goatgrass by early March prevented winter wheat grain yield loss. The interference data suggests that producers assess infestation levels and plan control measures in early March.

Inhibitors of acetolactate synthase (ALS) are important herbicides for control of wild mustard, a common weed of the north central United States and Canada. Wild mustard that survived treatments with the ALS inhibitors cloransulam, imazethapyr, and thifensulfuron was sampled from a North Dakota soybean field in 1999. The mechanism of resistance and response of this wild mustard biotype to ALS-inhibiting herbicides was investigated. In vitro enzyme-inhibition experiments confirmed a resistance mechanism associated with the ALS enzyme; imazethapyr or imazamox at 1 × 10−4 M caused only 10 to 11% and 12 to 16% reductions in ALS activity, respectively. ALS from a susceptible wild mustard biotype was inhibited 50% (I50) with imazethapyr at 8 × 10−7 M or imazamox at 1.1 × 10−6 M. Whole-plant greenhouse treatments confirmed cross-resistance across ALS-inhibitor classes. Treatment with twice-normal field rates of thifensulfuron, ethametsulfuron, triflusulfuron, imazamox, imazethapyr, flumetsulam, cloransulam, flucarbazone, and imazamethabenz reduced biomass of the susceptible biotype at least 96% 28 d after treatment. Biomass of the resistant biotype was reduced 49% by triflusulfuron and 35% by thifensulfuron, but was not reduced by other herbicides. DNA sequence analysis of ALS genes from resistant and susceptible biotypes revealed a point mutation inferring a Trp-to-Leu amino acid substitution in ALS of the resistant biotype. This mutation, corresponding to position 574 of the Arabidopsis ALS amino acid sequence, is known to confer cross-resistance to ALS-inhibiting herbicides and is the probable cause of resistance in the wild mustard biotype. Phylogenetic analysis of wild mustard and canola ALS sequences confirmed that the Trp574 mutation arose within wild mustard and was not derived via introgression from imidazolinone-resistant canola. The results of this research indicate a naturally occurring target-site point mutation responsible for conferring cross-resistance to ALS-inhibiting herbicides in this wild mustard biotype.