Selective control of Egyptian broomrape is extremely difficult because the close association between host crop and parasite limits the use of most mechanical and herbicidal approaches. However, this host–parasite interaction can also form the basis of the simplest control strategy: parasite-resistant crops. Although much work has been conducted to identify and characterize mechanisms of parasite resistance, varieties with stable resistance are still unavailable for most affected crops. The development of resistant crops can be accelerated by genetic engineering to the extent that important aspects of the host–parasite interaction are understood. In this study, we characterize a variety of gene promoter elements with respect to parasite induction and expression pattern. Studies were conducted using transgenic plants expressing fusions of the β-glucuronidase reporter gene with promoter elements from several genes. Promoters from genes known to have increased expression in response to pathogen attack or wounding showed localized, induced expression after parasitism. These included phenylalanine ammonia lyase, chalcone synthase, sesquiterpene cyclase, and HMG1 (3-hydroxy-3-methylglutaryl CoA reductase). In contrast, the systemic acquired resistance–associated gene PR-1a was not induced by parasitism. Non–defense-related genes varied in response, with squalene synthase being repressed, whereas farnesyltransferase was highly expressed in the region of parasite attachment. These results demonstrate a range of expression, both in intensity and tissue specificity, in response to parasitism.

Trifloxysulfuron controls green and false-green kyllinga more effectively than halosulfuron. Studies were conducted to evaluate the absorption, translocation, and metabolism of 14C-halosulfuron and 14C-trifloxysulfuron when foliar applied to green and false-green kyllinga. No differences were observed between the two kyllinga spp. with regard to absorption, translocation, or metabolism of either herbicide. The majority of 14C-halosulfuron and 14C-trifloxysulfuron was absorbed by 4 h, with an accumulation of 63 and 47% radioactivity, respectively. Accumulation of both herbicides occurred in the treated leaf and the primary shoot from whence the treated leaf was removed, with minor accumulation occurring in the roots and newly formed rhizomes. Of the total amount of 14C-halosulfuron absorbed into the plant, 77% remained in the form of the parent compound compared with 61% of 14C-trifloxysulfuron. The parent compound was distributed mainly in the treated leaf and primary shoot, whereas polar metabolites were concentrated in the roots and rhizomes. Nonpolar metabolites of 14C-trifloxysulfuron accumulated in the treated leaf and primary shoot. These data indicate that absorption, translocation, or metabolism could not explain the variation in green and false-green kyllinga control between halosulfuron and trifloxysulfuron.

Foramsulfuron (AE F130360) is a sulfonylurea herbicide for postemergence control of grasses and some broadleaf weeds in corn. Greenhouse and laboratory experiments were conducted to determine the physiological basis for differential tolerance of corn hybrids to foramsulfuron. Differences in corn tolerance were quantified by determining the herbicide rate required to cause injury and reduce corn height by 15% (GR15). Seven hybrids were screened in the greenhouse where GR15 values indicated that the some of the corn hybrids were sensitive to foramsulfuron. All but one of these hybrids exhibited an increase in tolerance when the safener, isoxadifen-ethyl, was applied with foramsulfuron. Experiments using 14C-foramsulfuron were conducted to determine whether isoxadifen-ethyl affected foramsulfuron absorption, translocation, and metabolism in two of the corn hybrids. There was no difference in absorption between a sensitive hybrid Novartis 58D1 and a more tolerant hybrid Novartis 59Q9, but the addition of isoxadifen-ethyl increased foramsulfuron absorption in both hybrids 24 h after treatment. Less than 1 and 3% of the 14C-foramsulfuron was translocated to plant portions above and below the treated leaf, respectively. Differences in hybrid tolerance were primarily due to differential herbicide metabolism. Foramsulfuron metabolism, with and without isoxadifen-ethyl, was similar at 4 h after treatment for both the sensitive and tolerant hybrids. However, by 24 h after treatment the more tolerant hybrid metabolized foramsulfuron to more polar compounds to a greater degree than the sensitive hybrid.

Previous research has shown that flumioxazin has the potential to cause peanut injury. In response to this concern, laboratory and greenhouse experiments were conducted to investigate the influence of temperature on germination of flumioxazin-treated peanut seed and the effect of interval between flumioxazin application and irrigation on peanut emergence and injury. Laboratory experiments using 14C-flumioxazin were also conducted to investigate differential tolerance exhibited by peanut, ivyleaf morningglory, and sicklepod to flumioxazin. Flumioxazin treatments containing either water-dispersible granular or wettable powder formulation at 1.4 μmol L−1 did not influence germination compared with nontreated peanut across all temperature regimes (15 to 40 C). Peanut treated with either formulations of flumioxazin preemergence and receiving irrigation at emergence and 2 and 4 d after emergence were injured between 40 and 60%. Peanut treated at 8 and 12 d after emergence were injured between 25 and 15%, respectively. Total 14C absorbed by ivyleaf mornigglory was 57% of applied whereas sicklepod absorbed 46%, 72 h after treatment (HAT). Peanut absorbed > 74% of applied 14C 72 HAT. The majority of absorbed 14C remained in roots of sicklepod, ivyleaf morningglory, and peanut at all harvest times. Ivyleaf morningglory contained 41% of the parent herbicide 72 HAT whereas sicklepod and peanut contained only 24 and 11% parent compound, respectively. Regression slopes indicated slower flumioxazin metabolism by ivyleaf morningglory (a susceptible species) compared with sicklepod and peanut (tolerant species).

Experiments were conducted in the North Carolina State University Phytotron greenhouse and field locations in Clayton, Rocky Mount, and Lewiston-Woodville, NC, in 2002 to determine the effect of glyphosate on pollen viability and seed set in glyphosate-resistant (GR) corn. Varieties representing both currently commercial GR corn events, GA21 and NK603, were used in phytotron and field studies. All glyphosate treatments were applied at 1.12 kg ai ha−1 at various growth stages. Regardless of hybrid, pollen viability was reduced in phytotron and field studies with glyphosate treatments applied at the V6 stage or later. Scanning electron microscopy of pollen from affected treatments showed distinct morphological alterations correlating with reduced pollen viability as determined by Alexander stain. Transmission electron microscopy showed pollen anatomy alterations including large vacuoles and lower starch accumulation with these same glyphosate treatments. Although pollen viability and pollen production were reduced in glyphosate treatments after V6, no effect on kernel set or yield was found among any of the reciprocal crosses in the phytotron or field studies. There were also no yield differences among any of the hand self-pollinated (nontreated male × nontreated female, etc.) crosses. Using enzyme-linked immunosorbent assay to examine CP4-5-enolpyruvlshikimate-3-phosphate synthase expression in DKC 64-10RR (NK603) at anthesis, we found the highest expression in pollen with progressively less in brace roots, ear leaf, anthers, roots, ovaries, silks, stem, flag leaf, and husk.

The root-parasitic broomrape species cause severe damage to field and vegetable crops worldwide. This study evaluated the relationship between small broomrape development and temperature with red clover as a host plant. Red clover plants were grown in soil artificially infested with small broomrape seed in temperature-controlled growth chambers. Parasite development was quantified at 48 different accumulated growing degree days (GDD). Small broomrape parasitism and temperature were strongly related. Small broomrape tubercle initiation was delayed by low temperature. Tubercle development initiated at about 750 GDD and peaked at about 1,100 GDD. Small broomrape biomass accumulation correlated with the increase in tubercle number over time. Parasitism stages were divided into lag, log, and maximum phases that were strongly related to GDD. Development of a predictive system for parasitism growth stage is needed to allow precise herbicide application for effective control before small broomrape shoot emergence.

The introduction of imazamox-tolerant winter wheat has created an interest in jointed goatgrass × winter wheat hybrids because of the potential for transferring resistance to jointed goatgrass. The literature is void of any information about the occurrence of hybrids in Oklahoma. Therefore, jointed goatgrass × winter wheat hybrids were identified and harvested for characterization and spikelet viability from 2000 to 2002. Mature hybrid height varied from 46 to 114 cm, and spike length varied from 5.0 to 13.8 cm. Hybrid spike color at harvest was darker than mature wheat spike color, and mature hybrid spikes disarticulated intact, unlike jointed goatgrass. More hybrid plants were produced when jointed goatgrass was grown with ‘Dominator’ wheat than with other cultivars. Hybrid spikelet germination was 0.42, 0.97, and 1.10% in 2000, 2001, and 2002, respectively.

Yellow starthistle is one of the most important alien invasive weeds in the western United States. It has been targeted for biological control based on the assumption that its abundance is limited by natural enemies in its native region but not in the United States. The geographic center of diversity for yellow starthistle appears to be in Turkey. This region is being explored to discover potential biological control agents; however, there is no quantitative information regarding the population density or dynamics of the plant in this region. Such information could help determine which natural enemies help suppress the plant in its land of origin. We measured densities of yellow starthistle plants and seeds during 2 yr at three locations in central Turkey. Densities of mature plants were about 4% of those measured at sites in California. Densities of capitula and seeds produced were about 60 and 65%, respectively, of those measured in California. The greatest difference between the two regions appears to be the densities of mature plants, which indicates the importance of focusing research on natural enemies that reduce plant survival.

Bulb onions are poor competitors and volunteer potato, commonly observed in western USA onion fields, is difficult to manage. To improve the understanding of onion and weed interactions, relationships were quantified among volunteer potato density, onion yield, and volunteer potato tuber production using hyperbolic or linear models. Onion yield losses because of volunteer potato interference occur at densities commonly observed in the field. A volunteer potato density as low as 0.067 plants m−2 resulted in a 10% reduction in crop yield. Asymptotic yield loss (A parameter) was 100% and achieved with 4 volunteer potato plants m−2. Volunteer potato competition limits onion bulb size, resulting in a lower quality and thus a less-valuable crop. Volunteer potato tuber density and biomass increased linearly with initial weed density as high as 8 volunteer potato m−2. Onion yield loss from volunteer potato competition occurs to a greater extent and at a lower weed density than demonstrated in previous research on small-seeded annual weed species.

Sexual reproduction is known to be an important means of propagation for native Japanese knotweed populations in Asia. For naturalized populations in the United States, however, its relative importance compared with propagation by rhizome and stem fragments has not been established. This article presents two related studies that address two basic questions regarding the potential for propagation by seed: (1) are seeds that are produced by Japanese knotweed in areas of Philadelphia, PA, capable of germinating in the field? If so, (2) when and how rapidly during the year do these seeds acquire germinability? Field germination experiments assessed germination of planted seeds at intervals during the growing season at two different study sites (one in 2000 and the other in 2002) and showed that germination was common at both sites. The 2002 experiment also demonstrated that germination occurred both within existing stands of Japanese knotweed and in areas well removed from existing stands, and that both planted and naturally occurring seeds germinated. Experiments on seasonal acquisition of seed germinability used batches of seeds collected weekly between early September and November 2000 from three different study sites. Results showed that at two of the study sites, a rapid increase in germinability from less than 10% to greater than 90% occurred between collection dates in mid-September and mid-October 2000. A smaller and more gradual change occurred at the third site, where maximum germinability was roughly 50%. Regarding management of Japanese knotweed, the field germination experiments suggest that spread of this plant by seed is a realistic possibility in the United States. Because of the rapidity with which germinability is acquired, the high germinability attained, and the large number of seeds produced, we recommend that female (male-sterile) plants or their inflorescences be removed from seed-producing populations before the formation of fruits to minimize spread by seed.

Studies were conducted to determine the relative fitness and competitive ability of an acetolactate synthase (ALS) inhibitor–resistant (R) downy brome biotype compared with a susceptible (S) biotype. In previous research, the mechanism of resistance was determined to be an altered ALS enzyme. Seed germination of the R biotype was compared with that of the S biotype at 5, 15, and 25 C. There were no different germination characteristics between R and S biotypes at 15 and 25 C. However, the R biotype germinated 27 h earlier than the S biotype and had reached over 60% germination when the S biotype initially germinated at 5 C. Under noncompetitive greenhouse conditions, growth of the R biotype was similar to that of the S biotype on the basis of shoot dry weight, leaf area, and plant height. Seed production of the R biotype was 83%, when compared with the S biotype, but seeds of the R biotype were larger than those of the S biotype. Replacement series experiments were conducted in the greenhouse to determine the relative competitive ability of R and S biotypes. No difference in competitive ability was observed between R and S biotypes on the basis of shoot dry weight, leaf area, or plant height. Thus, it appears that ALS-resistance trait is not associated with growth penalty in either noncompetitive or competitive conditions. In the absence of ALS inhibitors, these results suggest that the R biotype would remain at a similar frequency in a population of R and S biotypes.

A plant's ability to detect and adjust morphologically to changes in light quality (red–far-red [R:FR] ratio) is one mechanism by which a crop plant responds to weeds. To test this hypothesis, two experiments were conducted where corn was grown in growth cabinets under different light environments. First, to determine the effect of R:FR ratio on corn growth and development, treatments of high R:FR (1.37) and low R:FR (0.67) ratio were compared. These were established by planting corn in pots and then placing trays of either turface (a baked clay medium with high R:FR) or commercial grass sod (low R:FR) on each side of a row of corn pots. Grass sod was used to simulate low-growing weeds. The low R:FR sod treatment resulted in corn plants which were taller, had larger leaves, and greater shoot–root ratio than plants growing in the high R:FR turface treatment. In the second experiment, the effect of R:FR ratio on corn leaf azimuth position was examined. This was accomplished by adding a third treatment where each corn row had sod placed on one side and turface on the other. The proportion of leaves in four azimuthal classes was recorded. In the presence of sod, the proportion of leaves perpendicular to the corn row decreased, and this altered the proportion of leaves in other classes. Therefore, corn seedlings detected changes in light quality caused by the presence of sod (which simulated low-growing weeds) and responded by adjusting carbon allocation and leaf orientation to optimize the interception of light quantity and quality. These results support our hypothesis that low-lying vegetation can alter the growth of corn seedlings before competition for resources occurs. This change in growth may help explain the importance of early-season weed control in corn.

Because soil characteristics and weed densities vary within agricultural landscapes, determining which subfield areas are most favorable to weed species may aid in their management. Field and greenhouse studies were conducted to determine whether subfield environments characterized by higher soil organic carbon (SOC), or ridge vs. furrow microsites, affect common sunflower seed germination after winter burial, seedling emergence, or the control afforded by a preemergence herbicide in a ridge-tillage corn production system. Among seeds buried in situ during winter months and germinated in the laboratory, no differences in common sunflower seed germination or mortality were detected between high-SOC (1.8% mean) and low-SOC (1.1% mean) locations. However, seeds buried at 5-cm depth had about 40% laboratory germination compared with about 10% for seeds stratified on the soil surface or under crop residues. In field emergence and survival experiments, the SOC main-plot effect indicated 25% greater seedling survival in high- than in low-SOC locations. In the absence of herbicide, both emergence and survival were ≥ 35% greater in the ridge than in the furrow microsite, and seedling survival was 48% greater in high- vs. low-SOC furrow environments. However, common sunflower seedling survival was similar between herbicide-treated high- and low-SOC ridges. Greenhouse studies indicated a 13 to 24% increase in common sunflower seedling biomass per 1% increase in SOC under three atrazine doses. Altered or additional weed management tactics should be considered for common sunflower in high-SOC environments to offset the greater potential for seedling survival and growth.

Experiments were conducted to examine the use of spectral reflectance curves for discriminating between plant species across moisture levels. Weed species and soybean were grown at three moisture levels, and spectral reflectance data and leaf water potential were collected every other day after the imposition of moisture stress at 8 wk after planting. Moisture stress did not reduce the ability to discriminate between species. As moisture stress increased, it became easier to distinguish between species, regardless of analysis technique. Signature amplitudes of the top five bands, discrete wavelet transforms, and multiple indices were promising analysis techniques. Discriminant models created from data set of 1 yr and validated on additional data sets provided, on average, approximately 80% accurate classification among weeds and crop. This suggests that these models are relatively robust and could potentially be used across environmental conditions in field scenarios.

Field trials were conducted to investigate the influence of P application method on the critical period of smooth pigweed and common purslane interference in lettuce. Studies were carried out in low-P histosols, where supplemental P fertilization is needed for lettuce production. Phosphorus was either broadcast or banded 5 cm beneath the lettuce rows at rates of 250 or 125 kg ha−1, respectively. Seedlings of either smooth pigweed or common purslane were transplanted at a density of 16 plants per 5.4 m2 (6-m row by 0.9 m wide). Weed interference duration was achieved by manual removal 2, 4, 6, or 8 wk after lettuce emergence and subsequently keeping the plot weed free until harvest. A weed-free control within each P regimen was also established. Marketable head number, head fresh yield, and head diameter were measured at harvest. Weed-free lettuce fresh yield was 20% higher with banded P than broadcast applications. In the weed–lettuce mixtures, the P regimen by weed removal interaction affected lettuce fresh yield and head diameter but not head number. Compared with broadcast P application, banded P extended the time needed to cause significant weed interference in lettuce by 10 d: from 24 to 34 d for smooth pigweed and from 37 to 47 d for common purslane.

Field studies were conducted near Blackville, SC, in 2002 and 2003 and near Pendleton, SC, in 2003 to compare the critical period for weed control (CPWC) in narrow- (48 cm) and wide-row (97 cm) corn. A series of treatments of increasing duration of weed interference and length of weed-free period were imposed within each row spacing. Diversity and density of the weed spectrum were greater at Blackville than at Pendleton. Weed interference duration and weed-free period curves were similar between row widths for each of the 3 site-years. Averaged over row width, the CPWC was 36 to 40 d longer at Blackville than at Pendleton. The CPWC began 5 to 9 d after corn emergence (DAE) (one- to two-leaf stage) and ended 45 to 53 DAE (eight- to 10-leaf stage) at Blackville. At Pendleton, the CPWC was only 4 d, beginning 21 DAE (five-leaf stage) and ending 25 DAE (five- to six-leaf stage). Light interception by corn at Blackville at the end of the CPWC averaged 78%, but light interception averaged only 31% at Pendleton at the end of the CPWC, implying that the weed density or weed spectrum may be more of a determinate of the CPWC than canopy formation. Light interception was similar between row widths throughout the growing season, resulting in similar late-season weed biomass between row widths. The CPWC and crop competitiveness with late-emerging weeds was similar between wide- and narrow-row corn when corn light interception did not differ between row widths. Therefore, other strategies, such as increasing the population of narrow-row corn, are likely needed to provide a competitive advantage over wider rows.

Biological agents, including fungal pathogens, are being considered to control weeds such as dandelion. Compatibility of a fungal pathogen, Phoma herbarum, with nonformulated 2,4-D was tested in vitro. Moderate to high concentrations (828 to 2,451 ppm) of 2,4-D reduced fungal colony diameter by 50% or less, suggesting that practical concentrations may be chosen for tank mixes. Under growth room and field conditions, integration of 2,4-D and P. herbarum produced enhanced control of dandelion. In general, synergistic responses to sublethal doses of 2,4-D followed 4, 8, or 16 d later by P. herbarum inoculation were found. Synergistic interactions, as determined by the Colby method, ranged as high as 62% greater than expected for an additive interaction. In general, dandelion response was reduced under field conditions, suggesting that environmental conditions or other factors play a role in efficacy. Integration of chemical and biological agents shows potential for enhanced weed control.

The growth habits of the crop and competing weed species are important determinants of crop–weed competition. Three cowpea genotypes with similar vegetative vigor but different growth habit were assessed for their relative competitiveness with two weed species. ‘Iron-Clay’ (IC) grows erect, ‘IT89KD-288’ (288) is semierect, and ‘UCR 779’ (779) is prostrate. Common purslane, a short-statured weed, and common sunflower, a tall species, were planted within the cowpea rows. Cowpea canopy height and width, leaf area, dry weight, and light intensity above and below sunflower and cowpea canopies were measured weekly from 21 d after planting. Sunflower reduced the leaf area, amount of light received, and biomass of all cowpea genotypes. Purslane reduced the leaf area of 779 and the biomass of 288 and 779, but the biomass and leaf area of IC were not affected. The presence of sunflower increased the height of IC and 288, but the presence of purslane decreased the canopy height of 779. IC reduced sunflower biomass, whereas IC and 779 reduced purslane biomass. IC and 288 reduced sunflower leaf area, whereas IC and 779 reduced purslane leaf area. The growth analysis of biomass, leaf area, and canopy height of cowpeas and weeds showed similar results. The experiments indicate that cowpea genotypes differ in their ability to compete with purslane and sunflower. IC was the most competitive genotype, suggesting that an erect growth habit may be more effective in suppressing weeds than semierect or prostrate growth habits.

To minimize the chance of surface water contamination by herbicides, farmers need alternative ways to manage weeds in field crops, such as field corn, that reduce herbicide use. Zone herbicide application (ZHA) reduces herbicide use compared with conventional broadcast herbicide application by (1) banding low herbicide rates between corn rows (≤ 1× normal broadcast registered rate), (2) managing crops to favor crop competition, and (3) banding very low herbicide rates over crop rows (≪ 1× normal rate). The research goal was to compare the relative effectiveness of reduced-rate ZHA with broadcast herbicide application on in-row (IR) and between-row (BR) summer annual weed cover (chiefly giant foxtail and waterhemp species), grain yields, and net returns resulting from herbicide application in field corn. Preemergence ZHA of atrazine + metolachlor + clopyralid + flumesulam was made in zones (i.e., even width bands) at different rates between and over crop rows for three site-years in Missouri, and the 1× rate was 2.24 + 1.75 + 0.211 + 0.067 kg ai ha−1, respectively. Best ZHA treatments (0.29× to 0.30× IR herbicide rates + 0.74× to 0.80× BR herbicide rates) outperformed all reduced-rate broadcast herbicide treatments (0.25×, 0.5×, and 0.75×) based on net returns in partial budget analysis. Yields for highest yielding ZHA could not be distinguished from the 1× broadcast treatments in two of three site-years. Net returns due to herbicide application for the highest yielding ZHA were comparable with the 1× broadcast treatment in all three site-years. For the best ZHA, the 3-yr average for total herbicide applied per unit was 53% of the 1× broadcast rate. ZHA may provide row crop farmers with a new generic option for reducing herbicide rates and input costs while maintaining net returns and reducing the chance of surface water contamination by herbicides.

Glyphosate-tolerant spring wheat currently is being developed and most likely will be the first major genetically engineered crop to be marketed and grown in several areas of the northern Great Plains of the United States. The public has expressed concerns about environmental risks from glyphosate-tolerant wheat. Replacement of traditional herbicide active ingredients with glyphosate in a glyphosate-tolerant spring wheat system may alter ecological risks associated with weed management. The objective of this study was to use a Tier 1 quantitative risk assessment methodology to compare ecological risks for 16 herbicide active ingredients used in spring wheat. The herbicide active ingredients included 2,4-D, bromoxynil, clodinafop, clopyralid, dicamba, fenoxaprop, flucarbazone, glyphosate, MCPA, metsulfuron, thifensulfuron, tralkoxydim, triallate, triasulfuron, tribenuron, and trifluralin. We compared the relative risks of these herbicides to glyphosate to provide an indication of the effect of glyphosate when it is used in a glyphosate-tolerant spring wheat system. Ecological receptors and effects evaluated were avian (acute dietary risk), wild mammal (acute dietary risk), aquatic vertebrates (acute risk), aquatic invertebrates (acute risk), aquatic plants (acute risk), nontarget terrestrial plants (seedling emergence and vegetative vigor), and groundwater exposure. Ecological risks were assessed by integrating toxicity and exposure, primarily using the risk quotient method. Ecological risks for the 15 herbicides relative to glyphosate were highly variable. For risks to duckweed, green algae, groundwater, and nontarget plant seedling emergence, glyphosate had less relative risk than most other active ingredients. The differences in relative risks were most pronounced when glyphosate was compared with herbicides currently widely used on spring wheat.