Wild mustard (Sinapis arvensis L.) is a weed that frequently infests winter wheat (Triticum aestivum L.) fields in Golestan province, Iran. Tribenuron-methyl (TM) has been used recurrently to control this species, thus selecting for resistant S. arvensis populations. The objectives were: (1) to determine the resistance level to TM of 14 putatively resistant (PR) S. arvensis populations, collected from winter wheat fields in Golestan province, Iran, in comparison to one susceptible (S) population; and (2) to characterize the resistance mechanisms and the potential evolution of cross-resistance to other classes of acetolactate synthase (ALS)-inhibiting herbicides in three populations (AL-3, G-5, and Ag-Sr) confirmed as being resistant (R) to TM. The TM doses required to reduce the dry weight of the PR populations by 50% were between 2.2 and 16.8 times higher than those needed for S plants. The ALS enzyme activity assays revealed that the AL-3, G-5, and Ag-Sr populations evolved cross-resistance to the candidate ALS-inhibiting herbicides from the sulfonylureas (SU), triazolopyrimidines (TP), pyrimidinyl-thiobenzoates (PTB), sulfonyl-aminocarbonyl-triazolinone (SCT), and imidazolinones (IMI) classes. No differences in absorption, translocation, or metabolism of [14C]TM between R and S plants were observed, suggesting that these non-target mechanisms were not responsible for the resistance. The ALS gene of the R populations contained the Trp-574-Leu mutation, conferring cross-resistance to the SU, SCT, PTB, TP, and IMI classes. The Trp-574-Leu mutation in the ALS gene conferred cross-resistance to ALS-inhibiting herbicides in S. arvensis from winter wheat fields in Golestan province. This is the first TM resistance case confirmed in this species in Iran.

Experiments were conducted to determine the amount of time required for waterhemp to produce mature seeds after pollination. Female waterhemp plants were pollinated over a 24-h time period and then isolated from males. Two branches, each containing at least 500 flowers, were harvested from each female at the time of the initial pollination, designated as 0 d after pollination (DAP), as well as at multiple other times after pollination up to 62 DAP. One branch from each harvest was stored at 30 C for 48 h, while the other branch was stored at −20 C for 48 h. Branches were then stored at room temperature until all harvests were complete, at which time seeds from each branch at each time after pollination were collected, weighed, and stratified. Germination tests were then conducted to determine the time at which seeds become viable after pollination. Seeds that had not germinated by the end of the germination tests were subjected to tetrazolium testing for viability. Germination tests were also conducted on nonstratified seeds to investigate changes in seed dormancy that were expected to occur over the amount of time the seeds were allowed to remain on the plants. Seeds stored initially at 30 C postharvest became viable 7 to 9 DAP, whereas seeds stored initially at −20 C postharvest did not become mature until 11 DAP. Seed coat color was white soon after pollination and became dark brown to nearly black by 12 DAP, and seed weight increased until 12 DAP. Tetrazolium tests for seed viability correlated well with the germination tests. Germination tests on nonstratified seeds indicated that dormancy level was initially high in the population used, but began to decrease between 15 and 30 DAP. Results of this study have implications both for waterhemp management and research.

Knowledge of Palmer amaranth biology and physiology is essential for the development of effective weed management systems. The aim of this study was to investigate the response of Palmer amaranth gender to nutrient deficiency and light stress. Differential gender responses were observed for all the growth, phenology, and photochemistry parameters measured. Female plants, for example, invested more in height, stem, and total dry weight, whereas male plants invested more in leaf area and leaf dry weight. The growth rate of females was higher than that of male Palmer amaranth plants, although both followed similar declining trends as the experimental period progressed. Initiation of flowering of female plants occurred 6 to 8 d earlier compared with male plants. Nitrogen and to a certain extent phosphorous were the most influential nutrients that affected measured parameters in both Palmer amaranth genders, particularly under high light intensity. Electron transport rate and chlorophyll content of female Palmer amaranth plants compared with male plants was lower at high light intensity in combination with nitrogen and phosphorous deficiencies. There is a potential to manipulate Palmer amaranth population structure by altering microenvironments at the field level.

Studies were conducted of one of the structural factors that influences microroughness on johnsongrass leaves. Bicellular trichomes, 47 ± 5 μm long, represented 4 to 5% of all epidermal cells. They secreted a mucilagenous material that covered 8 ± 4% of the leaf surface. Bicellular trichomes occurred in longitudinal rows, intermixed with intercostal cork-silica cells, between rows of stomata. Numbers of bicellular trichomes present per unit area were inversely related to numbers of intercostal cork-silica cells. The trichomes were the panicoid type that are reported not to secrete salts. Johnsongrass trichomes, however, could be induced to discharge salt in the mucilage-type secretions when plants were grown in a soil mixture that was high in lime. Not all secretory constituents were identified, but carbohydrates and callose were found in addition to possible low concentrations of protein. The apical or cap cell of the trichomes stained positively for lipid, protein, and polysaccharide and negatively for pectin, polyphenols, steroids, and alkaloids. The presence of trichomes increases leaf surface microroughness, but the secretion covers wax crystals, decreasing leaf microroughness and likely providing another barrier to herbicide entry through the cuticle. Bicellular trichomes on grain sorghum were similar to those on johnsongrass and also discharged secretions on the leaf surface.

Field experiments were conducted near Boone, IA, to quantify postdispersal seed predation of common lambsquarters and common waterhemp in corn (2003) and soybean (2004) managed with conventional, reduced, and zero-tillage systems. Seed predation in each tillage regime was quantified using selective exclusion treatments during July through September 2003 and June through October 2004. In addition, the activity density of ground-dwelling invertebrates was estimated with pitfall traps. Choice and no-choice feeding trials were conducted in the laboratory using the most abundant weed seed predators found in the field to determine seed preferences of the potential predator organisms. The greatest seed loss occurred during July and August. In 2003, seed predation was lower in zero tillage than in conventional and reduced tillages, but no differences in seed predation between tillage regimes were observed in 2004. Maximum seed predation for common lambsquarters was 53% in 2003 and 64% in 2004. Common waterhemp seed predation reached 80% in 2003 and 85% in 2004. The majority of seed predation was by invertebrate organisms. The most common invertebrate species captured with pitfall traps were field crickets (Gryllus pennsylvanicus De Geer [Gryllidae, Orthoptera]) and ground beetles (Harpalus pensylvanicus Burmeister [Coleoptera, Carabidae]). In 2003, field crickets were relatively more abundant in conventional and reduced tillage than in zero-tillage plots. In 2004, field crickets were more abundant in the reduced tillage than in the other two tillage regimes. No differences were detected for ground beetles among tillage regimes (P = 0.57). Choice and no-choice feeding experiments confirmed the preferences of field crickets and ground beetles for common lambsquarters and common waterhemp seeds over the larger seeds of giant foxtail and velvetleaf. Under field conditions, the activity density of field crickets was a significant predictor of common lambsquarters (r2 = 0.47) and common waterhemp (r2 = 0.53) seed predation. Positive relationships were also detected between the activity density of ground beetles and common lambsquarters (r2 = 0.30) and common waterhemp (r2 = 0.30) seed predation. This research demonstrated that weed seed predation is an important component affecting weed seedbanks and that crop management practices that favor the activity of predators such as field crickets or ground beetles could influence weed populations. Also, the results suggested that tillage is more important in determining the number of weed seeds available on the soil surface to predators than directly affecting predator activity density.

Weed species or genera previously and currently targeted for biological control with phytophagous insects or phytopathogens in the United States (including Hawaii) and Canada are listed. The current status of and natural enemies involved in national, federal, state, and university biological control projects are reported.

An apparatus for trapping the vapors of ethyl N,N-dipropylthiolcarbamate (EPTC) and other herbicides froms soils under field conditions is described. The herbicide vapors were collected in cooled traps along with ice crystals, identified by gas chromatography and determined quantitatively. The amount of EPTC lost and trapped in 30 minutes ranged from 22 to 38 % of that applied to the surface of six different types of wet soils. The amount lost from dry soils was much less than from moist soils. Letting the surface of six different types of wet soils. The amount lost from dry soils was much less than from moist soils. Letting the surface of freshly worked moist soil dry out to depth of ½ inch before spraying with EPTC, greatly reduced the loss by vaporization. Immediate incorporation prevented any loss of EPTC from dry soil and greatly reduced the loss from moist soils. After spraying EPTC on dry soil, sprinkling with small amounts of water increased the loss. Comparison of several thiolcarbamate herbicides showed that n-propyl N,N-di-n-propylthiolcarbamate (R-1607) and n-propyl ethyl-n-butylthiolcarbamate (PEBC) were lost by vaporization at a slower rate than EPTC, and S-ethyl hexahydro-1-H-azepine-1-carbothioate (R-4572) was lost at a much slower rate than the other herbicides tested.

The herbicides mesotrione and topramezone inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD) and have efficacy against smooth crabgrass. Research was conducted to determine the impacts of soil-applied nitrogen (N) fertilizer on the effectiveness of single applications of mesotrione and topramezone for postemergence smooth crabgrass control. Field experiments in 2010 and 2011 evaluated the efficacy of mesotrione (280 g a.i. ha−1) and topramezone (9 g a.i. ha−1) for control of multitiller smooth crabgrass subjected to five N fertility treatments (0, 12, 25, 37, or 49 kg N ha−1). Greenhouse experiments evaluated the response of smooth crabgrass to mesotrione (0, 70, 140, 280, 560, and 1,120 g ha−1) and topramezone (0, 4.5, 9, 18, 36, and 72 g ha−1) with 0 or 49 kg N ha−1. Further research evaluated changes in smooth crabgrass leaf tissue pigment concentrations following treatment with mesotrione (280 g ha−1) and topramezone (18 g ha−1) with 0 or 49 kg N ha−1. In field experiments, N increased smooth crabgrass control with mesotrione and topramezone for 8 wk; however, increasing N rate above 25 kg ha−1 did not improve control on any rating date. In dose-response experiments, N application reduced I50 values for mesotrione and topramezone by 67 and 53%, respectively, 21 d after treatment (DAT). Reductions in aboveground biomass with both herbicides were greater when applied following N treatment as well. In leaf-response experiments, N decreased new leaf chlorophyll and carotenoid concentrations and new leaf production after treatment with topramezone. Future research should investigate whether increased translocation of these herbicides to meristimatic regions contribute to N-enhanced efficacy.

To simulate the effect of MSMA (monosodium methanearsonate) drift onto rice (Oryza sativa L. 'Starbonnet’ or ‘Labelle′), foliar applications at 0.20 to 0.78 kg/ha were made on rice at stages of maturity ranging from early tillering to panicle emergence. Foliage, sampled 1 to 2 weeks after application, and grain and grain parts harvested at maturity were analyzed by arsine generation atomic absorption spectrophotometry. Washed foliage contained 0.7 to 1.5 ppm background As, and 1 to 21 ppm As after MSMA application. The As content depended on application rate, stage of maturity at application, and the formulation of MSMA. Arsenic concentrations in grain increased exponentially as the time of application approached harvest, ranging from 0.25 to 7 ppm. There was little correlation between grain residues and rates, principally because dependence on the time of application was overriding. This study confirms previous observations that there is a specific stage in rice plant development, probably meiosis, during which rice is susceptible to sterilization by arsenic, and also indicates that significant yield reduction can occur from MSMA drift even when visual injury symptoms are not obvious.

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.

Shattercane interference in irrigated soybean was evaluated during 1987, 1988, and 1989 at Clay Center, NE, using ‘Rox’ forage sorghum to simulate shattercane. Soybean yield reduction did not occur if shattercane was removed by 2 wk after emergence in 1987 and 6 wk after emergence in 1988 and 1989. Shattercane interference with soybean began when shattercane height exceeded soybean height. Soybean yield was reduced up to 25% before the height differential reached 30 cm, the minimum difference required for selectively applying glyphosate with a wiper applicator. Soybean nodes per stem, pods per stem, and beans per pod decreased as duration of interference increased. A direct relationship between soybean yield loss and shattercane density fit a rectangular hyperbolic function. Yield loss per shattercane plant was highest at low shattercane densities. Soybean plant height, biomass, nodes per stem, pods per stem, pods per node, and beans per pod decreased as shattercane density increased. An interference model for estimation of soybean yield and economic loss based on shattercane density was developed.

The phenolic content of purple nutsedge (Cyperus rotundus L.) and yellow nutsedge (Cyperus esculentus L.) was determined. The major phenolic components were p-coumaric, ferulic, p-hydroxybenzoic, syringic, vanillic, salicylic, proto-catechuic, and caffeic acids, and eugenol. High concentrations of eugenol and salicylic acid inhibited the sprouting of nutsedge tubers. The lack of activity at lower concentrations argues against a major role for these compounds in maintaining nutsedge tuber dormancy. Exogenous abscisic acid inhibited nutsedge tuber sprouting and may be a natural dormancy factor in nutsedge.

The influence of mineral nutrition on the parasite-host relationship of witchweed (Striga lutea Lour.) and sorghum (Sorghum vulgare Pers. ‘Texas Bighead’) was investigated in greenhouse and growth chamber studies. Sorghum was grown in Eustis loamy sand, either infested or not infested with witchweed, and treated with one of three levels of mineral nutrients. Witchweed reduced host-shoot yields by about 70% at the low nutrient levels and by about 45% at the highest nutrient level. Witchweed produced seed at all nutrient levels, whereas the host produced seed at the highest level only. Increased nutrition also increased witchweed yields. Regardless of nutrient level, an early attack by witchweed was more destructive to the host than a late attack. Growth of witchweed in soil also was studied after the parasite was detached from the host or the host shoot was removed. When only the host shoot was removed, (a) witchweed growth was inhibited; (b) feeding of sucrose to witchweed did not restore parasite growth; (c) mineral nutrients were toxic to the parasite; and (d) high relative humidity around witchweed protected it from the nutrient toxicity. High relative humidity enhanced survival of the detached parasite but markedly inhibited growth of witchweed in the intact parasite-host relationship.

Field studies were conducted in 1979 and 1980 to determine yield losses caused by perennial sowthistle (Sonchus arvensis L. # SONAR) in rapeseed (Brassica napus L., B. campestris L.) fields in Saskatchewan and Manitoba. The relationship between percentage yield loss and density of perennial sowthistle was expressed by the linear regression equation y = −3.81 + 13.76 rdx. Weed survey data indicated that perennial sowthistle occurred in 39% of the rapeseed fields but infested only 7% of the hectares surveyed. Using both the survey data and the yield loss equation, the average annual loss in rapeseed yield was estimated to be 9.4 million kg in Saskatchewan and 6.1 million kg in Manitoba, with a value of 2.6 million and 1.7 million dollars (Cdn.), respectively. Perennial sowthistle occurs with a similar frequency and density in other crops so that the total loss from this weed would be much greater.

Three experiments were conducted in the greenhouse to study the effect of 1,1′-dimethyl-4,4′-bipyridinium salt (paraquat) when sprayed directly on seeds at the soil surface. Germination of alfalfa (Medicago sativa L., var. DuPuit) and red clover (Trifolium pratense L., var. Kenland) was not affected at 1 1b/A paraquat, but germination of Kentucky bluegrass (Poa pratensis L., var. Newport) and perennial ryegrass (Lolium perenne L., var. Linn) was severely reduced at that rate. A protecting layer of soil approximately 0.25 inch thick was completely effective in protecting the seeds from the effect of paraquat. Germination of seven other grass species was reduced by 0.5 1b/A paraquat.

A 2-year field study was conducted to determine the effects of fertilization on elodea (Elodea canadensis Michx.), eurasian watermilfoil (Myriophyllum spicatum L.), and heartleaf pondweed (Potamogeton pulcher Tuckerm.). Plants, water, and sediment were sampled and inorganic mineral contents determined. Water concentrations of NO3, NH4, P, and K increased sharply following fertilization and generally reached maximum values within 7 days after treatment. Concentrations of these nutrients decreased rapidly after reaching maximum values. In the first year, plant growth was not increased due to fertilization while in the second year only heartleaf pondweed produced significantly greater yields. Elodea and eurasian watermilfoil grew better in the control environment, presumably due to less competition from algae, whose growth was increased by the addition of fertilizer. At the end of the study the top 1.3 cm of bottom sediment had increased in cation exchange capacity (C. E. C.), organic matter, total N, and available P for all treatments while the underlying sediment remained relatively unchanged. Available P was greater in the surface of the fertilized sediment than in the surface of the control sediment.

Six herbicides including DNP (2,4-dinitrophenol) were tested for their effects upon nitrate metabolism in green leaves. It was found that these herbicides which are not classified as “photosynthetic inhibitors” permitted nitrite to accumulate within green leaves in darkness but not in light. It is postulated that these herbicides have an uncoupling effect, similar to that of DNP, which indirectly affects the process of nitrate reduction. A new technique which demonstrates this effect is presented and discussed.

Inhibition of growth and photosynthesis in sugarbeet (Beta vulgaris L. var. ‘USH10’) treated with desmedipham [ethyl m-hydroxycarbanilate carbanilate (ester)] was most severe between 25 and 30 C and decreased with higher or lower temperatures. Transfer of sugarbeet plants grown at temperatures from 10 to 35 C to higher temperatures after treatment increased injury and photosynthetic inhibition. Higher temperatures prior to treatment reduced injury at all posttreatment temperatures. When the temperature was changed from 25 to 40 C, inhibition was most severe immediately after treatment. Two days after treatment this 15 C temperature change did not cause additional injury. High posttreatment light intensities caused greater inhibition of photosynthesis than low light intensities.

The phenology of reproduction was highly variable among 23 selections of medusahead (Taeniatherum asperum (Sim.) Nevski). Selections with markedly early and late maturity were observed. The phenology generally was consistent during 4 years of testing at Reno, Nevada, and during 2 years at Davis, California. Individual selections differed greatly in phenology between two locations. Some of the selections exhibited phenotypic plasticity in phenology when grown in competition with other weeds.

Copper sulfate (CuSO4 · 5H20) was used as a tracer to determine soil sample variation and the effects of incorporation and seedbed preparation practices on the horizontal distribution of broadcast herbicide sprays. A Dubbs very fine sandy loam soil was sprayed with 14.1 kg/ha copper (as CuSO4 solution) and 19 mm diameter soil core samples were taken to a depth of 15 cm after spraying, after single or double disk-harrow incorporation, and after incorporation followed by bedding and row leveling. Cores were systematically located along and across the center two rows of the plots and numbered according to position. Soil cores were weighed, extracted with 0.05 M Na2 EDTA and the extract was analyzed by atomic absorption spectrophotometry. For all samples, cores had a range of copper between 50 and 1000 μg, with a coefficient of variation of 47%. Disk-harrow incorporation decreased the copper found in the center of the harrow path by nearly 50%. Bedding followed by row leveling did not change the tracer content of cores but decreased soil core densities. Disk-harrow incorporation may redistribute a herbicide that is initially applied evenly, producing areas of poor weed control and/or crop damage. This observation was confirmed in a field bioassay using trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) and wheat (Triticum aestivum L. ‘Coker 6815′).