In laboratory studies, absorption and translocation of glyphosate in velvetleaf was dependent on the concentration of both glyphosate and surfactant in the applied droplet(s). At equivalent total quantities applied, absorption by leaves was greater with concentrated droplets than with more dilute droplets in greater number. Increasing the surfactant concentration increased the relative absorption of dilute glyphosate droplets but did not overcome the negative effect of higher droplet number on total amount absorbed. More glyphosate was translocated from concentrated versus the most dilute droplets at equivalent total quantity applied. Conversely, more absorbed glyphosate from dilute droplet applications remained in the treated leaves. About 90% of translocated 14C glyphosate from the most concentrated drops was distributed evenly between roots and shoot tissue above the treated leaf and the remainder in shoot tissue below the treated leaf. As droplet number increased and surfactant concentration decreased, the relative amount of glyphosate distributed in roots declined in relation to shoots.

Jointed goatgrass (Aegilops cylindrica Host. ♯3 AEGCY) has a quantitative requirement for vernalization in order to flower. In greenhouse and field studies, increasing periods of vernalization progressively reduced the number of days needed for plants to mature following transfer from the cold treatment to favorable growing conditions. Plants that had been vernalized at 3 ± 2 C for 8 weeks as imbibed seed took 120 days to flower following transfer to the greenhouse. Unvernalized controls flowered 197 to 222 days after planting in the greenhouse. Lengthening periods of vernalization from 2 to 8 weeks increased the number of seedheads per plant and dry weight per seedhead. Vernalized plants partitioned more dry matter into seedheads than unvernalized controls. The ratio of seedhead dry weight to vegetative shoot dry weight increased with duration of vernalization, even though vernalization did not alter total shoot dry-matter production. In field studies, plants that were established in the fall flowered sooner and more synchronously after resumption of growth in the spring than those that were planted in the spring and flowered in the summer. Plants seeded after May failed to flower in the same summer.

Common cocklebur and sicklepod are troublesome weeds in soybean in the southern United States. A field experiment was conducted from 1991 through 1995 to determine (1) the influence of tillage (no-till and tilled after initial seed deposition) and intraspecific and interspecific interference on seed production potential, emergence pattern, and soil seedbank of common cocklebur and sicklepod, and (2) the dominant species after introduction into a weed-free field. Under intraspecific interference, 1,430 and 1,392 common cocklebur achenes m−2 and 1,827 and 5,435 sicklepod seed m−2 were deposited to the seedbank after 1 and 2 yr of seed production, respectively. For both species, approximately 11% of the initial seedbank emerged under tilled conditions the first year after deposition. Under no-till conditions, only 0.7% of common cocklebur and 1.6% of sicklepod emerged. The second year after deposition, common cocklebur emergence in no-till decreased to 0.25% of the initial seedbank, while sicklepod increased to 8% of the initial seedbank and remained higher than in tilled plots. Under tilled conditions, common cocklebur became the dominant species, and sicklepod became dominant under no-till conditions. Seedbank depletion was greater for both species under tillage. Three years after initial seed deposition, sicklepod seed was 100% viable but common cocklebur achenes were not viable. Under no-till conditions, common cocklebur was depleted in the seedbank but sicklepod was not. Thus, sicklepod poses a greater long-term weed problem than common cocklebur, especially under no-till conditions.

The allelopathic potential of germinating seed of velvetleaf (Abutilon theophrasti Medic.) was established in a laboratory study with germinating seed of turnip (Brassica rapa L. ‘Purple Top’). Two-day-old velvetleaf seedlings inhibited radicle growth of germinating turnip by 17% at 24 h after both were placed in the same petri plate. Alcoholic and aqueous extracts of velvetleaf seed, however, seriously inhibited germination of turnip seed. The extract of velvetleaf seed was fractionated by ion-exchange chromatography into acid, basic, and neutral fractions. Most of the allelopathic activity was located in the acid fraction, which includes the plant phenolics. The basic fraction (amino acids) inhibited radicle growth of turnip by 18% after 48 h. The amino acid profile of velvetleaf seed extracts is presented and discussed, because this fraction has previously been implicated as the allelopathic fraction.

Two 1-yr field studies were conducted to evaluate preemergence and preplant incorporated treatments of alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide], EPTC (S-ethyl dipropylthiocarbamate), fluridone {1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone}, and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] for the control of yellow nutsedge (Cyperus esculentus L.). Preemergence applications of metolachlor at 2.24, 3.36, and 4.48 kg/ha gave the best control with no significant differences observed among rates of metolachlor. Depth of herbicide incorporation did not result in any significant increase in control. Preplant incorporation of metolachlor at all rates, or fluridone at 0.56 and 0.84 kg/ha provided seasonal control of yellow nutsedge. Translocation of 14C-metolachlor applied to the root or shoot of yellow nutsedge was primarily acropetal with some limited basipetal movement. Application of 14C-metolachlor to the root or shoot of purple nutsedge (Cyperus rotundus L.) resulted in mostly acropetal translocation with little movement of 14C from the treatment site.

Experiments were conducted at Urbana, IL, in 1983 and 1984 to determine the effect of adjuvants, adjuvant rate, and carrier volume on postemergence weed control with bentazon [3-(1-methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)-one 2,2-dioxide] and sethoxydim {2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one} in soybeans [Glycine max (L.) Merr. ‘Williams’]. Little difference was observed between a petroleum oil:emulsifier blend (83:17, v/v) (POC) and a soybean oil: emulsifier blend (85:15, v/v) (SBOC) in enhancing control of velvetleaf (Abutilon theophrasti Medik. # ABUTH) with 0.6 or 1.1 kg ai/ha bentazon. Application of bentazon in a carrier volume of 94 L/ha provided velvetleaf control equivalent to that applied in 187 L/ha. Increasing the adjuvant rate from 2.3 to 11.7 L/ha increased visible soybean injury but had no effect on velvetleaf control with bentazon. Control of giant foxtail (Setaria faberi Herrm. # SETFA) with 0.1 kg ai/ha sethoxydim was enhanced more by POC than by SBOC. Phytotoxicity of sethoxydim was not altered by changes in carrier volume or adjuvant rate under conditions of adequate soil moisture in 1983. Under limiting soil moisture in 1984, giant foxtail control with sethoxydim increased slightly when the adjuvant rate was increased from 4.6 to 11.7 L/ha, and carrier volume was increased from 47 to 187 L/ha.

Field experiments were conducted at two locations in Georgia to evaluate wild radish control and soft red winter wheat tolerance of herbicides applied February 1 (one- to five-tiller stage) or March 1 (three- to seven-tiller stage). Bromoxynil controlled wild radish with no wheat grain or forage yield reductions in any experiment. Thiameturon controlled wild radish when applied at rates >0.02 kg/ha on March 1. Metribuzin, dimethylamine salt of 2,4-D, and dimethylamine salt of MCPA provided late-season control of wild radish. February 1 treatments of metribuzin reduced wheat stands at Plains. The difference was attributed to environmental conditions, wheat tiller number at application, and possibly to differences in soil fertility at planting. Metribuzin, thiameturon, dimethylamine salt of dicamba, MCPA, and 2,4-D reduced wheat forage yield at Tifton. Dicamba did not control wild radish and reduced grain yield when applied at a rate of 0.3 kg ai/ha on March 1.

Greenhouse experiments in central Texas assessed the relative importance of above- and belowground interactions of semidwarf Mit wheat and Marshall ryegrass during vegetative growth. One experiment used partitions to compare the effect of no (controls), aboveground only, belowground only, and full interaction for 75 d after planting (DAP) one wheat and nine ryegrass plants in soil volumes of 90, 950, and 3,800 ml. The results with the different soil volumes were similar. Wheat growth in the aboveground interaction only did not differ from controls. However, the full or belowground only interaction of wheat with ryegrass reduced wheat height, leaf number, tillering, leaf area, percent total nonstructural carbohydrates in shoot, and dry weights of leaves, stems, and roots 45 and 75 DAP compared to controls. Wheat in full and belowground interaction only did not differ from one another in growth. A replacement series experiment of 56 d also showed that the competitive advantage of ryegrass was relatively greater in root than in shoot growth. No allelopathic response of wheat to ryegrass occurred. While the tallness of the semidwarf wheat minimized aboveground interference by ryegrass, the root growth of the thinner and more fibrous roots of ryegrass greatly enhanced its belowground competitiveness.

The purpose of the paper is to permit the calculation of prospective costs before starting the biological control of a weed. The cost of the Canadian program from its start in 1950 to the end of 1976 was tabulated in scientist-years by target weed, agent, and the type of study done. This information and international data on the number of agents required to control a weed were used to calculate expected costs. A complete biological control program is likely to cost 18.8–23.7 scientist-years or currently $1.2 to 1.5 million and thus is only economic for use against major weeds. However, it is often possible to capitalize on studies done elsewhere, and under these circumstances biological control may be economic for solving more minor weed problems.

Annual bluegrass (Poa annua L.) was controlled in bermudagrass [Cynodon dactylon (L.) Pers.] turf when ethofumesate [(±)-2-ethoxy-2,3-dihydro-3,3-dimethyl-5-benzofuranyl methanesulfonate] was applied at 1.1 kg/ha in October and again in November, but not when applied in February and March. A single, October ethofumesate treatment, did not affect spring growth of nonoverseeded bermudagrass, but the growth of selected cultivars was delayed slightly from October + November treatment. Turf growth was severely delayed for all bermudagrass cultivars when ethofumesate treatments were delayed until February and March. The October and November ethofumesate treatments reduced the quality of overseeded perennial ryegrass [Lolium perenne (L.) ‘Medalist VI’] slightly, but did not injure it permanently and the turf recovered fully. The transition from ryegrass back to bermudagrass turf was good in the spring regardless of ethofumesate treatment. Even though turf cover was excellent throughout the transition period, bermudagrass cover during June and July was less when ethofumesate was applied in October than when application was delayed until after bermudagrass became dormant.

The effects of 8-, 12-, and 16-h photoperiods on the growth and reproductive biology of purple nutsedge (Cyperus rotundus L.) were investigated over a 3-month period. Plants under the 8-h photoperiod produced the least total, shoot, root, rhizome, and tuber dry weight. Flowering occurred only at the 12-h photoperiod; thus, purple nutsedge is intermediate in flowering response to daylength. Although the total dry weight varied with photoperiod, the relative amount of dry weight (as percent of total weight) partitioned to shoots, roots, rhizomes, and tubers was constant.

Shattercane seed survival during soil burial was highly dependent on attached glumes. Shattercane seed tightly enclosed by glumes averaged 5 and 53% germination after 4 mo of winter burial in 1988 and 1989, respectively. Shattercane caryopses (seed without glumes) survival averaged 0.5 and 17% compared to 17 and 41% when treated with seed fungicides in 1988 and 1989, respectively. Shattercane caryopses tannin and lignin contents were higher than for cultivated sorghum caryopses. Glume tannin was four times higher and glume lignin was five times higher than in the caryopsis. Shattercane seed survival was positively correlated with glume tightness, caryopsis lignin, and glume tannin. Tannin and lignin appear to function as barriers to microbial invasion resulting in higher shattercane seed resistance to overwinter demise. Attempts to isolate fungal inhibitors from shattercane seed were unsuccessful.

Systems of weed control composed of (a) herbicides only, (b) herbicides plus cultivation, or (c) cultivation only were devised. The most effective systems, involving herbicide sequences plus one “non-dirting” layby cultivation, controlled twelve troublesome species of weeds with acceptable yields of peanuts (Arachis hypogaea L.). On Greenville sandy clay loam, a particularly effective and economical system of weed control consisted of (a) N-butyl-N-ethyl-α,α,α-trifluoro-2,6-dinitro-p-toluidine (benefin) used as a preplanting incorporated treatment, then (b) S-propyl dipropylthiocarbamate (vernolate) either incorporated into the soil before planting or injected at planting, and finally, (c) a layby cultivation 5 to 6 weeks after planting. A similarly effective system on Tifton loamy sand involved vernolate injected at planting, 2-sec-butyl-4,6-dinitrophenol (dinoseb) at “cracking”, and 4-(2,4-dichlorophenoxy)butyric acid (2,4-DB) as a postemergence treatment followed by the layby cultivation. The “non-dirting” layby cultivation, averaged over both years and herbicide sequences, significantly increased yields of peanuts on both soil types. None of the systems of weed control caused consistent differences in market grade, average weight per seed, germination, or organoleptic quality of the peanuts.

Full-season interference from unicorn-plant [Proboscidea louisianica (Mill.) Thell. # PROLO] at densities ranging from 0 to 32 weeds/10 m row was measured on weed and cotton (Gossypium hirsutum L. ‘Westburn M′) parameters in three Oklahoma environments. Linear regression models using log10 transformations were superior to linear and comparable to quadratic models in estimating the impact of inter- and intraspecific competition. Weed measurements on an individual plant basis were not reliable indicators of interference. Weed dry biomass, ground cover, and seed capsule production/plot generally increased with increasing weed densities. In the three environments, densities of two, four, and eight weeds/10 m row initially reduced cotton plant height. Maximum height reductions averaged 20, 28, and 43% in the three environments but did not detrimentally affect the mechanical harvest of cotton. As unicorn-plant density (expressed in log10 units) doubled (within a range of 1 to 32 plants/10 m of row), lint yield reductions ranged from 84 to 146 kg/ha. Maximum lint yield losses averaged 59, 65, and 74% in the three environments. Transforming lint yield/plot to a percentage of that from weed-free plots resulted in a single quadratic equation applicable over environments. Unicorn-plant at the highest weed density reduced fiber fineness, uniformity, and length, but not fiber strength.

Experiments were conducted to investigate the effects of shading on and location of the mother plant on germination and hormone content of Palmer amaranth seed. Increasing the shading from 0 to 87% decreased germination of fresh, viable seeds in dark from 25 to 12%. Abscisic acid (ABA) content of seeds from plants in 0% shade increased from 13.3 ng g−1 dry seed to 19.1 ng g−1 dry seed with 87% shade. Shading of the mother plant did not influence the 1,000-seed weight of Palmer amaranth. Seeds that matured in the top and middle third of the mother plant had 67 to 78% greater germination than those that matured in the bottom third of the mother plant. Endogenous gibberellic acid (GA) content of seeds did not differ between locations on the mother plant; however, the ABA content of seeds produced on the bottom third of the plant was 46 and 59% higher than the ABA content of seeds produced at the middle and top third of the plant, respectively. Endogenous ABA or GA content of seeds and 1,000-seed weight had no relationship with seed germination over and above the treatment effects. This research suggests that shading and plant location can influence germination of fresh, viable seeds of Palmer amaranth, which will be a dormancy strategy for seed dispersal over time.

Rhizome initiation and growth is a contributing factor to the problem of controlling Johnsongrass (Sorgum halepense (L.), Pers.). Control methods by intensive cultivation or chemical applications are directed primarily toward the destruction of these underground structures. As described by Fletchall et al. rhizomes are fleshy, segmented underground stems having scales at the nodes. Their development is rapid; they function as organs for carbohydrate storage, and as reproductive structures which produce vegetative shoots from axillary and terminal buds. Information relative to development of rhizomes and their response to various treatments is reported; this is essential to understanding and evaluating methods of control.

Greenhouse studies were conducted to determine purple and yellow nutsedge response to selective placement of a 5-cm layer of pyrithiobac sodium-treated soil above, below, or above and below nutsedge tubers. Pyrithiobac sodium at 36 or 72 g ae ha−1 applied postemergence as foliar, soil, or foliar and soil treatments also was evaluated. Pyrithiobac sodium applied above, below, or above and below nutsedge tubers reduced yellow and purple nutsedge shoot number, shoot regrowth, and root-tuber dry weight at least 90%. Foliar-only treatment of pyrithiobac sodium was less effective at reducing emerged purple and yellow nutsedge numbers than application to soil only or to foliage and soil. Best reduction in yellow and purple nutsedge growth with pyrithiobac sodium was obtained with soil-incorporated treatments.

The witchweeds, members of the genus Striga, are noxious and persistent pests in farmers' fields and serious constraints to crop productivity throughout Africa, India, and Southeast Asia. Among the primary hosts for Striga are the major cereals (maize, sorghum, rice, and millet) and grain legumes (cowpea) that are important food staples worldwide. The negative impact of parasitic plants on crop productivity increases globally each year, and their potential for affecting domestic agriculture looms larger as the movement of seed resources expands on a global scale. At the present time there is a limited understanding of how Striga and other parasitic plants select a suitable host and overcome the innate defense responses of the host in order to complete their life-cycle. In the grasses most reported resistance to Striga appears to be polygenic with a large genotype by environment interaction. In contrast, resistance to S. gesnerioides in cowpea is conferred by single dominant genes functioning in a race-specific manner suggesting that a gene-for-gene mechanism similar to effector-triggered immunity (ETI) described in other host–pathogen interactions is likely operating in these parasite-host associations. A hallmark of ETI is the direct or indirect recognition of parasite-derived avirulence (Avr) factors and other effectors that interfere with plant innate immunity by host sensors (or R proteins) leading to activation of defense responses. The recent cloning and functional characterization of a race-specific R gene from cowpea encoding a canonical coiled-coil (CC)-nucleotide binding site (NBS)-leucine-rich repeat (LRR) type R-protein opens the door for further exploration of the mechanism of host resistance and provides a focal point for studies aimed at uncovering the molecular and genetic factors underlying parasite virulence and host selection. The potential for the development of novel strategies for parasite control and eradication based on parasite virulence factors is discussed.

Palmer amaranth, a dioecious summer annual forb, originating in Sonoran desert washes, compromises crop yields in much of the southern United States and its range is expanding northward. Appropriate tactics for managing this weed proactively in the Upper Midwest will depend on characterizing its damage niche, the geographic range in which it can reduce crop yields. We implemented a common garden study in 2011 and 2012, planting eight accessions of Palmer amaranth from the southern and midwestern United States, into soybean crops in southern, central, and northern Illinois, at a population density of 8 plants m−2 with a biocontainment protocol. Once Palmer amaranth plants initiated flowering, they were removed and burned. Weed survival, flowering, and weed biomass were measured, in addition to soybean yield and weather data. Analyses indicated that Palmer amaranth's damage niche in Illinois soybean was independent of weed genotype or maternal environment. Despite competing only briefly, Palmer amaranth reduced soybean yields in all site–years, indicating its damage niche in Illinois, and much of the Midwest, is limited primarily by seed immigration rate. These results highlight the urgent need for weed managers to learn Palmer amaranth identification, prevent seed introduction, and maintain a policy of zero seed return.

The environmental conditions under which parental plants are reared can affect the seed characteristics of the progeny. The variation originating from such maternal effects has rarely been incorporated into models of seed germination. Here, using Palmer amaranth (Amaranthus palmeri S. Watson), we examined the effects of water stress during the growth of parental plants on the progeny seed characteristics, including weight, size, final germination, and parameters of a hydrotime germination model. We grew two populations (from California and Kansas) under continuous water-deficit or well-watered conditions. In both A. palmeri populations, progeny seeds originating from water-stressed plants were heavier and larger than those from well-watered plants. Plants exposed to water stress also produced seeds that were ~30% less dormant than seeds from control plants. To test whether the maternal environment affects the parameters of a hydrotime model, progeny seeds were subject to five water potentials (0, −0.2, −0.4, −0.6, and −0.8 MPa) and incubated at 20 and 30 C; germination was monitored daily. The estimated median base water potential (Ψb(50)), that is, the water potential at which 50% of seeds cannot germinate, was consistently lower for seeds from water-stressed plants than for seeds from well-watered plants. Our results showed that A. palmeri plants experiencing drought during their growth produce seeds that are less dormant and can germinate from drier conditions—a maternal response that seems to be adaptive. These findings also call for development of germination models that incorporate the environmental conditions of both the current and past seasons to better describe the variability in germination of weed seeds.