Puccinia coronata infection increased absorption of 14C-glyphosate in quackgrass 3 days after inoculation (DAI) when compared to 0,6,9, or 12 DAI. Days after inoculation had no effect on glyphosate translocation out of treated leaves until 9 to 12 DAI when 89% of the absorbed 14C-glyphosate was retained within the infected leaf. In contrast, only 43% was retained in leaves treated at 0 DAI. In other studies, 14C-glyphosate absorption increased linearly from 21 to 35% 12 to 96h after treatment (HAT). Differences between control and rust-infected plants were not significant. Sixty-three percent of absorbed 14C-glyphosate translocated out of control leaves by 96 HAT, while only 10% translocated out of rust-infected leaves. Additionally, 28 and 4% of the 14C absorbed was recovered from the rhizomes of control and rust-infected plants, respectively.

More than 70% of all 14C-bentazon absorption occurred within 4 h after herbicide application regardless of adjuvant Moisture stress reduced 14C-bentazon absorption by common cocklebur and velvetleaf. Mature (second true leaf) and moisture-stressed leaves of velvetleaf had 50 and 17 μg cm−1 more epicuticular wax (ECW) than did juvenile and unstressed leaves, respectively. Common cocklebur had less 14C in the ECW and lower total 14C in treated mature leaves compared to juvenile leaves. The use of 28% urea ammonium nitrate (UAN) or crop oil concentrate (COC) increased 14C in ECW samples of both plant species, regardless of leaf age or moisture stress. More 14C in the ECW did not always correlate with more 14C in the leaf tissue. Adjuvants increased 14C-bentazon absorption into leaves of plants that had been stressed.

The locoweeds, woolly loco and silky crazyweed, contribute to livestock poisoning in the western United States. Differences in response of these two locoweeds to foliar-applied picloram and metsulfuron was investigated by evaluating differences in herbicide uptake, translocation, and metabolism. Silky crazyweed compared to woolly loco was more than 10 times as sensitive to increasing rates of either herbicide. The two species absorbed 8 to 15%, and 11 to 17% of applied picloram and metsulfuron, respectively. Translocation of picloram and metsulfuron out of treated leaflets of either species was less than 3% of that absorbed after 96 h. Approximately 70 and 100% of the absorbed herbicides remained as picloram and metsulfuron, respectively, in both species. Therefore, differences in picloram or metsulfuron uptake, translocation, and metabolism appear inadequate to account for the differential response to each herbicide by the two locoweed species. Selectivity differences between these locoweed genera to picloram and metsulfuron most likely are due to sensitivity differences at sites of action.

Disappearance of rye cover crop residue and allelochemicals from rye residue were evaluated in a field study at Clayton, NC, in 1992 and 1993. The aerial portion of rye biomass declined linearly over time with 50% of the residue disappearing by 105 d after clipping. Extrapolations indicated all residue would disappear approximately 200 d after rye clipping. Total content of DIBOA and two related compounds, DIBOA-glucoside and BOA, in the residue also was measured over time. Fifty percent of the 0-d content of the compounds disappeared from residue 10 and 12 d after kill for 1992 and 1993, respectively. Extrapolations indicated content reached 0121 and 168 d after kill for 1992 and 1993, respectively. Reported duration of weed suppression by rye cover crops more closely follows disappearance of allelochemicals from rye residue than disappearance of the residue.

Laboratory experiments were conducted to determine the physiological mechanism of tall morningglory resistance to the experimental cotton herbicide DPX-PE350. Tall morningglory, a resistant species, was compared with entireleaf morningglory, a sensitive species, to evaluate inhibition at the site of action, the acetolactate synthase (ALS) enzyme (E.C.4.1.3.18), by DPX-PE350 as well as uptake, translocation, and metabolism of DPX-PE350. No differences were found between species in the concentration required to inhibit the ALS enzyme by 50% (I50), or in uptake and translocation of the herbicide. Tall morningglory metabolized the herbicide more rapidly than did entireleaf morningglory. Tall morningglory contained 3.6 and 1.4 times more metabolites of DPX-PE350 than did entireleaf morningglory 6 and 24 h after treatment, respectively. Tall morningglory produced an O-desmethyl metabolite from the 3,5-dimethoxypyrimidine moiety of DPX PE350 that was not found in entireleaf morningglory. These data suggest that the ability of tall morningglory to more rapidly metabolize DPX-PE350, possibly through the production of the pyrimidinyldesmethyl metabolite, may be the mechanism of resistance to DPX-PE350.

Goosegrass, wide spread throughout the tropics and subtropics, is one of the most noxious weeds known. Recently, biotypes of goosegrass have been found resistant to the dinitroaniline herbicides. An alteration in the structure/composition of a tubulin protein has been postulated as an explanation for the hyperstable microtubules and the resistant phenotype. Our study was initiated to investigate the structure of the alpha (α)-, beta (β)- and gamma (γ)-tubulin related gene sequences in resistant, intermediately resistant, and susceptible biotypes. Heterologous tubulin gene clones were used as probes of restriction endonuclease-digested genomic DNA from each biotype, to determine gene size and copy number and to screen for restriction fragment length polymorphisms. The tubulin genes are organized into gene families. There are three to five α-tubulin genes, four to seven β-tubulin genes, and four to eight γ-tubulin genes. There was no evidence of multiple copies or tandem repeats of any individual gene sequence. Although RFLPs were observed, no significant difference in the banding pattern between the resistant and the susceptible biotypes was found for either α-, β-, or γ-tubulin gene families. Therefore, it is unlikely that the difference between the herbicide-response phenotypes can be attributed to large deletions or insertions in a tubulin gene.

Picloram uptake, translocation, and efficacy were studied using greenhouse-grown Russian knapweed plants. Uptake of 14C-picloram, applied as discrete droplets to adaxial leaf surfaces of well-watered plants, averaged less than 10% of that applied. Most uptake occurred within 30 min of application. Uptake increased linearly with external picloram concentrations from 6.2 to 74.5 mM and was proportional to picloram concentration. Only about 10% of absorbed picloram was translocated out of the treated leaf of well-watered plants within 96 h, with approximately equal acropetal and basipetal translocation. Water stress before, at the time of, and after picloram application did not affect picloram uptake, but reduced total translocation and increased the relative amount translocated basipetally. Water stress also reduced picloram efficacy. Although Silwett L-77 increased picloram uptake into Russian knapweed leaves, it did not increase efficacy under water stress conditions.

Studies determined the effect of sodium bisulfate (NaHSO4) on the phytotoxicity, retention, uptake, and translocation of the suspension concentrate formulation of imazamethabenz in wild oats. NaHSO4 completely solubilized this herbicide formulation when added in an equimolar concentration and did not affect herbicidal activity at NaHSO4 concentrations below 28 mM when used in a carrier volume of 100 L ha−1. NaHSO4 improved phytotoxicity at a carrier volume of 33 L ha−1. NaHSO4 at 28 mM increased the efficacy of imazamethabenz applied as individual drops on growth chamber-grown wild oats. The herbicide retention was not changed but foliar absorption and acropetal translocation were increased, while basipetal translocation was decreased. The increase in phytotoxicity of imazamethabenz with NaHSO4 was related to absorption of imazamethabenz by the target plants.

Laboratory and greenhouse studies were conducted to determine the mode of action of soil- and foliar-applied UCC-C4243. Experiments demonstrated that UCC-C4243 required light for phytotoxicity, phytotoxic symptoms were similar to inhibitors of porphyrin synthesis such as acifluorfen, and UCC-C4243 potently inhibited protoporphyrinogen oxidase. Germination and emergence of field pennycress and lentil in the dark were not affected by soil-incorporated UCC-C4243 at rates more than 10 times greater than like treatments that killed all plants in the light. Soil-incorporated UCC-C4243 required light for activity and killed seedlings within 1 d after emergence; sublethal doses caused desiccation, veinal necrosis, and leaf deformation. Field pennycress and lentil were susceptible to soil-incorporated UCC-C4243 and acifluorfen in the light, but were 5 to 93 times less sensitive to the herbicides in the dark. Wheat was not affected by either herbicide in the light or dark. Injury symptoms from UCC-C4243 applied POST to redroot pigweed were similar to symptoms from diphenyl ether and bipyridinium herbicides: rapid, light-dependent chlorophyll bleaching, desiccation, and necrosis. UCC-C4243, acifluorfen-methyl, and acifluorfen acid caused light- and concentration-dependent chlorophyll bleaching and electrolyte leakage from cucumber leaf disks (I50 = 1.0, 1.8, and 4.3 μM, respectively). An inhibitor of the porphyrin synthesis pathway, 4,6-dioxoheptanoic acid, almost completely inhibited herbicide-induced electrolyte leakage. δ-Aminolevulinic acid, a tetrapyrrole precursor and stimulator of the porphyrin synthesis pathway, caused synergistic effects with each herbicide. Protoporphyrinogen oxidase from barley etioplast preparations was inhibited 50% by 40 nM UCC-C4243. Barley leaf sections treated with 100 μM UCC-C4243 accumulated protoporphyrin IX in vivo to levels > 75 times non-treated controls. These data indicate the light-requiring herbicide activity of UCC-C4243, like acifluorfen, is due to inhibition of protoporphyrinogen oxidase.

Thirteen biotypes of rigid ryegrass were screened for trifluralin resistance. From these, the two most resistant biotypes, SLR 31 and SLR 10, were chosen for further studies involving exposure to other dinitroanilines, mitosis-inhibiting herbicides and 14C-trifluralin. SLR 31, and SLR 10 exhibited an approximate 10-fold reduced sensitivity to trifluralin in comparison to susceptible biotypes. Resistance to five other dinitroaniline herbicides was observed, with reduced sensitivity varying from 32-fold for ethalfluralin to 2.5-fold for isopropalin. The resistance in rigid ryegrass to other herbicides and drugs that affect mitosis were tested. Resistance comparable to that of trifluralin was recorded for the herbicides terbutol and DCPA, while low levels of cross-resistance to amiprophosmethyl was present. Trifluralin affected mitotic indices at a much lower level in the susceptible biotypes than in the resistant biotypes. No differences in the uptake and translocation of 14C-trifluralin were observed between resistant and susceptible biotypes. Most of the 14C detected in the plant material was in the root tissue. A small level of 14C was detected in the seeds, and no substantial increases were noted in coleoptile tissue. The resistance spectra in SLR 31 and SLR 10 were phenotypically similar to those occurring in an intermediate trifluralin-resistant goosegrass and trifluralin-resistant green foxtail.

Russian thistle plant movement and seed dispersal were studied in 1991 and 1992 by placing Russian thistle plants in the center of wheat fields in eastern Washington. Three adjacent site treatments, with 24 plants on each site, were used each year; wheat stubble, summerfallow planted to winter wheat, and a “stationary” site. Plants in the “stationary” site were anchored to the ground to prevent tumbling. Plants in the stubble and summerfallow sites were allowed to tumble naturally. Individual plant movement was monitored and recorded weekly by satellite global positioning systems technology. Average estimated seed number per plant at the beginning of the experiment was 57,400 in 1991 and 66,000 in 1992. The direction plants moved correlated highly with wind direction. Some plants moved a maximum distance of 4069 m in 6 wks, while other plants moved only 60 m because of variable winds and being compressed by snow or frozen into wheat stubble. Average percentage seed loss in 1991 and 1992 for stationary plants was 15 and 26%, and for tumbling plants was 48 and 66%, respectively.

A replacement series experiment was conducted in the field to quantify the interaction between Canada thistle and buckwheat, and to determine if allelopathy was the mechanism of interference. Plant biomass data indicated that buckwheat only responded to intraspecific competition and not interspecific competition from Canada thistle. Canada thistle responded to interspecific competition from buckwheat plants but not to intraspecific competition. The absence of one of the interspecific competition components (no measurable effect of Canada thistle density on buckwheat) indicates that the value for niche differentiation is probably greater than unity, signifying that there is no mutual antagonism between buckwheat and Canada thistle. Because allelopathy is one mechanism that causes mutual inhibition, data do not support the hypothesis that buckwheat interferes with Canada thistle by allelopathy.

Wild, ball, and dog mustard growth and development were investigated by mathematical growth analysis in a greenhouse experiment. Plant height and total plant biomass over the growth period followed the trend wild mustard > ball mustard > dog mustard. Dog mustard plants had lower leaf areas than either wild or ball mustard. In a replacement series experiment, wild mustard was more competitive than either ball or dog mustard, and ball mustard was more competitive than dog mustard.

Studies were conducted in conventional and no-tillage corn in 1990, 1991, and 1992 at Wooster, OH, to measure corn yield and velvetleaf seed production in response to density of early and late emerging velvetleaf, and to estimate economic thresholds. The percent reduction in corn yield fit a hyperbolic function over velvetleaf densities from 1 to 30 plants m2. The percent yield loss and velvetleaf seed production were higher in a warm, wet year (1990) than in a dry (1991) or cold, wet year (1992). The percent corn yield reduction was generally greater in no-tillage than in conventional tillage and from early rather than late emerging velvetleaf. Maximum velvetleaf seed production ranged from about 18,000 seeds m2 for early emerging weeds in no-tillage in 1990 to 100 seeds m2 for late emerging weeds in no-tillage in 1992. The single year economic threshold for early emerging velvetleaf ranged from 0.40 to 14.0 velvetleaf m2 in conventional tillage and 0.13 to 3.13 in no-tillage. Economic thresholds that were predicted using yield goal information deviated from actual thresholds (using actual yields) for a given year by −43 to 30%. Single year economic thresholds were similar in both tillage treatments, but their value for management decisions is questionable due to variation among growing seasons and weed seed production from subthreshold populations.

Field studies were conducted to determine the effects of cultivated oats planting pattern on early canopy shape and growth of cultivated oats and wild oats, in part to test the assumption of radial plant canopy expansion on which previous theoretical models of crop-weed interference models have been based. Cultivated oats density was kept constant as the pattern rectangularity was varied, and single wild oats plants were centered within each pattern. Individual plant canopies, photographed from above 31 days after emergence (DAE), were radial for wild oats in all crop planting patterns and for cultivated oats planted in triangular and square planting patterns. Canopy radius perpendicular to the crop row axis in rectangular patterns was similar to canopy radius along the same cardinal axis in equidistant patterns, but was reduced along the crop row axis, resulting in a rectangular canopy shape and decreased canopy area in rectangular compared to equidistant patterns. Cultivated oats dry weight and leaf area at crop flowering (64 DAE) also decreased with increasing rectangularity of crop planting pattern. Reductions in cultivated oats growth in rectangular patterns were associated with earlier intraspecific interference and delayed crop canopy closure in rectangular compared to equidistant patterns. Wild oats leaf area and tiller number 64 DAE decreased with more equidistant crop planting patterns, consistent with reduced canopy area 31 DAE and earlier crop canopy closure in equidistant patterns. The data suggest that individual oats canopy expansion during early growth is essentially radial and also support previous theoretical predictions of crop planting pattern effects on weed suppression.

The movement of sulfonylurea herbicide-resistant (R) kochia pollen was investigated in a spring barley field near Moscow, ID, using a Nelder plot design in 1991 and 1992. Each 61 m diameter plot had 16 rays spaced 22.5° apart and contained 211 kochia plants. There were 12 susceptible (S) plants and one R plant along each ray. The R and S plants were 1.5 m and 3.0 to 30.5 m from the center of the plot, respectively. Wind direction and speed in the 16 vectors, air and soil temperature, and rainfall were monitored continuously. Mature kochia seed was collected from individual plants, planted in the greenhouse, and sprayed with chlorsulfuron to test for resistant F1 progeny. Results from the 2-yr study showed outcrossing of R pollen onto S plants at rates up to 13.1% per plant 1.5 m from the R plants and declining to 1.4% per plant or less 29 m from the R plants. At least 35% of the total R x S crosses occurred in the direction of prevailing southeastward winds. Predicted percentages of R x S crosses per plant ranged from 0.16 to 1.29 at 1.5 m, and 0.00 to 0.06% at 29 m. Thus, resistant kochia pollen can spread the sulfonylurea-resistant trait at least 30 m during each growing season.

Wild poinsettia germination was at least 95% when exposed to solution pH of 2.5 to 10 or solution osmotic potential of up to −0.8 MPa. No germination was observed at pH 12 or osmotic potential of −1.5 MPa. Light was not required for germination. Seed germinated at temperatures ranging from 20 to 40 C with maximum germination of 97% at 35 C. Wild poinsettia had the highest emergence (80%) from seed placed 2 to 4 cm below the soil surface, while 20% of the seed planted at 14 cm emerged. Wild poinsettia germinated over a wide range of conditions, which explains why it is becoming an increasingly serious problem.

Experiments conducted in North Carolina and Virginia compared weed control, peanut yield, and net returns with systems using imazethapyr applied at various times and the regional standard treatment of paraquat applied at the ground-cracking stage of peanut (GC) followed by acifluorfen plus bentazon applied POST. Imazethapyr was applied PPI, PRE, GC, or POST at 70 g ae ha−1. Imazethapyr also was applied sequentially PPI plus GC, PPI plus POST, and PRE plus POST at 35 + 35 and 70 + 70 g ha−1. Late-season control of common ragweed and a mixture of entireleaf, ivyleaf, and pitted morningglories by the standard treatment ranged from 85 to 100%. Spurred anoda was controlled 80%, and common lambsquarters and prickly sida were controlled completely. Control of common lambsquarters, prickly sida, and morningglory by imazethapyr applied one or more times was similar to control by the standard. Control by imazethapyr exceeded that by the standard only for spurred anoda. The most effective time for applying imazethapyr varied by species and locations. Imazethapyr was equally effective on common lambsquarters and spurred anoda when applied PPI, PRE, or GC. Prickly sida and morningglory were controlled best when imazethapyr was applied PPI or PRE and GC, respectively. Common ragweed was controlled poorly with single applications of imazethapyr. Applying imazethapyr sequentially improved consistency of control across the range of species. In most cases, imazethapyr applied sequentially at 35 + 35 g ha−1 controlled all weeds as well as or better than when applied once at 70 g ha−1. Overall, imazethapyr at the registered rate of 70 g ha−1 was most effective when applied PPI at 35 g ha−1 followed by 35 g ha−1 at GC. Except for common ragweed, weed control with this treatment was similar to that by the standard. Peanut yield and net returns with this treatment were similar to those with the standard at three of four locations.

Italian ryegrass interference in broccoli was measured in field experiments and the data fit a rectangular hyperbolic competition model. The model predicted 58% of broccoli yield loss related to Italian ryegrass density when pooled over three y. An economic threshold value of 4.9 Italian ryegrass plants m−1 of crop row was determined to be the density required to cause a 3.6% yield loss, equal to postemergence weed control costs. Italian ryegrass densities of 600 to 1000 plants m1 of broccoli row caused 100% yield loss.

Experiments were conducted at Lexington and Quicksand, KY, in 1989 and 1990 to determine the effect of preplant incorporated clomazone on weed control, crop injury, and yield of broccoli, cauliflower, green cabbage, red cabbage, and pak choi. Oxyfluorfen and trifluralin were included as standard treatments. Greater than 80% weed control was observed at both locations with 0.8 kg ai ha−1 clomazone, with the exception of smooth pigweed at Lexington. All clomazone treatments caused crop injury 2 WAT. Injury was still evident 8 WAT at clomazone rates of 1.7 and 3.4 kg ha−1, but was minor with rates of 0.8 kg ha−1 or less on broccoli, cauliflower, and red cabbage. Injury was less at Quicksand in both years and seasons over all clomazone rates, most likely due to higher soil organic matter content Yields of broccoli and cauliflower treated with 0.8 kg ha−1 clomazone were similar to yields with oxyfluorfen on a 3% organic matter soil at Lexington in 1989. However, clomazone at 0.8 kg ha−1 at Lexington reduced 1989 spring yields of green cabbage, red cabbage, and pak choi and 1990 spring yields of all cole crops as compared to oxyfluorfen. Clomazone at 0.8 kg ha−1 at Quicksand reduced yield of green cabbage and pak choi in spring 1990 only on a 5.2% organic matter soil. Fall yields of broccoli and cauliflower in both years were not reduced by clomazone at 0.8 kg ha−1 or less at either location. Our studies indicated potential for utilization of clomazone on cole crops in higher organic matter soils, especially if some early season crop injury and occasional yield loss can be tolerated.