Nonradioactive and 14C-organic arsenical herbicides were applied to foliage of young cotton (Gossypium hirsutum L., var. Acala SJ-1) at various temperatures. An application of 3.36 kg/ha of monosodium methanearsonate (MSMA), with and without 0.5% surfactant, severely injured plants exposed at 13 C. Similar injury resulted when plants were exposed and treated at 20 C with MSMA containing 0.5% surfactant. Applications of MSMA did not injure plants exposed at 31 C. Disodium methanearsonate (DSMA) was less injurious to cotton than MSMA. Only slight to moderate injury resulted when plants were exposed and treated at 13 C with 3.36 kg/ha of DSMA containing 0.5% surfactant. Applications at higher temperatures did not injure cotton. Cotyledons absorbed 14C-methanearsonic acid (MAA) and 14C-MSMA more readily than 14C-DSMA. Contact injury from these herbicides was slight at 13 C but was pronounced at 20 and 29 C. Generally, translocation of herbicides into developing true leaves was slight. The exception resulted from applications of 14C-MSMA to plants at 13 C when contact injury was negligible.

Field studies on the persistence of simazine [2-chloro-4,6-bis(ethylamino)-s-triazine] showed less persistence under no-tillage corn (Zea mays L. ‘Pioneer 3369A’) culture than conventionally tilled corn. Bioassay studies conducted in the greenhouse using oats (Avena sativa L. ‘Compact’) as indicator crop showed the lowest level of simazine remaining in soils of low pH (5.4 and lower). Persistence of simazine in the soil increased with increasing soil pH.

Partially purified glutathione-s-transferase (GST) isolated from corn (Zea mays L.) seedlings was about 14-fold higher in specific activity than GST isolated from giant foxtail (Setaria faberi Herrm. # SETFA) seedlings. Greater amounts of GST activity were present in leaf tissue than in stem tissue. Four-leaf giant foxtail seedlings contained about threefold more reduced glutathione per gram fresh weight in the leaves than one- to two-leaf giant foxtail seedlings. When atrazine and reduced glutathione were used as substrates, tridiphane [2-(3,5-dichlorophenyl)-2-(2,2,2-trichloroethyl)oxirane] inhibited isolated GST from corn with an I50 of about 5 μM and from giant foxtail with an I50 slightly lower. Tridiphane inhibited the metabolism of atrazine [6-chloro-N-ethyl-N′-(1-methylethyl)-1,3,5-triazine-2,4-diamine] to water-soluble metabolites in giant foxtail leaves but not in corn leaves. Unmetabolized atrazine levels tended to be greater in giant foxtail seedlings treated with tridiphane plus atrazine than in plants treated with atrazine alone. Tridiphane applied 12 h before atrazine increased the uptake of atrazine in both corn and five-leaf giant foxtail seedlings. The amount of atrazine metabolism to water-soluble metabolites increased in corn leaves but remained constant in giant foxtail leaves when atrazine uptake was increased. Tridiphane and atrazine combinations impaired net photosynthetic rates more than atrazine alone in giant foxtail seedlings but did not increase chlorophyll fluorescence. There was no decrease in net photosynthetic rate 12 h following applications of atrazine or tridiphane plus atrazine on corn leaves. Tridiphane did not decrease photosynthesis or increase chlorophyll fluorescence in either species when applied alone.

High Avena fatua control costs have caused some Hordeum vulgare growers to use reduced rates of herbicides without fully understanding the consequences. Field studies near Moscow and Genesee, ID, were conducted to determine the effect of A. fatua density and PP-604 rate on A. fatua seed production in H. vulgare and on H. vulgare yield. PP-604 treatments were 25, 50, 100, 150, and 200 (minimum labeled rate) g ha−1, and five A. fatua densities ranged from 0 to 386 plants m−2. Visual A. fatua control was greater than 85% with 100 g ha−1 PP-604 at all locations. Data from 1998 were used to construct nonlinear exponential decay and parabolic models to describe the effect of reduced herbicide rates on viable A. fatua seed production and relative H. vulgare grain yield, respectively. At A. fatua densities of 42 to 138 plants m−2, 46 to 71% of the minimum labeled rate of PP-604 reduced seed production 95%. However, an estimated 140 to 235 seeds m−2 were produced at this level of control, which may not ensure a decline in the A. fatua population over the long-term. Hordeum vulgare grain yield was maximum when 70 to 85% of the minimum labeled rate was applied to A. fatua densities of 42 to 138 plants m−2. A higher rate of PP-604 likely will be required to ensure a decline in A. fatua populations over the long-term than needed to obtain maximum H. vulgare grain yield in a single growing season.

The hypothesis that plants treated with 2,4-dichlorophenoxyacetic acid (2,4-D) die as a consequence of abnormal growth has been examined. Cocklebur (Xanthium sp) plants spot treated on one leaf with 2,4-D show three phases of growth toward death. For the first 2 days after treatment, there is a net weight increase, largely due to abnormal growth of the axis (apex, stem, and tap root). Root and leaf growth are drastically curtailed. Between 2 and 7 days, axis growth continues primarily at the expense of leaf tissue which is induced to senesce, especially the cotyledons. The last phase between 7 and 10 days leads to collapse and withering of plants, and was not examined in detail.

Analyses for nitrogenous constituents show the usual mobilization to the axis, with early and large increases in nucleic acid. Both photosynthesis and ion absorption are initially stimulated but decline sharply after the first day. Translocation to leaves and roots is drastically reduced in favor of the proliferating axis.

The death of the plant appears to be due to suppression of normal apical growth coupled with induction of abnormal axis growth. Failure to produce new root and leaf tissue plus inadequate nutritional maintenance of existing roots and leaves lead to lack of autotrophism and eventual death. The biochemical basis for these responses appears to be with aberrant nucleic acid metabolism.

Feral rye is an agricultural and ruderal weed of the western United States. We investigated how it has phenotypically diverged from its cultivated ancestor, domesticated cereal rye, and across its range since the introduction of its progenitor. Vegetative growth, flowering phenology, and reproductive characters of feral populations from across a 13° range in latitude in the northwestern United States were compared to that of rye cultivars under both vernalized (cold-treated) and nonvernalized conditions. Feral populations as a whole had smaller seeds, thinner culms, and a delay in flowering relative to cultivars, regardless of cold treatment. Vernalized feral populations from northern latitudes (northern California and eastern Washington) produced more, but smaller leaves and more tillers than both vernalized rye cultivars and southern California feral populations. Northern feral populations also flowered significantly later, irrespective of vernalization treatment. We conclude that feral rye is phenotypically distinct from domesticated cereal rye and that feral populations have diverged regionally from one another. Reproductive isolation from domesticated rye, due both to the loss in popularity of the crop and to phenological shifts in feral rye relative to cultivars, may be contributing to the rapid evolution of this weed away from its domesticated ancestor in less than 120 yr since its introduction.

Gene flow via pollen and inheritance of acetyl-coenzyme A carboxylase (ACCase) inhibitor resistance in giant foxtail have not been previously characterized. Therefore, experiments were conducted to quantify outcrossing rates and flowering periods of giant foxtail accessions resistant or susceptible to ACCase inhibitors and to determine the inheritance pattern of gene(s) that confer resistance. In greenhouse experiments, outcrossing rates between resistant and susceptible parents ranged from 0.24 to 0.73%, as gauged by the response of F1 plants to fluazifop-P. Resistant plants and susceptible plants flowered at similar times, with peak flowering occurring 63 to 67 d after planting. During this period, flower initiation was nearly twice as great for resistant plants as for susceptible plants. In inheritance experiments F2 plants segregated in a 1:2:1 ratio of resistant–intermediate–susceptible phenotypes after exposure to fluazifop-P, indicating that resistance was associated with a single nuclear, incompletely dominant allele. The very low rates of outcrossing among giant foxtail plants in the greenhouse indicate that gene flow of resistance traits in the field is likely to occur primarily by seed immigration rather than by pollen movement. However, gene flow of herbicide resistance traits via pollen provides a mechanism in which multiple resistance may develop among giant foxtail populations that are resistant to other classes of herbicides.

Evidence that β-alanine or calcium pantothenate partially reversed the inhibitory action of several chlorinated aliphatic acids on yeast growth (4) suggested the hypothesis that synthesis of pantothenate is one of the metabolic pathways sensitive to this group of herbicides. The objectives of the experiments reported herein were (a) to determine the mechanism by which the enzymatic synthesis of pantothenate was inhibited by these compounds, (b) to seek evidence for the contributions of this mechanism, in vivo, to the herbicidal effects of 2,2-dichloropropionic acid (dalapon), (c) to determine whether pantothenate treatments could overcome the effects of dalapon in plants and (d) to test chloro-substituted analogs of pantoate for herbicidal activity. To achieve the last objective it was necessary to synthesize several new chemicals.

Goss's bacterial wilt and leaf blight of corn is caused by the bacterium Clavibacter michiganensis subsp. nebraskensis (Cmn). This disease has recently re-emerged as an important disease in the Midwestern United States (US) and continues to spread. Cultural practices are currently the only methods available for controlling the disease. Weedy species in the genera Echinochloa, Setaria, and Sorghum have previously been described as alternative hosts of Cmn. The objective of this research was to use an isolate of Cmn from the eastern Midwest to examine the host status of previously confirmed hosts, as well as test whether additional weedy or cover crop species are alternative hosts of the bacterium. Plants were inoculated with a suspension of 1 × 108 colony-forming units of Cmn per milliliter in a greenhouse experiment. Leaves were observed for typical symptoms of Goss's wilt 7 d after inoculation. Pathogen presence was determined by observing bacterial streaming microscopically, and isolating Cmn from symptomatic plants. Putative colonies of Cmn were confirmed with the use of morphological and molecular methods. Koch's Postulates were completed on populations of new plant species that showed symptoms. Results revealed three new hosts of Cmn: annual ryegrass, johnsongrass, and large crabgrass. In contradiction to previous reports, barnyardgrass was not a host of Cmn in this study. Results also confirm that giant foxtail, green foxtail, shattercane, and yellow foxtail are hosts of Cmn. These results redefine the known host range of Cmn and are important in identifying additional sources of inoculum to improve our understanding of the epidemiology of Goss's wilt.

The percentage composition of fat, protein, nitrogen-free extract (NFE), fiber, and ash was determined for 50 weed and crop species. Data on 16 additional species were compiled from the literature. Generally, within a family there was a tendency toward a certain type of storage material. Seeds of the Polygonaceae and Gramineae were low in fat, high in NFE and, with few exceptions, low in protein. Seeds of the Leguminosae family were the highest in protein and, with the exception of peanut (Arachis hypogaea L.) and soybean [Glycine max (L.) Merr] were low in fat and high in NFE. Seeds of the Compositae, Cruciferae, Cucurbitaceae, and Solanaceae were high in fat, moderately high in protein, and low in NFE. The data may be valuable in the interpretation of the mode of action or basis for selectivity of herbicides.

Pronounced differences in the tolerance of tomatoes and bell peppers to clomazone observed in field studies were confirmed in a greenhouse experiment. In greenhouse studies, preemergence clomazone rates causing 50% visible injury on bell pepper and tomato seedlings 10 days after application were 9.4 and 0.1 kg/ha, respectively. Based on growth inhibition, bell peppers were 40-fold more tolerant of clomazone than tomatoes 20 days after clomazone application. In laboratory studies investigating the basis for differential clomazone tolerance, no differences in uptake of 14C-clomazone from nutrient solutions between tomato and bell pepper plants were observed after 24 h. Minor differences were observed in the distribution of 14C label within plants; a higher percentage of 14C was recovered in bell pepper roots than in tomato roots, while the opposite was true for the shoots. Clomazone was metabolized to two products in roots of both bell peppers and tomatoes within 48 h after treatment. Tomato shoots were more active in converting clomazone to these metabolites than were tomato roots. Bell pepper roots converted more clomazone to metabolites than did tomato roots 24 h after treatment. However, by 72 h, differences in clomazone metabolite levels between species were negligible in both roots and shoots. Enzymatic and acid hydrolysis of soluble, polaf clomazone metabolites indicated that these metabolites may be sugar conjugates of clomazone.

Lack of understanding the effects of single- and multiple-weed interference on soybean yield has led to inadequate weed management in Primorsky Krai, resulting in much lower average yield than neighboring regions. A 2 yr field experiment was conducted in a soybean field located in Bogatyrka (43.82°N, 131.6°E), Primorsky Krai, Russia, in 2013 and 2014 to investigate the effects of single and multiple interference caused by naturally established weeds on soybean yield and to model these effects. Aboveground dry weight was negatively affected the most by weed interference, followed by number of pods and seeds. Soybean yield under single-weed interference was best demonstrated by a rectangular hyperbolic model, showing that common ragweed and barnyardgrass were the most competitive weed species, followed by annual sowthistle, American sloughgrass, and common lambsquarters. In the case of multiple-weed interference, soybean yield loss was accurately described by a multivariate rectangular hyperbolic model, with total density equivalent as the independent variable. Parameter estimates indicated that weed-free soybean yields were similar in 2013 and 2014, i.e., estimated as 1.72 t and 1.75 t ha−1, respectively, and competitiveness of each weed species was not significantly different between the two years. Economic thresholds for single-weed interference were 0.74, 0.66, 1.15, 1.23, and 1.45 plants m−2 for common ragweed, barnyardgrass, annual sowthistle, American sloughgrass, and common lambsquarters, respectively. The economic threshold for multiple-weed interference was 0.70 density equivalent m−2. These results, including the model, thus can be applied to a decision support system for weed management in soybean cultivation under single and multiple-weed interference in Primorsky Krai and its neighboring regions of Russia.

The discovery of the herbicidal properties of isopropyl N-phenylcarbamate (IPC) and isopropyl N-(3-chlorophenyl)carbamate (CIPC) and other organic chemicals has created an impetus in the development of new herbicides. Since the discovery of 2,4-dichlorophenoxyacetic acid (2,4-D), considerable effort has been devoted to the development of herbicides that are as effective and selective for the control of annual grasses as is 2,4-D for the control of many broad-leaved weeds.

Movement of pesticides on soil thin-layer chromatographic (TLC) plates was detected by a bioassay employing a green alga, Chlorella sorokiniana Shihira and Krauss. After leaching the soil TLC plates with water, a Chlorella suspension in an enriched agar medium was aspirated onto the soil to give ca. 8 × 106 cells/cm2. Plates were incubated at 100% relative humidity at 30 C, with illumination of ca. 40 klux. Zones of pesticide movement were visible within 24 to 48 hr. Mobility in Hagerstown silty clay loam was assessed for two methylcarbamate insecticides and for nine phenylurea, 11 s-triazine, and 13 miscellaneous herbicides. Among analogous triazines, the mobility order was: -OCH3 > -Cl > -SCH3. Movement of 6,7-dihydrodipyrido[1,2-a:2′,1′-c]pyrazinediium ion (diquat) and 1,1′-dimethyl-4,4′-bipyridinium ion (paraquat), normally immobile in soils, was directly proportional to rate when applied in high doses.

Horseweed is a common pest in vineyards of the San Joaquin Valley (SJV) of California. Interest in controlling this weed has increased with the recent discovery of a glyphosate-resistant (GR) biotype that has been observed to be more vigorous than a glyphosate-susceptible (GS) biotype in the SJV. However, the impact that either biotype may have on grapevine growth has not been assessed. Therefore, two glasshouse experiments were conducted to characterize the competitiveness of GR and GS horseweed biotypes from the SJV with young grapevines. ‘Syrah’ grapevines grafted to Freedom rootstocks were planted in 8-L plastic pots, alone, or with a single GR or GS horseweed. Additional GR and GS horseweeds were also planted separately in individual pots, and all plants were grown for 14 and 16 wk in 2006 and 2007, respectively. Grapevines grown with either biotype of the weed produced fewer leaves and amassed approximately 20% less dry mass (DM) than vines grown alone. The GR biotype reduced grapevine stem DM and length by 30%, but the GS biotype did not. The GR biotype accumulated more than twice the DM as the GS biotype, whether in competition with grapevine or not. Grapevines reduced the total leaf number of both horseweed biotypes by almost 50% and aboveground DM of GR and GS biotypes by 50 and 75%, respectively. These preliminary findings indicate that competition from horseweed can substantially reduce the growth of young grapevines and that the GR biotype may be more competitive than the GS biotype.

The toxicity of 1,1'-dimethyl-4,4'-bipyridinium ion (paraquat) in soil can be related to the degree of bonding strength to the soil aggregates. Tightly bound material is unavailable to plants, while loosely bound material can be potentially available to plants. On the basis of growth of corn (Zea mays L., var. Golden Cross Bantam) and beans (Phaseolus vulgaris L., var. Idaho 1-11) in sand, loamy sand, loam, and muck treated with paraquat, it is concluded that planned and careful application would not lead to accumulation of lethal dosages in medium to heavy soils.

Few studies report animal grazing effects on weed populations. A study was conducted to assess weed populations in annual and perennial forage grasses grazed at various intensities by cattle over a 4-yr period. The perennial forages were Bromus inermis and Bromus riparius, and the annual forages were winter Triticosecale and a mixture of Hordeum vulgare and winter Triticosecale. With few exceptions, results from the two annual pastures could be adequately described as a group, as could the results from the two perennial pastures. The two most prevalent weed species were Capsella bursa-pastoris and Taraxacum officinale; other species encountered over the course of the study were analyzed as a group. Tillage (seedbed preparation) in the annual system supported a proliferation of annual weeds in the spring. In the perennial pasture system, a lack of tillage and spring MCPA allowed T. officinale to increase as the study progressed, especially at the highest grazing intensity. In the perennial pastures, each unit increase in grazing intensity led to 51 more C. bursa-pastoris m−2 and 4 more T. officinale m−2. At lower levels of grazing intensity, C. bursa-pastoris and other species were most abundant in the annual pastures. Weed population shifts in response to grazing pressure in the annual pasture systems were restricted because of annual tillage and MCPA. Therefore, pasture managers may subject annual pastures to heavy grazing pressure with less negative weed population consequences than perennial pastures where herbicides are not applied.

Field experiments were conducted in 1998 and 1999 at Five Points, CA, in the San Joaquin Valley under irrigated conditions to study competition between four commonly grown tomato cultivars and velvetleaf and to identify cultivar characteristics associated with greater tolerance to velvetleaf. The effect of velvetleaf competition varied with both year and tomato cultivar. When grown with 5 velvetleaf plants m−1 of row, marketable yield of tomato was reduced 8% in 1998 and 60% in 1999 for cultivar H8892 and 58% in 1998 and 80% in 1999 for cultivar H9661, compared to cultivars grown in monoculture. Across velvetleaf densities, height of tomato cultivars was not reduced compared to that of cultivars grown in monoculture. However in 1999, canopy width of tomato cultivars grown with velvetleaf was less than that of cultivars grown in monoculture. At early stages of growth, the leaf area index of tomato cultivars grown with velvetleaf was less than that of cultivars grown in monoculture. Crop growth rate and aboveground dry biomass of tomato cultivars grown with velvetleaf were generally less than those of cultivars grown in monoculture. Yield loss at high weed density was similar among cultivars, whereas yield loss at low weed density varied among cultivars. Cultivar tolerance to velvetleaf varied with year. However, cultivar H8892 had low yield loss and cultivar H9661 high yield loss at low weed density in 1998 and 1999. For cultivar H8892, leaf area expansion rate was also among the greatest for both years.

Virginia buttonweed seeds germinated at temperatures between 15 and 40 C, with optimal germination occurring at 25 or 30 C in 12 h light alternated with 15 to 25 or 20 to 30 C in darkness, respectively. Germination rate was lower in continuous darkness than in light. Germination was reduced under simulated water stress conditions with less than 10% of seeds germinating at osmotic potentials below −0.3 MPa. Overall seed germination was reduced under simulated flooding conditions; greater germination occurred in aerated water than in unaerated water. Maximum germination occurred at pH 6. Seedlings emerged from depths as great as 8 cm, with maximum emergence occurring from 2 cm.

Field experiments were conducted in 1987 and 1988 at Frederick, MD, to determine established horsenettle (Solanum carolinense L.) interference with snapbean. Horsenettle grown for 3 yr prior to planting snapbeans reduced yield 65% in 1987 and 48% in 1988. Horsenettle established for 1 yr reduced snapbean yield 20 and 18% in 1987 and 1988, respectively. As the distance of the snapbean row from the horsenettle row increased, there was less effect on the yield of snapbean. In 1987, snapbean yield in rows 15, 30, or 60 cm from horsenettle was reduced 43, 29, and 15%, respectively, while during 1988 in the same plots snapbean yields were reduced 38, 26, and 11%. Eight horsenettle planted in a 4.6-m row the first year and 16 horsenettle per row for the second year reduced snapbean yield 36 and 55% respectively. Horsenettle fruit number was 16% lower the first year and 45% lower the second year when snapbeans were grown at a 15-cm row spacing instead of a 60-cm row spacing.