Unique long-term historical emergence records were used to assess the association between weed seedling emergence and various elements of meteorological data. These elements included both temperature-based and rainfall-related variables in the 7-d periods before and during which emergence occurred. Five weed species (Stellaria media, Chenopodium album, Capsella bursa-pastoris, Matricaria perforata, and Veronica hederifolia) with contrasting emergence patterns were studied in disturbed soil. Logistic regression analysis was used to identify meteorological variables of interest and allowed their relative importance to be assessed and ranked. Logistic regression was further used to associate probabilities of emergence with observed levels of important individual meteorological elements. This approach enabled prediction of the probability of emergence following given meteorological conditions and hence an assessment of the risk of omitting weed control measures. Predictions were made based on single meteorological variables and compared with observed data. Results indicated that temperature was the dominant factor in predicting emergence. Soil moisture, while also important, was a secondary factor only becoming important once the species-specific temperature requirement had been satisfied. The potential for further development of the model is discussed.

Competition studies were conducted with soybeans Glycine max. (L.) Merr. ‘Clark 63’ and Venice mallow (Hibiscus trionum L.). One Venice mallow per 7.5 cm of soybean row reduced soybean seed yield 632 kg/ha after 85 days competition. Thirty to 40-cm weed bands in and between soybean rows reduced yields 270 to 651 kg/ha with 35 to 40 days of competition. A natural stand of 215 Venice mallow plants per square meter reduced soybean yield 454 kg/ha after 30 days of competition, and competition up to 110 days reduced yield as much as 1490 kg/ha. Weed competition affected the number of pods per soybean plant more than any other seed-yield component, and soybean height was reduced. Soybean yields were reduced more when soil moisture was abundant early in the growing season and limited in late summer than when moisture was limited early in the growing season and above average until soybeans matured.

The effects of varying concentrations and duration of alachlor [2-chloro-2′,6′diethyl-N-(methoxymethyl)acetanilide] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)acetamide] treatment on root growth, cell division, and cell enlargement were studied. Peas (Pisum sativum L. ‘Alaska’) and oats (Avena sativa L. ‘Victory’) were treated from 0 to 48 h with concentrations ranging from 1 × 10-8 to 1 × 10-3 M of each herbicide. After 48 h, average growth rates were significantly inhibited at concentrations of 1 × 10-7 M alachlor and 5 × 10−8 M metolachlor, and 5 × 10−7 M alachlor and 1 × 10-6 M metolachlor for peas and oats, respectively. When growth inhibitions were examined across time at concentrations greater than these, the degree of growth inhibition was a function of both concentration and duration of treatment. Often the greatest decrease in growth occurred between 0 and 12 h. Mitotic indices of root tip squashes from pea roots and paraffin sections from oat roots were determined. There was a significant reduction in the mitotic indices of pea roots treated for 48 h with 5 × 10−6 M alachlor or 1 × 10-5 M metolachlor. After a 30-h treatment, the mitotic indices of oat roots were significantly reduced by 1 × 10−7 M metolachlor and 1 × 10−6 M alachlor. Significant inhibition of elongation of etiolated oat coleoptiles were observed at 5 × 10−6 M alachlor (27%) and 5 × 10−5 M metolachlor (30%). Inhibition of pea hypocotyl elongation did not occur at concentrations below 5 × 10−4 M. It was concluded that the growth inhibition of plants caused by alachlor and metolachlor results from both an inhibition of cell division and cell enlargement.

Weeds spread through movement of seeds and vegetative reproductive propagules. Pollen movement can spread weedy traits, such as herbicide resistance, between related weed and crop species. Weed seeds can spread short or long distances by natural plant dehiscence mechanisms, wind, water, animals, and man&s activities. This symposium paper is a practical review of short-distance spread of weed seeds in and between nearby arable fields and noncrop lands, examining some of the causes of spread and subsequent effects on crop production. Pollen movement, as it affects the spread of herbicide resistance, also is considered a component of short-distance weed spread. Specific weed management options can be used to reduce man-caused weed seed spread within and between nearby fields, thus reducing potential crop yield losses. Long-term management will be more difficult for weed seed spread by natural dispersal mechanisms.

Densities up to 100 downy brome m2 were established in winter wheat in southeastern Wyoming and west-central Kansas to quantify wheat yield loss from downy brome interference and to approximate economic threshold levels. A quadratic equation best described wheat yield loss as a function of weed density when downy brome emerged within 14 days after wheat emergence. Densities of 24, 40, and 65 downy brome m2 reduced wheat yield by 10, 15, and 20%, respectively. Wheat yield was not reduced when downy brome emerged 21 or more days later than wheat. Economic thresholds varied with changes in downy brome density, cost of control, wheat price, and potential wheat yield. In a greenhouse experiment, dry weight of 72-day-old wheat plants grown in association with downy brome was not affected by the distance between the weeds and wheat, whereas downy brome plant dry weight increased with increasing distance between the weeds and wheat.

Yellow nutsedge (Cyperus esculentus L.) produced an average of 1227 and 6685 flowers per inflorescence from eight California fields sampled during 1974, 1975, and 1976. Up to 17% of the flowers produced seeds and up to 78% of the seed germinated. There was a linear relationship between seed weight and germination percentage. Purple nutsedge (C. rotundus L.) averaged between 186 and 1354 flowers per inflorescence from three California fields sampled in 1974 and 1976. Only 43 purple nutsedge seed were collected and none germinated. Yellow nutsedge produced viable seed about 2 weeks after anthesis. A day/night temperature of 38/32 C was best for seed germination, especially when accompanied by a light period. Yellow nutsedge has potential to reproduce by seed, but probability of purple nutsedge reproducing by seed is low.

Experiments were conducted in 2006 to 2008 to study growth, phenology, and competitive ability of glyphosate-resistant (GR) and -susceptible (GS) biotypes of horseweeds from San Joaquin Valley (SJV), CA. When grown alone, in pots, the GR horseweeds consistently developed more rapidly than the GS weeds, as evidenced by their earlier bolting, flowering, and seed set; the GR horseweeds set seeds nearly 25 d (approximately 190 fewer growing degree days) sooner than the GS horseweed. At seed set, the relatively slow-developing GS horseweeds had amassed 40% more shoot dry matter than the GR weeds at the same phenological stage, but neither biotype was consistently more fecund than the other. Although the GR biotype had lower shoot dry mass than the GS biotype when grown alone, in mixed populations under increasing levels of competition (in a replacement series design) and limited resources (mainly moisture), the GR weeds were not only taller, but also accumulated more dry matter than the GS weeds. Thus, the GR biotype was more competitive than the GS biotype, particularly when grown at high densities and under moisture-deficit stress. Therefore, under California conditions there is no apparent fitness penalty for this particular GR horseweed biotype, and it is likely to persist in the environment and outcompete the GS biotypes regardless of further glyphosate selection pressure. If so, this biotype of GR horseweed is likely to become increasingly common in the SJV until effective management strategies are developed and adopted.

Young vegetative clones of yellow nutsedge (Cyperus esculentus L.) were propagated by subdivision of older clones and then grown 6 months under photoperiods of 8 to 24 hr. Early stages of development of basal bulbs, tubers, and flowering structures were characterized in terms of apical meristem activity and differentiation of various foliar appendages: cladophylls, prophylls, leaf primordia, foliar tube development, and involucral leaves. Ramification of the axial stem system was interpreted as a repeating phylogenetic sequence: viz., undifferentiated axial meristem (from basal bulb) å primitive stem (rhizome) å advanced stem (new basal bulb). New photosynthetic leaves differentiated every 4.5 to 5 days, and each exhibited a sigmoid pattern of growth for 24 to 40 days. As photoperiods increased from 14 to 24 hr, certain active vegetative processes—total peripheral shoot development, rhizome proliferation, and rate of higher order shooting—were progressively promoted. The rate of differentiation of indeterminate rhizome tips into basal bulbs (new shoots) was maximum at 16 hr and into tubers at 8 to 12 hr. Delayed tuberization, however, occurred even at the longest photoperiod. Flowering occurred only at photoperiods of 12 and 14 hr. Active vegetative processes were competitive with tuberization, and flowering was competitive with both active and dormant vegetative development.

Orobanche aegyptiaca is a chlorophyll-lacking holoparasite that subsists on the roots of plants, inflicting severe damage to legumes and other crops in the Mediterranean region. Common vetch and purple vetch are important legume forage crops in Israel and the Middle East. Experiments in pots containing soil inoculated with O. aegyptiaca seeds have shown that common vetch genotypes ‘Yovel’ and ‘473-A’ are susceptible to the parasite, whereas purple vetch genotypes ‘Popany’ and ‘Sadot’ are resistant. When grown in association with O. aegyptiaca in polyethene bags, purple vetch genotypes stimulated a significantly higher rate of parasite seed germination than that stimulated by common vetch genotypes. However, the number of successful Orobanche attachments developed on the host root was lower on resistant genotypes compared to susceptible ones. This differential response between purple and common vetch genotypes was evident at diverse temperature regimes (17/12, 22/17, and 27/22 C day/night). Microscopic observations revealed that resistant genotypes rapidly developed necrotic lesions surrounding the contact point of the parasite radicle (germ tube) with the host root, hence preventing further development of the parasite. In the sensitive genotypes, attachments developed without necrotic lesions. These data may indicate involvement of host defense mechanisms induced by the parasite attachment.

Germination, growth, and potential phytotoxicity of sicklepod (Cassia obtusifolia L.) were studied. Seed germination occurred in the temperature range from 18 to 36 C, but rapid seedling growth occurred only between 30 and 36 C. Seed dormancy caused by a waxy coat resulted in only 15% germination in soil over a 12-month period. A 1:15 (g/ml) water extract of sicklepod tops inhibited germination of cotton (Gossypium hirsutum L.) and oat (Avena sativa L.) seed. Incorporation of sicklepod residues in soil reduced germination of cotton seed but increased persistence of sericea (Lespedeza cuneata (Dumont) G. Don.) and crimson clover (Trifolium incarnatum L.) because damping-off was almost eliminated. There was evidence in nutrient cultures that sicklepod released a phytotoxic residue from its roots, reducing growth of cotton but having no effect on soybeans (Glycine max (L.) Merr.). Sicklepod grew well in soils ranging from pH 3.2 to 7.9. Sicklepod response to N, P, and K levels was similar to cotton and higher than that of soybeans.

Tracer studies employing 2,2-dichloropropionic acid (dalapon)-2-C14 and -Cl36 were conducted on barley (Hordeum vulgare L.), bean (Phaseolus vulgaris L.), and three hypostomatous species—zebrina (Zebrina pendula Schnizl.), coleus (Coleus blumei Benth.), and nasturtium (Trapoelum majus L.). Count data and radioautographic evidence showed that greater amounts of dalapon were absorbed and translocated at high (88 ± 3%) than medium (60 ± 5%) or low (28 ± 3%) post-treatment relative humidity (herein called R.H.). Also, greater uptake and translocation of C14 occurred in bean at 43 ± 1 C than at 26 ± 1 C post-treatment temperature, with constant R.H. and light conditions. Rate of droplet drying, and stomatal distribution and behavior, apparently contributed to the R.H. effect. Droplets dried less rapidly at high than at low R.H., thus prolonging the period of effective absorption. At low R.H., periodic rewetting of the droplet area enhanced absorption and translocation but never to the extent achieved with high ambient R.H. (in growth chambers) or in saturated atmosphere (polyethylene bagging in the greenhouse). High R.H. (95 ± 3%) favored stomatal opening in zebrina, and uptake of dalapon was greater through the abaxial than adaxial surface of leaf disks of zebrina, coleus, and nasturtium. Rewetting of droplets or high ambient R. H., however, promoted uptake through astomatous as well as stomated surfaces. Absorption and translocation were markedly greater in bean plants grown at 95 ± 3% R.H. than at 28 ± 3% R.H., both treated under high R.H. conditions. Thus cuticle hydration and ontogeny also were implicated in the R.H. phenomenon. Several solution additives also were studied in an attempt to substitute for the R.H. effect. At low R.H., a surfactant (X-77) markedly enhanced uptake of dalapon in bean, glycerol (a humectant) and hexadecanol (a filming agent) were less advantageous, and Plyac (a sticker-spreader) decreased penetration. Results of these studies aid in explaining the greater toxicity of dalapon in regions of high R.H. and, conversely, its sometimes erratic performance in drier climates. The results also confirm the distribution and accumulation patterns of dalapon absorbed by plants and the usefulness of an effective spray adjuvant in promoting foliar absorption.

Two or more pesticides together may produce a growth response in plants that is not predictable by their individual or independent toxicities. This unpredicted (dependent) response results from an interaction, a concept that usually is not easily interpreted. Dependent responses are further complicated by the fact that they can be either synergistic or antagonistic. Several methods exist for identifying and measuring phytotoxic interactions. Nearly all methods have certain shortcomings, however. Additive and multiplicative models (mathematical expressions) are the two basic approaches to determining pesticide interactions. The two-parameter, isobole, and calculus methods axe additive; whereas, Colby and regression estimate are multiplicative models. Regression estimate analysis considers deviations due to experimental errors, and a statistical significance can be attached to the interaction magnitude, thereby overcoming the deficiencies of the Colby method, but both methods seem to be limited to response data in which the combined pesticide concentration is the sum of the individual pesticide concentrations. The two-parameter method seems to be limited to response data in which the combined concentration is equal to the individual pesticide concentration and to response data in which a pesticide concentration necessary to produce a 50% of control value is interpolated rather than extrapolated. The calculus method is a mathematical expression of the growth response, and interaction is measured by derivation of the equation obtained. The calculus method is difficult to interpret and has a major weakness because it depends upon the multiple regression equation of the observed data. The regression estimate method is recommended as a reasonable approach to interpretation of interaction type data, with a SAS language computer program available from the author.

Ragweed parthenium is a highly invasive weed species in several countries, including Australia. Laboratory experiments were conducted to evaluate the effect of temperature, light, salinity, pH, and moisture on germination of two Australian biotypes of ragweed parthenium: Clermont (highly invasive) and Toogoolawah (noninvasive). Although seeds of both biotypes could germinate under complete darkness, germination was improved by 20% to 49% under a 12-h photoperiod. Both biotypes germinated over a wide range of constant (8 to 35 C), and alternating day/night (15/5 to 35/25 C) temperatures. However, the Clermont biotype exhibited significantly higher germination than Toogoolawah biotype over the range of temperatures studied. Highest germination of Clermont (100%) and Toogoolawah (97%) was observed at constant temperatures of 14 to 23 C and 23 C, respectively. The best alternating day/night temperature for germination of both biotypes was 25/15 C. Clermont also germinated better than Toogoolawah under osmotic- and salt-stress conditions. Osmotic stress had moderate negative effects on germination, with 52% and 36% of the Clermont and Toogoolawah seeds able to germinate at −0.60 MPa, respectively. Complete germination inhibition for both biotypes was observed at an osmotic potential of −1.2 MPa. Both biotypes also germinated at a very high sodium chloride (NaCl) concentration of 250 mM. A 50% reduction in germination of Toogoolawah and Clermont was caused by 99 and 154 mM NaCl, respectively. Germination of the Clermont biotype was not affected by a wide range of pH (4.0 to 10.0), whereas the strong acidic and alkaline pH levels (4.0 and 10.0) caused 18% and 25% reductions in germination of the Toogoolawah biotype compared with control. The Clermont biotype had a higher ability to germinate across all treatments compared with the Toogoolawah biotype, which might be a contributing factor toward the high invasive ability of the former compared with the latter.

Competition of yellow nutsedge (Cyperus esculentus L.) with corn (Zea mays L.) was evaluated in the field at various yellow nutsedge densities over a 3-yr period. A relationship between yellow nutsedge density (shoots/m2) and percentage yield reduction revealed an 8% yield reduction for every 100 shoots/m2. Two 3-yr studies were conducted to determine the most effective combination of preplant-incorporated, postemergence, or postemergence-directed treatments for yellow nutsedge control in corn. The preplant incorporated treatments were alachlor [2-chloro-2′,6′-diethyl-N-(methoxymethyl)acetanilide], EPTC (S-ethyl dipropylthiocarbamate), or nothing; postemergence treatments were bentazon [3-isopropyl-1H-2,1,3-benzothiadiazin-(4) 3H-one 2,2-dioxide], two cultivations, or nothing; and the postemergence-directed treatments were ametryn [2-(ethylamino)-4-(isopropylamino)-6-(methylthio)-s-triazine] or nothing. One preplant-incorporated treatment of EPTC or alachlor prevented yield reductions from yellow nutsedge competition. When no control was practiced, yields were reduced 17% in a moderate yellow nutsedge infestation (initially infested with 300 tubers/m2) and 41% in a heavy infestation (initially infested with 1200 tubers/m2). Yields were reduced 7 to 8% in the moderate infestation when no preplant-incorporated treatments were used regardless of whether postemergence or postemergence-directed treatments were also used. After 1 yr, all control measures resulted in less tuber density than no control measures, but all control treatments had essentially similar tuber densities. After the second year, several herbicide treatments were as effective as hand weeding in reducing tuber density. At least 2 yr of effective control treatments were required to reduce tubers to 20% of the original density, and 3 yr of treatment to reduce the density to 15% of the original density. No combination of treatments, including hand weeding, eliminated tubers after 3 yr.

Multiple regression and response surface plots were used to analyze the effects of common lambsquarters population density and interference duration on weed growth and soybean seed yield. Under favorable growing conditions in 1986, weed biomass production at all population densities and interference durations was four to five times that produced in 1987, under less favorable conditions. However, there was no significant treatment by year interaction for soybean seed yield reduction by common lambsquarters, and production of each kg/ha weed biomass resulted in an average soybean yield reduction of 0.26 kg/ha. Utilizing 5% yield loss as an arbitrary threshold level, the regression equation predicted a common lambsquarters density threshold of 2 plants/m of row for 5 weeks of interference after crop emergence and 1 plant/m of row for 7 weeks. Seed production by individual common lambsquarters plants was highly correlated (r=0.92) with weed dry weight, and seed production ranged from 30 000 to 176 000 seeds/plant.

When herbicide-resistant weeds are highly mobile across farms, delaying resistance becomes a common-pool resource (CPR) problem. In such situations, it is in the collective long-term interest of farmers to conserve an herbicide's usefulness. Yet, each farmer has an individual short-run incentive to use the herbicide without considering effects on resistance. This study considers the potential for community-based (CB) approaches to address problems of herbicide-resistant weeds. Here, growers actively participate in designing, financing, and implementing programs, usually in collaboration with industry, government, and universities. CB approaches have certain advantages over top-down regulatory or subsidy-based approaches. Scholars and practitioners have developed effective governance mechanisms for many CPRs that could aid in developing effective resistance management programs. Successful CB management programs for insect pest eradication and areawide invasive weed control offer additional lessons about the potential and challenges of such efforts. Lessons from these examples can inform the design and implementation of successful, voluntary programs for herbicide-resistance management. Key research, education, and outreach priorities to help implement successful CB programs are identified at the close.

A greenhouse experiment was conducted to evaluate the herbicidal activity of five aliphatic (ethyl, propyl, butyl, allyl, and 3-methylthiopropyl) and three aromatic (phenyl, benzyl, 2-phenylethyl) isothiocyanates on Texas panicum, large crabgrass, and sicklepod. All isothiocyanates were applied to soil at 0, 10, 100, 1,000, and 10,000 nmol g−1 of soil and incorporated. Weed emergence was generally stimulated at the lower isothiocyanate concentrations, but all isothiocyanates provided 37% or more suppression of each species at the highest concentration. Propyl and allyl isothiocyanate were most effective in suppressing Texas panicum, with 50% effective dose (ED50) values of 345 and 409 nmol g−1 of soil. All aliphatic isothiocyanates reduced Texas panicum density by at least 98%. Allyl and 3-methylthiopropyl isothiocyanate were the most effective aliphatics on large crabgrass, with density reductions of 98 and 100%, respectively. All aromatic isothiocyanates reduced large crabgrass density by 86 to 96%. Sicklepod was generally the most tolerant of the three species evaluated, with ED50 values for ethyl, propyl, and butyl isothiocyanate being greater than the evaluated concentrations. Maximum reduction in sicklepod density was 72, 68, 65, and 62%, which was achieved with allyl, benzyl, 3-methylthiopropyl, and phenyl isothiocyanate, respectively. This research shows that soil-applied and incorporated isothiocyanates are effective in suppressing some important weeds of the southeastern United States, but effectiveness of each isothiocyanate varies among species. Application techniques that minimize loss of volatile isothiocyanates may further improve their potential as an effective means of controlling these and other troublesome weeds.

Of 15 hard red winter wheat (Triticum aestivum L.) cultivars grown on several soil types and treated in the spring with several rates of metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H)-one], ‘Lindon’ and ‘Vona’ were the most sensitive and ‘TAM W 101’ and ‘Osage’ the most tolerant. Differences in cultivar response to metribuzin occurred in 9 of 13 locations, but were not observed when precipitation was very light or when the soil was saturated at the time of treatment. TAM W 101 was also more tolerant than Vona in the greenhouse.

Downy brome is a grassy weed that infests millions of acres of winter wheat in the Western United States and costs farmers millions of dollars in lost yields. It can not be controlled consistently using chemicals or cultural practices. Isolates of naturally occurring rhizobacteria have shown potential as biological control agents for downy brome. Centrifuged supernatant from aerobic shake cultures of Pseudomonas fluorescens strain D7 suppressed germination of seeds and reduced root and shoot growth of downy brome in agar diffusion assays. The present article relates to the isolation and characterization of active compounds from strain D7. The active compounds were denatured and lost activity with commonly used techniques of extraction, concentration, and purification of microbial products. Two unusual techniques for isolation and partial purification of the active compounds from supernatant of strain D7 were developed. With one technique, supernatant was concentrated at 30 to 35 C under vacuum to 10% of the initial volume. Subsequently, the concentrate was chromatographed on sephadex gels to separate the active fraction. With the second technique, cell-free supernatant was frozen, with or without prior concentration, at −12 C for 12 to 24 h. The frozen supernatant was then thawed at 2 to 4 C, resulting in precipitation and crystallization of the active fraction. The active fraction isolated with either method inhibited downy brome. Inhibition was complete at concentrations as low as 1 mg total dry matter L-1 agar. Also, the active fraction inhibited the plant pathogenic fungus Gaeumannomyces graminis var. tritici. Chemical analysis of the active fraction complex revealed the presence of chromopeptides and other peptides, fatty acid esters, and a lipopolysaccharide matrix. Separation of any of the components from the complex resulted in nearly complete loss of activity against downy brome.

Plant canopy reflectance over the 0.45- to 1.25-μm wavelength (WL) of weed species and crops was recorded with a field spectroradiometer to evaluate the possible use of remote sensing to distinguish weeds from crops. Weed and weed-crop species reflectance differences were generally greater at the 0.85 μm WL in the near-infrared spectral region than at the 0.55 μm WL in the visible region, indicating that color infrared (CIR) aerial photography may be useful to detect weed populations in crops. Canopy reflectance data were more directly related to photographic differences in weed-crop images than were single leaf or inflorescence reflectance data. Aerial photography at altitudes of 610 to 3050 m distinguished climbing milkweed (Sarcostemma cyancboides ♯ SAZCY) in orange [Citrus sinensis (L.) Osbeck. ‘Valencia’) trees; ragweed parthenium (Parthenium hysterophorus L. ♯ PTNHY) in carrot (Daucus carota L., var. sativa ‘Long Imperator’); johnsongrass [Sorghum halepense (L.) Pers. ♯ SORHA) in cotton (Gossypium hirsutum L. ‘CP 3774’) and in sorghum (Sorghum bicolor L. Moench. ‘Oro’); London rocket (Sisymbrium irio L. ♯ SSYIR) in cabbage; and Palmer amaranth (Amaranthus palmeri S. Wats. ♯ AMAPA) in cotton. Johnsongrass was also detectable with CIR film in maturing grain sorghum from 18 290 m. Detection of weed species in crops was aided by differential stages of inflorescence and senescence, and by the chlorophyll content, color, area, intercellular space, and surface characteristics of the leaves. Discrete plant community areas were determined by computer-based image analyses from a 1:8000-scale positive transparency with the efficiency of 82, 81, 68, and 100% for Palmer amaranth, johnsongrass, sorghum, and cotton, respectively. The computer analyses should permit discrete aerial surveys of weed-crop communities that are necessary for integrated crop management systems.