Yellow nutsedge (Cyperus esculentus L.) was studied to determine the relationship between physical character of tubers and sprouting habits. All tuber lots studied, which averaged from 157 to 662 mg per tuber, sprouted and grew well under greenhouse conditions. Longevity (the time between initial planting and death) increased with tuber weight. Weight of the third and fourth sprouts equalled that of the first and second sprouts for heavy tubers, but sprout weight decreased for light tubers. Detaching the plants at 2-week intervals from the tubers which produced them allowed all buds present to sprout. However, when the plants were detached at 4-week intervals, a reduction in the number of sprouts and a decrease in longevity were observed. Allowing plants to grow for 8 weeks before detaching did not cause any further decrease. Number of buds, number of sprouts, number of multiple sprouts, the time interval between first planting and first sprout, and the time interval between successive sprouts did not vary with initial tuber weight. Prolonged refrigerated storage of tubers caused an increased number of multiple sprouts, a decreased initial and subsequent sprouting interval, and a short life.

In a 2-year study, five densities of kochia (Kochia scoparia (L.) Schrad.) competed with sugarbeets (Beta vulgaris L.) for the entire growing season. The yield of sugarbeet roots and pounds of sucrose per acre decreased as the intensity of competition from kochia plants increased. One kochia plant per 25 ft of row reduced the average yield of roots by 2.6 T/A and sucrose by 960 lb/A. A density of one kochia plant, or more, per 2 ft of row in the first year, or one kochia plant per 1 ft of row in the second year, significantly reduced the sucrose content of the roots. Individual kochia plants significantly reduced the weight of sugarbeet tops and roots within a radius of 31 inches from the center of the sugarbeet row.

In citrus, weedy grasses compete for moisture, nutrients, and light and can inhibit the growth of young trees and delay fruit production. These weeds are difficult to control, either because of their tolerance to available herbicides or due to growth habits that enable them to resist other control practices. Control of seven such weedy grasses (southern sandbur, large crabgrass, crowfootgrass, guineagrass, Texas panicum, johnsongrass, and yellow foxtail) with a mixture of three fungal pathogens, termed the multiple-pathogen strategy, was field tested in 1996 and 1998. Three fungi indigenous to Florida, Drechslera gigantea, Exserohilum longirostratum, and E. rostratum, isolated from large crabgrass, crowfootgrass, and johnsongrass respectively, were used. Two separate field studies were conducted: one study with seven grasses transplanted and grown within each plot (grass mixture field trial) and another study on a population of guineagrass alone present in a naturally infested field (guineagrass field trial). The objectives of this study were to (1) evaluate the field performance of D. gigantea, E. longirostratum, and E. rostratum individually and in a mixture to control the seven transplanted weedy grasses (grass mixture) and a population of guineagrass in a naturally infested field, respectively, and (2) compare the effectiveness of three carriers (water, Metamucil®, and an invert emulsion) on the bioherbicidal efficacy under field conditions. The fungi were applied as foliar sprays, each pathogen alone or in a mixture of the three fungi (1:1:1, v/v/v, for a total of 5 × 105 spores ml –1) in water, 0.5% aqueous Metamucil®, or an emulsion containing Sunspray® 6E. During the 14-wk experimental period, one or two additional sprays of all treatments were applied. Disease severity was recorded weekly for 4 to 6 wk after the initial spray (WAI). Maximum disease severities were obtained in emulsion-inoculum treatments, and were higher than those in the water-inoculum and the Metamucil-inoculum treatments. The pathogen mixture was equally effective as the individual pathogens in controlling the weeds tested. In the 1996 trial, 6 WAI, disease severity on grasses inoculated with D. gigantea spore suspensions in emulsion ranged from 78 to 100%, with E. longirostratum 90 to 100%, E. rostratum 79 to 100%, and the mixture 74 to 100%. In the 1998 trial, 4 WAI, disease severity on grasses inoculated with D. gigantea spore suspensions in emulsion ranged from 45 to 98%, with E. longirostratum 45 to 98%, E. rostratum 34 to 98%, and the mixture 32 to 98.5%. Thus, it was possible to manage all seven weedy grasses under field conditions using an emulsion-based inoculum preparation with the individual pathogens as well as the mixture of pathogens. The same three fungal pathogens were field tested for their ability to manage populations of guineagrass in a naturally infested field. The experimental design and treatments were identical to the field testing with the seven transplanted grasses. Two applications of an emulsion-based inoculum preparation of each pathogen or the mixture of pathogens effectively controlled guineagrass for up to 10 wk, with no regrowth.

To avoid major negative impacts of the widespread adoption of biofuel species, whether they are exotic species, natives, or novel constructs, we need a system for screening their weed potential. Australia is an important global center of biodiversity and also has major cropping industries to protect. Prevention of the entry of further weeds is therefore a major national priority. The Weed Risk Assessment (WRA) system was developed and implemented for importation decisions in 1997; it has since been introduced into other countries and is probably as good as any system currently in operation. However, we need to be aware of the limitations of any system, to address these, and to work toward improved or alternative systems. WRA is a very simple spreadsheet requiring answers to questions about a species' life-history traits, dispersal, habitat suitability, impacts on other species, and history overseas, which are then added together and compared with numerical decision criteria. Its predictive powers are limited by this simplicity and by the complexity of human attitudes toward risk and impact. Alternative risk-management methods are available but, even so, the capacity for improvement is limited. It is quite possible, therefore, that in using any trait-based system to assess the negative risks of importation or interstate translocation of biofuel species, we will wrongly reject a valuable species or approve a species that turns out to be a major weed. It is suggested that, rather than attempting to improve a single-tiered decision-support system (the quarantine “sieve”), a multitiered system (nested sieves) would lead to a more effective system and greater cost-effectiveness. The key to this would be a postentry screening process for those species that have successfully passed through the WRA system.

Because of the large number of potentially invasive species, and the time required to complete weed risk assessments (WRAs) with the use of the current, mandated system in the United States, species need to be prioritized for assessment and possible listing as Federal Noxious Weeds. Our objective was to rank the potential invasiveness of weedy or pest plant species not yet naturalized in the United States. We created a new model of invasiveness (hereafter the U.S. weed-ranking model) based on scoring factors within four elements: (1) invasiveness potential, or likelihood to exhibit invasive behavior; (2) geographic potential, or habitat suitability; (3) damage potential, or likely impact; and (4) entry potential, or likelihood to be introduced. The ranking score was the product of the four elements. We scored 250 species satisfactorily, from a list of 700 +. We analyzed model sensitivity to scoring factors, and compared results to those from a WRA model for Hawaii. For species not in cultivation in the United States, the top 25 species included a mix of annuals, perennials, sedges, shrubs, and trees. Most had exhibited invasive behavior in at least several other countries. Because of greater entry potential scores, the highest-scoring species were weeds in cultivation. Twenty-nine such species, out of 44 total, had scores greater than the highest scoring species not in cultivation. In comparison to the Hawaii WRA model, correlation and regression analyses indicated that the U.S. weed-ranking model produced similar, but not exact, results. The ranking model differs from other WRAs in the inclusion of entry potential and the use of a multiplicative approach, which better suited our objectives and United States regulations. Two highly ranked species have recently been listed as Federal Noxious Weeds, and we expect most top-tier species to be similarly assessed.

Wheat cultivars resistant to imazamox will facilitate selective chemical control of many winter annual grass weeds, including jointed goatgrass, downy brome, and feral rye. These three weed species respond differently to imazamox postemergence treatments with feral rye, demonstrating more tolerance than jointed goatgrass or downy brome; therefore, growth chamber studies were conducted to evaluate imazamox absorption in all three weed species and translocation and metabolism in jointed goatgrass and feral rye. Adding nonionic surfactant (NIS) or methylated seed oil increased absorption in jointed goatgrass and feral rye but not in downy brome, compared to imazamox applied alone. Imazamox applied with NIS and urea ammonium nitrate resulted in the highest absorption in each species: 97, 91, and 92% of applied 14C for jointed goatgrass, downy brome, and feral rye, respectively, 48 h after treatment (HAT). Imazamox translocation from the treated leaf was similar for jointed goatgrass and feral rye across seven harvest intervals between 0 and 96 HAT. Shoot tissues of jointed goatgrass and feral rye accumulated 17 and 14% of applied 14C, respectively, by 96 HAT. Differential translocation of imazamox into root tissue was observed within 12 HAT; by 96 HAT, 20% of applied 14C translocated to jointed goatgrass roots compared to 27% for feral rye. Imazamox was readily metabolized in both weed species. At 96 HAT, 73 and 98% of the applied 14C was metabolized in the treated leaves of jointed goatgrass and feral rye, respectively. Metabolism was consistently higher in feral rye than in jointed goatgrass in all plant parts 96 HAT. On a whole-plant basis, metabolism was 25% greater in feral rye than in jointed goatgrass. The differential response of jointed goatgrass and feral rye to foliar applications of imazamox appears to be related to differences in translocation and metabolism but not in absorption.

Survival and dormancy of purple nutsedge tubers has not been studied quantitatively. Yet this is fundamental to our understanding of the population dynamics of this highly noxious weed. Field studies were conducted to determine the effect of age on tuber survival and dormancy. A modified exponential decay function accurately described the age-dependent decline in tuber survival. This model is biologically meaningful, has good statistical properties, and can describe a wide range of responses. Tuber population half-life was 16 mo, and the predicted longevity (99% mortality) was 42 mo. Burial depth at 8 and 23 cm had no significant effect on survival or dormancy. Tuber dormancy increased with age. After 18 mo, the proportion of dormant tubers in the surviving population was two-and-one-half times higher than in the 3-mo-old population. We report for the first time that tubers were able to enter a state of secondary dormancy after sprouting. The finding that tubers persist after sprouting has important implications for population dynamics of this species. This is also the first report of tuber predation by the larvae of a billbug.

Silverleaf nightshade is a widespread, deep-rooted, summer-growing perennial that significantly reduces production in Australian crop and pasture systems. It has an extensive root system, which competes both directly and indirectly with summer and winter pastures and crops through depletion of soil moisture and nutrients. Long-distance dispersal of seeds is an important mechanism for its spread and management. A range of experiments was conducted to determine the factors influencing seed production and seedbank dynamics. Seed production ranged from 1,814 to 2,945 m−2. Diurnally fluctuating temperatures of 25/15 C provided the optimal thermal conditions for germination, with germination not affected by light. Osmotic stress reduced germination, with no germination occurring at −1MPa. Germination was reduced to 5% at 160 mM NaCl, suggesting some salt sensitivity. Germination occurred over a pH range of 4 to 10, but declined with increasing acidity. Viability of buried seed declined to around 20% after 3 yr, with seed buried at 10 cm remaining the most viable. The prolonged seed persistence in the soil indicates a long-term control program is necessary for depleting the soil seedbank.

Dodder (Cuscuta) species are obligate shoot parasites that attach to stems and leaves of plants belonging to diverse families, diverting water, minerals, and metabolites, resulting in suppression of crop plants and yield reduction. Lespedeza dodder has the most diverse and numerous host range among the Cuscuta genus and is a major weed problem in tomato production in certain parts of the world, including California. At present, few resistant varieties of normally susceptible plant species have been developed or identified, and none are available in California. In this study, greenhouse and field studies were conducted to test and characterize the tolerance of commercial hybrid tomato varieties to lespedeza dodder. In greenhouse studies, Heinz varieties ‘9492’, ‘9553’, and ‘9992’ exhibited tolerance to the parasite, compared to the susceptible variety, ‘Halley 3155’. Lespedeza dodder germinated, made contact, twined around tomato stems, and adhered to them, but in most cases, haustoria failed to penetrate into the stem, eventually leading to the death of the parasite. In field studies, lespedeza dodder attachments were 75% less on tolerant varieties, and dodder growth was reduced by more than 70%.

Cover crop residues and other biologically based approaches often provide incomplete and inconsistent weed control. This research was conducted to evaluate interactions between hairy vetch residue on the surface of soil and the herbicide metolachlor. Herbicide was applied and incorporated with simulated rainfall before residue placement, residue was applied to the soil surface at precise rates, and potentially confounding variables such as nitrogen and soil moisture were controlled in a greenhouse experiment. Emphasis was placed on the use of suboptimal rates of both residue and metolachlor to explore the potential synergistic interactions between these factors. Deviation from a multiplicative model that included a quadratic response to hairy vetch residue and a log-logistic response to metolachlor was used to demonstrate the presence or absence of synergism. This model effectively showed that emergence of smooth pigweed, common lambsquarters, giant foxtail, and velvetleaf and early growth of smooth pigweed and common lambsquarters were reduced synergistically by the combination of hairy vetch residue and metolachlor. For example, smooth pigweed emergence was reduced 13% by 500 g m−2 of hairy vetch residue alone and was reduced 16% by 10 g ha−1 of metolachlor alone, but together, they reduced smooth pigweed emergence by 86%. This model could be used to determine synergistic interactions between any combination of a phytotoxin and a biologically based weed management approach that could be expressed in quantitative units.

Barnyardgrass [Echinochloa crus-galli (L.) P. Beauv.] is acknowledged to be the most troublesome weed in rice fields in Anhui and Jiangsu provinces of China. It cannot be effectively controlled using certain acetolactate synthase (ALS)-inhibiting herbicides, including penoxsulam. Echinochloa crus-galli samples with suspected resistance to penoxsulam were collected to identify the target site–based mechanism underlying this resistance. Populations AXXZ-2 and JNRG-2 showed 33- and 7.3-fold resistance to penoxsulam, respectively, compared with the susceptible JLGY-3 population. Cross-resistance to other ALS inhibitors was reported in AXXZ-2 but not in JNRG-2, and occasionally showed higher sensitivity than JLGY-3. In vitro ALS activity assays revealed that penoxsulam concentrations required to inhibit 50% of ALS activity were 11 and 5.2 times greater in AXXZ-2 and JNRG-2, respectively, than in JLGY-3. DNA and predicted amino acid sequence analyses of ALS revealed Ala-205-Val and Ala-122-Gly substitutions in AXXZ-2 and JNRG-2, respectively. Our results indicate that these substitutions in ALS are at least partially responsible for resistance to penoxsulam.

The Harrington Seed Destructor (HSD), a novel weed control technology, has been highly effective in Australian cropping systems. To investigate its applicability to conditions in western Canada, stationary threshing was conducted to determine the impact of weed species, seed size, seed number, chaff load, and chaff type on efficacy of seed destruction. Control varied depending on species, with a range of 97.7% to 99.8%. Sieve-sized volunteer canola seed had a linear relationship of increasing control with increasing 1,000-seed weight. However, with greater than 98% control across all tested seed weights, it is unlikely that seed size alone will significantly influence control. Consistently high levels of control were observed at all tested seed densities (10 seeds to 1 million seeds). The response of weed seed control to chaff load was quadratic, but a narrow range of consistently high control (>97%) was again observed. Chaff type had a significant effect on weed seed control (98% to 98.6%); however, seed control values in canola chaff were likely confounded by a background presence of volunteer canola. Overall, the five parameters studied statistically influence control of weed seeds with the HSD. However, small differences between treatments are unlikely to affect the biological impact of the machine, which provides high levels of control for those weed seeds that can be introduced into the harvester.

The influence of two weed management systems was determined on weed seed and seedling populations and on yields of cantaloupe (Cucumis melo L. var. reticulatus Naudin ‘Perlita′), bell pepper (Capsicum annuum L. var. grossum ‘Grande Rio 66′), cotton (Gossypium hirsutum L. ‘CP 3374′), onion (Allium cepa L. ‘1015Y′), and cabbage (Brassica oleracea, var. capitata L. 'Sanibel′) sequentially grown in two 3-yr cycles. Palmer amaranth (Amaranthus palmeri S. Wats. # AMAPA) did not exist initially, but hurricane-introduced seed populations increased to 1.1 billion/ha as seed populations of common purslane (Portulaca oleracea L. # POROL) decreased from 786 million/ha to 124 million/ha in the 6-yr period, without weeding or herbicide. Use of herbicides and handweeding reduced Palmer amaranth seed populations 98%, but 18 million/ha still remained after 6 yr. The use of herbicides and Palmer amaranth interference decreased the seed populations of common purslane by 84%, but handweeding was inefficient. Yields of all but the first crop of cantalouple were almost totally eliminated by season-long interference of Palmer amaranth. Savings with the utilization of herbicides rather than handweeding ranged from $62/ha for cotton to $4703/ha for bell pepper.

Five field experiments were conducted with snap bean (Phaseolus vulgaris L.) row spacings of 15, 25, 36, 46, and 91 cm, all of which were seeded at a constant density of 43 plants/m2. When weeds were allowed to emerge with the crop, row spacings of 15 to 36 cm suppressed weed growth by 18% compared to the conventional 91-cm row spacing. When weeds were controlled for the first half of the season, row spacings of 15 to 36 cm suppressed weed growth by 82% compared to the 91-cm row spacing. The effect of the 46-cm row spacing on weed growth was variable. Weed suppression by narrow rows was explained by a higher rate of snap bean canopy closure in narrow rows. Snap beans in row spacings of 15 to 46 cm produced similar yields and were higher by an average of 23% than yields produced by snap beans in 91 - cm rows.

Comparative growth, development, and resource allocation of triazine - susceptible and triazine - resistant biotypes of common groundsel (Senecio vulgaris L.), grown under two light regimes, were studied over a 7 - week period. Dry-matter production, height, number of leaves, and leaf area of the susceptible biotype were greater than those of the resistant biotype at all harvests under both light regimes. Root/shoot ratios were lower in the resistant than in the susceptible biotype under high light. Lower values for these parameters in resistant plants are due to lowered photosynthetic capacity, which limits growth and lowers relative root production. Net assimilation rate (NAR) was lower in resistant than susceptible plants when grown under high light, but mean leaf area ratio over a harvest interval (L) of resistant plants was higher than that of susceptible plants under high light. Relative growth rate (RGR) was higher for susceptible plants before day 42 and higher for resistant plants after day 42 under high light. A lower plastochron index (PI) of resistant plants up to 5 weeks old, relative to susceptible plants of the same age, suggests that, when grown under high light, resistant plants are developmentally at least 2 days behind susceptible plants. Shading lowered dry-weight production, height, number of leaves, leaf area, NAR, RGR, PI and root/shoot ratios of both biotypes. Values for these growth parameters for susceptible plants were similar to or larger than for resistant plants when grown under low light. In both biotypes, resource allocation patterns and L shifted towards increased shoot and leaf production in response to shading.

Growth chamber and field studies were conducted to investigate the possibility of an interaction between the herbicides 4-(methylsulfonyl)-2,6-dinitro-N,N-dipropylaniline (nitralin), a,a,a-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine (trifluralin), 3- (m-tri-fluromethylphenyl)-1,1-dimethylurea (fluometuron), and 2,4-bis(isopropylamino)-6-methylmercapto-s-triazine (prometryne), and the seedling disease organism Rhizoctonia solani Kuehn in cotton (Gossypium hirsutum L.). When high levels of trifluralin or prometryne were used in the growth chamber, an interaction with the pathogen that was injurious to the cotton occurred. Prometryne and fluometuron produced an antagonistic effect on R. solani.

Under field conditions, R. solani usually caused cotton injury. Interactions injurious to the cotton occurred between trifluralin and a low level of R. solani and between nitralin and a high level R. solani infestation. No interactions occurred with fluometuron or prometryne. In all instances, the effect occurred only when the herbicides were being used at high rates.

Metribuzin [4-amino-6-tert-butyl-3-(methylthio)-as-triazine-5(4H) one] injured and reduced growth of tomato (Lycopersicon esculentum Mill.) but did not inhibit germination or emergence of several other species. In cucumber (Cucumis sativus L.), metribuzin did not have a specific site of uptake, being equally toxic when placed in the root, shoot, or seed zone. Tomato was more susceptible to metribuzin when the plants were grown under high humidity, high temperature, maintained under low light intensities before spraying, had their leaves rubbed with glass wool, or had a surfactant in the spray solution. The increased toxicity under low light was not overcome by exogenous sucrose. About 4% of the available metribuzin was absorbed by tomato leaves or roots and was translocated in the xylem. Tomato plants 13 cm in height were less susceptible to metribuzin injury than 8-cm high plants.

Oxadiazon (10 μM) caused rapid, light-dependent membrane damage to cucumber cotyledon discs. Electrolyte leakage was detected within 1 h of exposure to light, as were cytoplasmic vesiculation and breakage of the tonoplast and plasmalemma. The ultrastructure of chloroplasts was not affected until the cytoplasm was dispersed. Photosynthetic inhibitors had no effect on activity and, after a period of dark incubation with oxadiazon, there was little effect of temperature on the light-caused membrane destruction. Porphyrin synthesis inhibitors (gabaculine and 4,6-dioxoheptanoic acid) almost completely prevented the herbicidal activity of oxadiazon. Oxadiazon treatment caused accumulation of protoporphyrin IX, a photodynamic pigment. Oxadiazon caused physiological effects on cucumber cotyledons that were virtually identical to those of p-nitro-diphenyl ether herbicides like acifluorfen and its methyl ester, which have recently been shown to also cause protoporphyrin IX accumulation.

We studied the effectiveness of glyphosate [N-(phosphono-methyl)glycine] for the control of Orobanche spp. in the field when applied to the crop foliage prior to the emergence of the parasite. The best Orobanche control in October-sown carrots (Daucus carota L.) parasitized by Orobanche crenata Forsk. and O. aegyptiaca Pers. and in December-sown broad bean (Vicia faba L.) and peas (Pisum sativum L.) parasitized by O. crenata was obtained by spraying twice, 2 weeks apart, in late January and in February, respectively. Carrot root yield remained unaffected by glyphosate at rates up to 200 g/ha. In heavily infested fields, carrots were irreversibly damaged by early Orobanche parasitism. The highest yields of pods of broad beans occurred after two or three applications of 150 g/ha glyphosate. Peas were seriously damaged by 150 g/ha. Two or three sprays of 50 g/ha glyphosate resulted in the highest green-kernel yield of pea where the soil was not heavily infested. Glyphosate effectively reduced O. cernua Loeffl. emergence in non-irrigated tomatoes (Lycopersicum esculentum Mill.), but was very phytotoxic to the crop. Glyphosate was highly effective in controlling Orobanche spp.; however, marginal selectivity may be sufficient in some crops and not in glyphosate-susceptible ones.

Field experiments were established in Greece from 1990 to 1993 to study the effect of littleseed and short-spiked canarygrass density on wheat and barley yield, as well as the influence of littleseed canarygrass removal time on wheat yield. Competitive ability of both canarygrass species in wheat was similar, though littleseed canarygrass showed faster growth rate and formed more panicles than short-spiked canarygrass. Grain yield of wheat was not significantly affected by the presence of 76 plants m−2 of either canarygrass species, but it was reduced by 36 to 39% by the presence of 304 canarygrass plants m−2. None of the canarygrass species at any density had any effect on grain yield of wheat when cold and dry conditions prevailed during the early stages of growth. Grain yield of barley was not affected by any density of either canarygrass species. Growth and consequently panicle number of both canarygrass species were severely reduced by the interference of barley. The littleseed canarygrass removal time study indicated that emergence of this weed was completed by the middle of February. Grain yield of wheat grown with 150 littleseed canarygrass plants m−2 was not affected even when these plants were removed early in April. However, the presence of these canarygrass plants until harvest reduced grain yield of wheat by 23 to 28%.