Experiments were conducted to evaluate a biotype of smooth pigweed that had survived applications of sulfonylurea (SU) and imidazolinone (IMI) herbicides in a single season. The source field had a history of repeated acetolactate synthase (ALS)-inhibiting herbicide use over several years. Whole-plant response experiments evaluated the resistant (R11) biotype and an ALS-inhibitor susceptible (S) smooth pigweed biotype to herbicides from the SU, IMI, pyrimidinylthiobenzoate (PTB), and triazolopyrimidine sulfonanilide (TP) chemical families. The R11 biotype exhibited 60- to 3,200-fold resistance to all four ALS-Inhibiting herbicide chemistries compared with the S biotype. Nucleotide sequence comparison of ALS genes from R11 and S biotypes revealed a single nucleotide difference that resulted in R11 having an amino acid substitution of aspartate to glutamate at position 376, as numbered relative to the protein sequence of mouseearcress. This is the first report of an amino acid substitution at this position of an ALS gene isolated from a field-selected weed biotype. To verify the role of this mutation in herbicide resistance, the ALS gene was cloned and expressed in Arabidopsis. Transgenic Arabidopsis expressing this ALS gene exhibited resistance to SU, IMI, PTB, TP, and sulfonylaminocarbonyltriazolinone ALS-Inhibiting herbicide classes.

In spring 2005, a downy brome population with possible resistance to fluazifop-P, an acetyl-CoA carboxylase (ACCase) inhibitor (group 1) herbicide was found in a commercial creeping red fescue seed production field, near La Grande, OR, where fluazifop-P had been used to control downy brome repeatedly over 7 yr. Greenhouse experiments were conducted to confirm resistance to a number of group 1 herbicides. The suspected resistant downy brome accession was tested for resistance to fluazifop-P and tested for cross-resistance to other aryloxyphenoxy propionate (APP) and cyclohexanedione (CHD) herbicides, including quizalofop-P, sethoxydim, and clethodim. Data recorded included plant-mortality counts and biomass. Tests revealed that the La Grande downy brome accession was highly resistant to fluazifop-P and sethoxydim at all tested rates. The La Grande accession suffered no mortality from fluazifop-P or sethoxydim treatments up to the maximum tested rate of eight times (8×) the labeled recommendation. The La Grande accession was resistant to quizalofop-P and clethodim at the labeled rate or less but was susceptible to application rates higher than the labeled rate. The control downy brome accession was completely susceptible to fluazifop-P, quizalofop-P, and clethodim at all rates and exhibited increasing susceptibility with increasing sethoxydim rate. This pattern of cross-resistance differs from that of a previously reported case of ACCase resistance in downy brome.

Weed genetic diversity is important for understanding the ability of weeds to adapt to different environments and the impact of herbicide selection on weed populations. Genetic diversity within and among six wild oat populations in China varying in herbicide selection pressure and one population in North Dakota were surveyed using 64 polymorphic alleles resulting from 25 microsatellite loci. Mean Nei's gene diversity (h) for six wild oat populations from China was between 0.17 and 0.21, and total diversity (HT) was 0.23. A greater proportion of this diversity, however, was within (Hs = 0.19) rather than among (Gst = 0.15) populations. For the wild oat population from the United States, h = 0.24 and HT = 0.24 were comparable to the values for the six populations from China. Cluster analysis divided the seven populations into two groups, where one group was the United States population and the other group included the six Chinese populations. The genetic relationships among six populations from China were weakly correlated with their geographic distribution (r = 0.22) using the Mantel test. Minimal difference in gene diversity and small genetic distance (Nei's distance 0.07 or less) among six populations from China are consistent with wide dispersal of wild oat in the 1980s. Our results indicate that the wild oat populations in China are genetically diverse at a level similar to North America, and the genetic diversity of wild oat in the broad spatial scale is not substantially changed by environment, agronomic practices, or herbicide usage.

Weeds compete with crops for light, soil water, and nutrients. Nitrogen (N) is the primary limiting soil nutrient. Forecasting the effects of N on growth, development, and interplant competition requires accurate prediction of N uptake and distribution within plants. Field studies were conducted in 1999 and 2000 to determine the effects of variable N addition on monoculture corn and velvetleaf N uptake, the relationship between plant N concentration ([N]) and total biomass, the fraction of N partitioned to leaves, and predicted N uptake and leaf N content. Cumulative N uptake of both species was generally greater in 2000 than in 1999 and tended to increase with increasing N addition. Corn and velvetleaf [N] declined with increasing biomass in both years in a predictable manner. The fraction of N partitioned to corn and velvetleaf leaves varied with thermal time from emergence but was not influenced by year, N addition, or weed density. With the use of the [N]–biomass relationship to forecast N demand, cumulative corn N uptake was accurately predicted for three of four treatments in 1999 but was underpredicted in 2000. Velvetleaf N uptake was accurately predicted in all treatments in both years. Leaf N content (NL, g N m−2 leaf) was predicted by the fraction of N partitioned to leaves, predicted N uptake, and observed leaf area index for each species. Average deviations between predicted and observed corn NL were < 88 and 12% of the observed values in 1999 and 2000, respectively. Velvetleaf NL was less well predicted, with average deviations ranging from 39 to 248% of the observed values. Results of this research indicate that N uptake in corn and velvetleaf was driven primarily by biomass accumulation. Overall, the approaches outlined in this paper provide reasonable predictions of corn and velvetleaf N uptake and distribution in aboveground tissues.

Pitted morningglory seed were collected in the fall of 2003 from Blackville, SC, and 2004 from Pendleton, SC, to assess the effect of After-Ripening and burial on light and temperature requirements for germination. Pitted morningglory germination was evaluated over a 12-mo period after maturation. Germination was neither stimulated by red light or inhibited by far-red light, nor was it reversible by red or far-red light. Light was not essential for germination of buried seed. Direct exposure to sunlight prevented germination of recently mature seed, but not once seed had sufficiently after-ripened. Pitted morningglory was capable of germination in darkness over a wide range of constant and fluctuating temperatures immediately after maturation. Germination in response to temperature varied with time of year after maturation, with the population from Pendleton having increased germination in May. Thermal fluctuations increased germination of both populations at suboptimal temperatures. Thermal amplitude regulation of germination varied over time and appeared to play a more important role in germination of after-ripened seed than recently mature ones. The ecological significance of changes in germination requirements with After-Ripening is discussed.

Large proportions of nonhybrid progeny result from controlled crosses between Palmer amaranth and common waterhemp, both dioecious weeds. Agamospermy was proposed as an explanation for this phenomenon, and here we provide evidence in support of this hypothesis. We evaluated 60 nonhybrid offspring from two interspecific crosses, and all plants were females and had DNA content values similar to the female (Palmer amaranth) parent. Among nine hybrids resulting from these crosses, eight were nonviable (lethal or neuter), and only one hybrid allowed for continued gene movement. Cytogenetic evaluation of this hybrid revealed triploidy, further supporting the occurrence of unreduced gametes in these species. In light of this new evidence, we examine earlier data regarding Palmer amaranth by common waterhemp hybridization and suggest some prior conclusions may be premature.

Weed seed persistence in the soil seedbank is central to weed population dynamics; however, limited knowledge of mechanisms regulating seed survival in soil remains an obstacle to developing seed-bank management practices. Weed seeds are rich in carbon and nitrogen, and therefore may represent an important nutritional resource to soil microbes. The objective of this study was to test the hypothesis that weed seed mortality due to microbial predation is limited by soil inorganic N availability and soil C:N ratio. A factorial of N fertilizer rate (0, 14, and 28 mg N kg soil−1) and corn stover addition rate (0 and 3,000 mg stover kg soil−1) was applied to bioassay units containing Illinois field soil (silt loam, 3.8% organic carbon) and seeds of one of eight annual weed species common to Illinois field crops: giant foxtail, green foxtail, yellow foxtail, wooly cupgrass, giant ragweed, redroot pigweed, velvetleaf, and Venice mallow. Seeds were incubated for 2 mo, after which they were recovered from the soil and tested for viability. Only three of the eight species, velvetleaf, giant ragweed, and wooly cupgrass, responded to the experimental treatments. Velvetleaf seed mortality was 40% lower in the corn stover–amended treatment than in the unamended treatment. Both giant ragweed and wooly cupgrass showed a more complex interaction between N fertilizer and corn stover treatments. Path analysis supported the hypothesis that the influence of soil N on seed mortality in velvetleaf was because of the direct effect of soil N on microbial predation of velvetleaf seeds, whereas for giant ragweed and wooly cupgrass, the effect on seed mortality appeared to be mediated through soil N effects on germination. Mechanisms underlying soil N fertility effects on weed seed mortality appear to be species-specific. Future investigations of this phenomenon should include quantitative measures of seed coat composition and quality.

The importance of managing weeds in seminatural habitats that are adjacent to farm fields is unclear. Weedy-margin vegetation may harbor pests or pathogens and may ALSo serve as source populations for ongoing immigration of weeds into the field. It is ALSo possible, however, that margin vegetation provides habitat for organisms that consume weed seeds or suppress the likelihood of pest or pathogen outbreak. We examined the nature of margin habitat using spatial-scaling of Weed-Species richness as an ecological assay. In 2003, we recorded the occurrence of weedy species along the perimeters of 63 fields in Wisconsin. The fields were distributed within six counties that differed in topography, geological history, local climate, and soil type and which spanned the range of variability in the agricultural landscape. We identified seven habitats that differed in geology and land use. The relationship between species richness and margin class was estimated using an analog of the power law. Additionally, we investigated broadscale correlates of habitat heterogeneity at the field level, using a modeling strategy that included additional explanatory factors logically connected to plant diversity. Using a model-confrontation approach, the survey supported the inclusion of two topographical diversity indices, elevation gradient and a field-shape index, into our model. Our broadscale survey provides information on one of a suite of important considerations needed to make decisions about the importance of managing weeds in field margins.

The exotic Brazilian peppertree is a serious invader of both disturbed and natural areas in central and south Florida, forming fast-growing, impenetrable thickets that dominate entire ecosystems. Brazilian peppertree has been targeted for biocontrol, and two defoliating insect species may eventually be released. This study was done to consider the possible effectiveness of defoliating biocontrol agents. The research investigated the effects of different frequencies of defoliation on height, crown diameter, and berry production of young Brazilian peppertrees. All the foliage was manually clipped from 36 trees in field plots once or twice per year for ≥ 1 yr. The effect on berry production of clipping 100% of the leaves from scattered individual branches of one large Brazilian peppertree was ALSo examined. Trees that were completely defoliated five times at 6-mo intervals were significantly smaller and produced significantly fewer fruits than undamaged controls. Plants defoliated one time only, two times in 1 yr, and two times in each of 2 yr were comparable to the undamaged controls. From this simulated herbivory study, we infer that multiple defoliations by insect defoliators have the potential to significantly suppress the growth and fruit production of Brazilian peppertree in Florida.

Southern Root-Knot nematode and common cocklebur interfere with cotton growth and yield. A greater understanding of the interaction of these pests with cotton growth and yield is needed for effective integrated pest management (IPM). An additive design was used in outdoor microplots with five common cocklebur densities (0, 1, 2, 4, and 8 plants per plot) growing in competition with cotton, with and without the presence of southern Root-Knot nematode. Differences in cotton height could not be detected among common cocklebur densities or nematode presence at 3 wk after transplanting (WAT); however, differences in crop height were observed at 5 WAT between nematode treatments. In the absence of nematodes, the relationship between cotton yield loss and common cocklebur density was described by a rectangular hyperbolic regression model (P < 0.0001). Maximum yield loss from common cocklebur in the absence of nematodes exceeded 80%. In the presence of nematodes, there was a linear relationship between cotton yield loss and common cocklebur density (P = 0.0506). The presence of nematodes at each common cocklebur density increased cotton yield loss 15 to 35%. Common cocklebur plant biomass was 25% greater in nematode treatments, likely because of the reduced competitiveness of the cotton plants in these plots. This study demonstrates that multiple pests can interact to cause an additive reduction in crop yield.

Experiments were conducted to determine the efficacy of glyphosate on four common lambsquarters populations collected from Kansas, Nebraska, North Dakota, and Ohio. Glyphosate dose-response studies for common lambsquarters treated at 2.5-, 7.5-, and 15-cm heights showed that glyphosate at 1.1 kg ae ha−1 caused more than 80% injury to 2.5-cm plants but less than 55% injury to 7.5- and 15-cm plants. All populations were susceptible to glyphosate at the 2.5-cm height. The glyphosate rate required to cause 50% injury (GR50) was 430, 500, 500, and 560 g ha−1 for the Kansas, North Dakota, Ohio, and Nebraska populations, respectively. Differential response of common lambsquarters populations was evident with 15-cm plants where the GR50 was glyphosate at 1,010, 1,230, 1,650, and 2,770 g ha−1 for the Kansas, North Dakota, Nebraska, and Ohio populations, respectively. Reduced injury on 15-cm common lambsquarters plants by glyphosate may be partly attributed to reduced glyphosate accumulation per unit of plant tissues and enhanced calcium content in more-developed plants. All four common lambsquarters populations at the early seedling stage were susceptible to glyphosate, but tolerance increased as the plant developed and the extent of tolerance differed among populations.

Broomrapes (Orobanche spp.) are chlorophyll-lacking root parasites of many dicotyledonous species that cause severe damage to vegetables and field crops worldwide. Egyptian broomrape is common throughout Israel, where it parasitizes a wide range of crops. The main damage caused by this parasite is to processing tomatoes. The objectives of this study were to determine (1) the relationship between the parasitism process on tomato roots and Egyptian broomrape seed depth, and (2) the effect of herbicide application on Egyptian broomrape control at depths of 0 to 30 cm. The study was conducted in controlled conditions in 25-L pots containing soil that was artificially infested with Egyptian broomrape seeds. Seeds (1-cm layer) were infested at soil surface and at depths of 6, 12, 18, 24, and 30 cm. Sulfosulfuron was applied PRE to Egyptian broomrape shoots, 14 and 42 d after tomato planting. Egyptian broomrape emergence was highly correlated to the depth of the seed infestation: the deeper the seeds were buried, the more pronounced the delay in Egyptian broomrape emergence. A four-parameter sigmoid equation was found that describes the relationship between Egyptian broomrape parasitism and seed depth. No Egyptian broomrape shoots were observed aboveground when sulfosulfuron was applied on tomato foliage. However, live and dead attachments were observed at different levels in all seed-depth treatments. No significant difference in the efficacies of sulfosulfuron at rates of 37.5 and 75 ai ha−1 was observed at depths of 6, 12, and 18 cm. The best control was obtained at a depth of 6 cm at both sulfosulfuron rates. At depths of 24 and 30 cm, no broomrape control was achieved at 37.5 ai g ha−1 sulfosulfuron, and only low control efficacy was obtained at 75 ai g ha−1.

Two polymeric anion-exchange resins and one sorbent resin were evaluated for their propensity to adsorb, and subsequently desorb, oryzalin. The intent was to determine whether these resins could adsorb and subsequently release oryzalin in a manner that would render these resins as an option for slow-release herbicide delivery. The dinitroaniline herbicide oryzalin is weakly acidic with a dissociation constant (pK a) of 8.6. An additional objective was to determine whether altering the pH between sorption and desorption would enhance the desired performance. Maximum oryzalin sorption by the two anion-exchange resins was between 127 and 132 mg g−1 ai. The sorbent resin was adsorbed at a maximum concentration of 191 mg g−1 ai. Maximum sorption occurred with the pH 10 solutions with all resins. Average oryzalin desorption by the anion-exchange resin was between 0.12 and 3.84 mg g−1 per desorption event. Maximum desorption occurred at pH 6.0. Results reveal that the resins evaluated may have merit for slow-release herbicide delivery.

Toadflax invasion into natural areas has prompted interest in weed management via biological control. The most promising biological control agent currently available for the control of Dalmatian toadflax is Mecinus janthinus, a stem-boring weevil that has been shown to significantly reduce toadflax populations. Some land managers, however, are reluctant to release approved weed biological control agents based on concerns about possible nontarget impacts. Few postrelease examinations of biocontrol impact and host specificity have been performed, despite the call for such information. This study examined the host specificity of Mecinus janthinus, postrelease, in relation to Colorado sites to provide information to managers about its relative safety as a weed biological control agent. This study employed three components: (1) greenhouse choice and no-choice experiments; (2) no-choice caged field experiments; and (3) release-site evaluation of nontarget use of native plant species where this weevil has been released and has established. Both greenhouse and field experiments failed to demonstrate nontarget use of native plant species by M. janthinus in the region where it was studied, even in no-choice starvation tests. We found no evidence of nontarget herbivory on native plants growing at toadflax sites where M. janthinus was well established. These results support the continued use of M. janthinus as a low-risk biological control agent for the management of toadflax in the Rocky Mountain Front Range.

Quinclorac drift has been speculated as the cause of injury to tomato crops throughout northeast Arkansas. In this study, we set out to determine whether tomato plant injury and yield reduction were correlated with simulated quinclorac drift. Experiments were carried out at Fayetteville, AR, in 1999 and 2000. Maximum plant injury (visual ratings) was about 20% when plants were treated with one, two, or three quinclorac applications (weekly intervals beginning at first flower) at 0.42 g ai ha−1 (0.001 times the normal use rate to simulate drift). Maximum plant injury ranged from 48 to 68% with quinclorac simulated drift treatment of 42 g ha−1. Overall, increasing quinclorac rate and number of applications increased tomato injury. In both years, tomato plant fresh-weight accumulation was not influenced by one, two, or three applications of quinclorac at 0.42 g ha−1 compared with the untreated control. In 1999, increasing the rate of quinclorac from 0.42 to 4.2 g ha−1 reduced plant fresh-weight accumulation. In 2000, there was no significant difference in plant fresh weight when plants were treated with quinclorac at 2.1 to 4.2 g ha−1. Evaluation of the herbicide rate effect indicated that quinclorac at 0.42 g ha−1 did not reduce tomato fruit yield (total weight of edible fruit) compared with the untreated control, but yield decreased as rate increased above 0.42 g ha−1. Increasing the number of applications generally decreased tomato yield, and overall as maximum visual plant injury increased, tomato yield reduction ALSo increased linearly. We conclude that quinclorac at simulated drift rates can adversely affect tomato plant growth and yield.

Invasive plant species have adversely affected rangelands throughout the world and continue to invade previously uninfested lands at an alarming rate. Previous efforts have focused on eradication and control; however, recent efforts have recognized that preventing invasive plant species from infesting new areas is more cost-effective and efficient than trying to restore the system after it is infested. One of the major components of prevention is limiting the introduction of the invasive plant to uninfested areas. Guidelines to limit the introduction of invasive plants into new areas are usually general and not developed to address differences in dispersal vectors among invasive plants. To limit the dispersal of invasive plants, land managers need a framework that assists them in identifying major spatial dispersal vectors and management strategies based on those vectors. We propose an initial conceptual framework that integrates the ecology of invasive plant dispersal with prevention management. The framework identifies major potential vectors by incorporating invasive plant seed adaptations for dispersal through space and infestation locations relative to vector pathways. The framework then proposes management strategies designed to limit dispersal by those specific vectors. The framework ALSo identifies areas where research could improve the effectiveness of dispersal-prevention strategies by providing additional management tools.

A recent paper (Shields et al. 2006) reports trapping seed of horseweed in the planetary boundary layer (PBL) of the lower atmosphere at heights up to 140 m above ground level. While it claims that seeds have not been trapped from the PBL before, it is shown here that seeds of other weedy composites were trapped at altitudes up to 610 m above ground level in southern Australia more than 40 yr ago.