Invasive species and acid rain cause global environmental problems. Creeping daisy, an invasive exotic allelopathic weed, has caused great damage in southern China, where acid rain is prevalent. The impact of the acidity of simulated acid rain (SAR) on soil nutrients, the decomposition of creeping daisy litter, and on the allelopathic potential of the surrounding soils was investigated. Litter was treated with SAR at different acidity (pH 2.5, 4.0, 5.6) or with water (pH 7.0) as a control. After 70 d, the remaining amount of creeping daisy litter, nutrient contents, and allelopathic potentials in the surrounding soil were determined. The litter decomposition was commensurate to the increase in the acidity of the SAR. Total C and N contents, NO3 −-N and available P increased, levels of NH4 +-N, the ratio of C/N and soil pH values decreased, water contents increased and then decreased, whereas available K did not significantly change in the soil surrounding the litters in response to the increase in the acidity of the SAR. Bioassays showed that SAR promoted the allelopathic activity in the soil surrounding the litter, as measured by seedling growth of turnip and radish. In conclusion, our results indicated that SAR influenced soil nutrient status, accelerated creeping daisy litter decomposition, and enhanced the allelopathic potential of its litter in the surrounding soil, suggesting that acid rain may enhance the invasiveness of creeping daisy plants.

Rapid adoption of glyphosate-resistant (GR) corn hybrids has led to the reemergence of volunteer corn as a problematic weed in soybean and has made controlling the initial stand of corn in a replant situation more difficult. If volunteer corn in soybean or the initial corn stand in a replant situation is not controlled, yield loss can occur. Clethodim and glufosinate are often used to control GR corn in corn replant situations and in soybean. The objectives of this research were to evaluate the response of two hybrid corn varieties and their F2 progeny to clethodim and glufosinate and to evaluate the effect of plant nitrogen (N) concentration on clethodim and glufosinate efficacy. First, a dose-response study was conducted with clethodim and glufosinate on DeKalb 60-18 and 60-18F2, and DeKalb 63-42 and 63-42F2 to compare the response of the hybrids and their F2 progeny to the herbicides. DeKalb 63-42 was more tolerant to clethodim than 60-18 and 60-18F2. No differences were found between the hybrids and their respective F2 progeny in the response to clethodim or glufosinate. In a second dose-response study assessing the effect of N conditions on herbicide efficacy, both clethodim and glufosinate were less injurious to plants growing in low N than in high N availability.

Striga is a major constraint to food production in Africa. Most technologies developed for the eradication of Striga asiatica from the United States are not adaptable to Africa. Imazapyr and pyrithiobac coated imidazolinone-resistant (IR)-resistant maize seed prior to planting at rates of 30 to 45 g ha−1 provide near season long control of Striga and can increase maize yields three- to fourfold if supplied with fertilizer. Slow release seed coatings reduce maize injury when post-planting rains are sparse and improve Striga control when there is excessive rainfall early in the season. Models suggest that herbicide resistance may not be a significant threat in short season maize, but vigilance in removing flowering Striga plants that are not controlled is recommended due to the known risk of evolution of resistance to these herbicides. Stacking the IR gene with glyphosate resistance and using imazapyr treated seed and applying glyphosate mid-season would provide season long Striga control and delay the evolution of resistance to both herbicides. To date, adoption of this technology has been limited by a number of factors. However, it should be included as one component of a multi-factor approach to increasing maize productivity in areas of Africa where Striga is problematic.

The increasing use of synthetic chemicals for pest control in rice has become an overwhelming economical border, and more important, it could pose a serious threat of the environment. In addition, the extensive use of synthetic herbicides has been the cause for the evolution of herbicide-resistant barnyardgrass worldwide. This weed species is the most competitive weed in rice after the red rice. Thus, this study was conducted to examine the combination effects of aqueous sunflower leaf extracts with lower rate of pretilachlor on barnyardgrass emergence and growth in Marang (sandy loam) and Seberang (silt loamy) soil series under glasshouse conditions. Interestingly, the ED95 values (rate that causes 95% inhibition) of pretilachlor for emergence and shoot fresh weight (SFW) of barnyardgrass were reduced by 79 and 82%, respectively, when being mixed with sunflower leaf extracts in Marang series. In contrast, the addition of sunflower leaf extracts increased ED95 value of pretilachlor in Seberang series. Rice seedlings at 4 and 8 d after sowing (DAS) were found to be tolerant to this mixture treatment. However, root growth of rice seedlings were inhibited at 0 and 2 DAS. These results suggest that sunflower leaf extracts have potential to reduce rate of pretilachlor for inhibiting emergence and growth of barnyardgrass without injuring rice seedlings in rice fields depending on soil types and growth stage of rice.

Using an oriental mustard root length bioassay, thiencarbazone bioavailability and dissipation in five Saskatchewan soils was investigated under laboratory conditions. Thiencarbazone bioavailability was assessed at 0 to 3.9 µg ai kg−1. Thiencarbazone concentrations corresponding to 50% inhibition (I50 values) obtained from dose-response curves varied from 0.56 to 1.71 µg kg−1. Multiple regression analysis indicated that organic carbon content (P = 0.018) and soil pH (P = 0.017) predicted thiencarbazone bioavailability. Thiencarbazone dissipation was examined in soils incubated at 23 C and moisture content of 85% field capacity. Thiencarbazone half-lives estimated from dissipation curves were 9 to 50 d, and organic carbon content (P = 0.002) and soil pH (P = 0.008) were significant factors affecting thiencarbazone dissipation. Thiencarbazone bioavailability decreases and dissipation rate is slower in Canadian prairie soils of high organic matter content and low soil pH. Because root length of oriental mustard plants also was reduced by ammonium, therefore ammonium-containing or -producing fertilizers can cause false positive results for thiencarbazone soil residues. Canaryseed roots had the same sensitivity to ammonium as oriental mustard roots but were not inhibited by thiencarbazone. Therefore canaryseed root length bioassay was effective in identifying inhibition caused by ammonium toxicity. Use of oriental mustard root and canaryseed root bioassays together can aid in interpreting bioassay results for detection of thiencarbazone residues.

Glyphosate is widely used for weed control in the grape growing industry in southern Australia. The intensive use of glyphosate in this industry has resulted in the evolution of glyphosate resistance in rigid ryegrass. Two populations of rigid ryegrass from vineyards, SLR80 and SLR88, had 6- to 11-fold resistance to glyphosate in dose-response studies. These resistance levels were higher than two previously well-characterized glyphosate-resistant populations of rigid ryegrass (SLR77 and NLR70), containing a modified target site or reduced translocation, respectively. Populations SLR80 and SLR88 accumulated less glyphosate, 12 and 17% of absorbed glyphosate, in the shoot in the resistant populations compared with 26% in the susceptible population. In addition, a mutation within the target enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS) where Pro106 had been substituted by either serine or threonine was identified. These two populations are more highly resistant to glyphosate as a consequence of expressing two different resistance mechanisms concurrently.

In a 4-yr field study, “weed suppressive” rice cultivars provided 30% greater control of barnyardgrass and sustained 44% less yield loss (relative to weed-free) compared to “nonsuppressive” tropical japonica rice cultivars. 13C analysis revealed that rice root mass predominated vertically and laterally within the soil profile of plots infested with barnyardgrass. Among all cultivars, rice roots accounted for 75 to 90% of the total root mass in samples, and this was most concentrated in the surface 5 cm of soil in the row. Barnyardgrass roots were most prevalent in the surface 5 cm between rows where they accounted for 30% of total root mass. Overall, barnyardgrass root mass was about twice as high in nonsuppressive rice compared to suppressive rice. Weed suppression by indica/tropical japonica rice crosses generally was intermediate between that of the other two rice groups. At the 0- to 5-cm depth, between-rows, barnyardgrass root mass was correlated negatively with rice height (r = −0.424), yield (r = −0.306), and weed control ratings (r = −0.524) in weedy plots. Control ratings in weedy plots also were negatively correlated with rice percent height reduction (r = −0.415) and % yield loss (r = −0.747) relative to weed-free plots, and with barnyardgrass root mass as a percent of total root mass (r = −0.612). Control ratings were positively correlated with rice yield under weed pressure (r = 0.429) but were correlated with rice root mass in-rows only (r = −0.322). Clearly, rice root mass could not have been the major cause of the differences in barnyardgrass control between cultivars.

Parasitic weeds such as Phelipanche and Orobanche are obligate holoparasites that attack roots of almost all economically important crops in semiarid regions of the world. A wide variety of parasitic weed control strategies (chemical, biological, cultural, and resistant crops) has been tried. Unfortunately, most are partially effective and have significant limitations. The current mini review will discuss the needs for alternative methods and will summarize current and new biotechnology-based approaches for broomrape control. At present, we have generated transgenic tobacco plants expressing a cecropin peptide (sarcotoxin IA), under the control of the inducible HMG2 promoter. Transgenic lines enhanced host resistance to the parasitic weed; transgenes showed higher numbers of aborted parasitization events, reduced Phelipanche biomass, and increased host biomass. Sarcotoxin IA had no obvious effect on growth and development of transgenic host plants. Mannitol content in the parasite is regulated by Mannose 6-Phosphate Reductase (M6PR) gene, an essential process to broomrape species for water and nutrient uptake from the host. In our study, we used the inverted repeat technique to silence the parasite target gene, M6PR. In this study it was shown that the endogenous M6PR mRNA from P. aegyptiaca tubercles or shoots grown on transgenic tomato plants harboring the M6PR silencing construct was reduced by 60 to 80%. The number of dead tubercles was also increased significantly on transgenic plants as compared with the control plants. The strategies presented here are potentially superior to other methods in that they are effective, have a low cost of implementation for producers, and are safe for the environment.

The herbicides mesotrione and topramezone inhibit 4-hydroxyphenylpyruvate dioxygenase (HPPD) and have efficacy against smooth crabgrass. Research was conducted to determine the impacts of soil-applied nitrogen (N) fertilizer on the effectiveness of single applications of mesotrione and topramezone for postemergence smooth crabgrass control. Field experiments in 2010 and 2011 evaluated the efficacy of mesotrione (280 g a.i. ha−1) and topramezone (9 g a.i. ha−1) for control of multitiller smooth crabgrass subjected to five N fertility treatments (0, 12, 25, 37, or 49 kg N ha−1). Greenhouse experiments evaluated the response of smooth crabgrass to mesotrione (0, 70, 140, 280, 560, and 1,120 g ha−1) and topramezone (0, 4.5, 9, 18, 36, and 72 g ha−1) with 0 or 49 kg N ha−1. Further research evaluated changes in smooth crabgrass leaf tissue pigment concentrations following treatment with mesotrione (280 g ha−1) and topramezone (18 g ha−1) with 0 or 49 kg N ha−1. In field experiments, N increased smooth crabgrass control with mesotrione and topramezone for 8 wk; however, increasing N rate above 25 kg ha−1 did not improve control on any rating date. In dose-response experiments, N application reduced I50 values for mesotrione and topramezone by 67 and 53%, respectively, 21 d after treatment (DAT). Reductions in aboveground biomass with both herbicides were greater when applied following N treatment as well. In leaf-response experiments, N decreased new leaf chlorophyll and carotenoid concentrations and new leaf production after treatment with topramezone. Future research should investigate whether increased translocation of these herbicides to meristimatic regions contribute to N-enhanced efficacy.

The Parasitic Plant Genome Project has sequenced transcripts from three parasitic species and a nonparasitic relative in the Orobanchaceae with the goal of understanding genetic changes associated with parasitism. The species studied span the trophic spectrum from free-living nonparasite to obligate holoparasite. Parasitic species used were Triphysaria versicolor, a photosynthetically competent species that opportunistically parasitizes roots of neighboring plants; Striga hermonthica, a hemiparasite that has an obligate need for a host; and Orobanche aegyptiaca, a holoparasite with absolute nutritional dependence on a host. Lindenbergia philippensis represents the closest nonparasite sister group to the parasitic Orobanchaceae and was included for comparative purposes. Tissues for transcriptome sequencing from each plant were gathered to identify expressed genes for key life stages from seed conditioning through anthesis. Two of the species studied, S. hermonthica and O. aegyptiaca, are economically important weeds and the data generated by this project are expected to aid in research and control of these species and their relatives. The sequences generated through this project will provide an abundant resource of molecular markers for understanding population dynamics, as well as provide insight into the biology of parasitism and advance progress toward understanding parasite virulence and host resistance mechanisms. In addition, the sequences provide important information on target sites for herbicide action or other novel control strategies such as trans-specific gene silencing.

Microstegium vimineum is a widespread invasive grass that poses significant threats to forests and disturbed areas throughout the United States. Often, the large-scale, rapid spread of Microstegium prohibits management by traditional methods. Control of Microstegium may be possible through the use of a pathogen (referred to here as Bipolaris Mv) that causes leaf blight on Microstegium. Members of the fungal genus Bipolaris are known pathogens of many plants, including important agronomic crops. However, little is known about the biology and host range of Bipolaris Mv. We used a series of growth chamber and light bank experiments to determine the variation in Bipolaris Mv from different geographic origins and its ability to cause foliar lesions and chlorosis on Microstegium. We used petri plate and soil infestation assays to determine the effects of Bipolaris Mv on Microstegium emergence from seed, biomass, and root necrosis. Finally, we tested the host range of these fungi on economically and ecologically important plant species. All isolates increased disease on Microstegium foliage relative to controls, although the effects varied among isolates. Isolates increased root necrosis by 97% in petri plate assays and by 4% in soil infestation trials compared to controls. Infestation of soils with Bipolaris Mv reduced emergence of Microstegium from seed by 31% compared to controls, but did not affect root or stand biomass. Bipolaris Mv produced lesions on a range of grasses, including corn, sorghum, rye, and wheat, although lesion size varied with isolate. These results indicate that Bipolaris Mv may be an effective pathogen on Microstegium, but its use as a bioherbicide may be impractical because of its effects on a wide range of grasses.

The witchweeds, members of the genus Striga, are noxious and persistent pests in farmers' fields and serious constraints to crop productivity throughout Africa, India, and Southeast Asia. Among the primary hosts for Striga are the major cereals (maize, sorghum, rice, and millet) and grain legumes (cowpea) that are important food staples worldwide. The negative impact of parasitic plants on crop productivity increases globally each year, and their potential for affecting domestic agriculture looms larger as the movement of seed resources expands on a global scale. At the present time there is a limited understanding of how Striga and other parasitic plants select a suitable host and overcome the innate defense responses of the host in order to complete their life-cycle. In the grasses most reported resistance to Striga appears to be polygenic with a large genotype by environment interaction. In contrast, resistance to S. gesnerioides in cowpea is conferred by single dominant genes functioning in a race-specific manner suggesting that a gene-for-gene mechanism similar to effector-triggered immunity (ETI) described in other host–pathogen interactions is likely operating in these parasite-host associations. A hallmark of ETI is the direct or indirect recognition of parasite-derived avirulence (Avr) factors and other effectors that interfere with plant innate immunity by host sensors (or R proteins) leading to activation of defense responses. The recent cloning and functional characterization of a race-specific R gene from cowpea encoding a canonical coiled-coil (CC)-nucleotide binding site (NBS)-leucine-rich repeat (LRR) type R-protein opens the door for further exploration of the mechanism of host resistance and provides a focal point for studies aimed at uncovering the molecular and genetic factors underlying parasite virulence and host selection. The potential for the development of novel strategies for parasite control and eradication based on parasite virulence factors is discussed.

Broomrapes (Orobanche and Phelipanche spp.) are obligate root parasites that spend most of their life cycle in the soil subsurface, making them hard to detect. In these underground developmental stages, broomrapes are highly sensitive to herbicides, and therefore knowledge of the dynamics of their parasitism is essential to precisely apply herbicide for their control. To address these complexities, two approaches have been proposed: (1) estimating the temporal variation in parasitism dynamics and predicting broomrape parasitism on its host by thermal time; (2) characterizing the spatial variation in infestation within and between fields by using a geographical information system and a global positioning system. In addition, the use of molecular markers to identify broomrape infestation (species and amount) in the field can contribute to determining its spatial distribution, which can then be used for site-specific weed management. In this paper, we discuss how technology can be optimized for control of the root-parasitic broomrapes. Special attention is given to the development of integrative approaches. An example of a decision support system for the rational management of Egyptian broomrape in processing tomato is given.

Herbicides are the basis for conventional management of purple nutsedge, one of the world's most troublesome weeds. However, as concern rises over their environmental impact, farmers are being required to reduce herbicide usage. Herbicide efficacy is strongly affected by weed growth stage and density at application, and when herbicides are applied under optimal conditions, low rates can provide maximal control efficacy (CE). Therefore, this study aimed to determine the time window for control of purple nutsedge using a low rate of herbicide, based on an effective degree days (EDDs) model, at low (one tuber) and high (10 tubers) densities. Two experiments were performed under field conditions, in the summers of 2009 and 2010. Rate of 3.75 g a.i. ha−1 trifloxysulfuron was applied once on each of five individual application dates. The growth of both treated and untreated plots was evaluated by means of leaf cover area (LCA) and biomass, which were then used to establish the time window for control. Results showed differences in both growth parameters between low and high tuber densities. The high-density patches reached LCA and fresh biomass values of 1,367 g and 1.12 m2, respectively, compared to 604 g and 0.69 m2, respectively, in the lower density patches. The favorable control periods based on biomass and LCA for the lower density patches were set to later dates than those for the higher density patches, 626 EDD compared to 483 EDD for biomass, and 786 EDD compared to 502 EDD for LCA, respectively. Although differences between the biomass- and LCA-based favorable control periods were observed at both tuber densities, the computed linear relations between the two growth parameters enabled adjusting them and setting the appropriate control period.

Previous research has shown that both the density and spatial pattern of wheat have an influence on crop growth and weed suppression, but it is not clear what degree of uniformity is necessary to achieve major improvements in weed suppression. Field experiments were performed over 3 yr to investigate the effects of crop density and different spatial distributions on weed suppression. The spatial pattern of spring wheat sown in five patterns and three densities in small weed-infested plots were analyzed with the use of digitized photographs of field plots to describe the locations of individual wheat plants as x and y coordinates. We used a simple quantitative measure, Morisita's index, to measure the degree of spatial uniformity. Increased crop density resulted in reduced weed biomass and increased crop biomass every year, but crop pattern had significant effects on weed and crop biomass in the first year only. Weather conditions during the second and third years were very dry, resulting in very low weed biomass production. We hypothesize that water deficiency increased the importance of belowground relative to aboveground competition by reducing biomass production, making competition more size symmetric, and reducing the effect of crop spatial pattern on weed growth. The results indicate that increased crop density in cereals can play an important role in increasing the crop's competitive advantage over weeds, and that spatial uniformity maximizes the effect of density when low resource levels or abiotic stress do not limit total biomass production.

Volunteer corn (VC) in hybrid corn has become more prevalent in recent years and can reduce grain yield. Nitrogen (N) management can influence VC interference in corn. Field experiments were established to determine the effects of N fertilizer management and VC interference on hybrid corn growth and grain yield. Treatments consisted of three VC densities (control, 0 plants m−2; low density, 1 plant m−2; high density, 4 plants m−2) and six N fertilizer treatments (0 kg N ha−1, 67 kg N ha−1 at planting, 67 kg N ha−1 at planting + 133 kg N ha−1 at V5 corn growth stage, 67 kg N ha−1 at planting + 133 kg N ha−1 at V10 corn growth stage, 200 kg N ha−1 at V5 corn growth stage, and 200 kg N ha−1 at V10 corn growth stage). The effect of VC on hybrid corn was dependent on N rate. When 200 kg N ha−1 was applied, regardless of application timing, hybrid corn dry weight, hybrid corn N content, and hybrid corn grain yield were reduced by the high VC density. However, when VC grain yield was added to hybrid corn grain yield, VC density did not affect total grain yield. When 0 and 67 kg N ha−1 were applied, neither hybrid corn dry weight nor hybrid corn N content was affected by either VC density, but the high VC density reduced hybrid corn grain yield for both N rates by 19% and total grain yield by 9 and 10%, respectively. Application timing of N fertilizer had no effect on hybrid corn dry weight, N content, or grain yield. However, late N fertilizer applications (200 kg N ha−1 at V10 and 67 kg N ha−1 at planting +133 kg N ha−1 at V10) resulted in greater VC N content, VC grain yield, and total yield. Assuming the harvestability of VC, the ability of a late N treatment (V10) to maximize total grain yield allows growers to use a late N application to reduce the competitive effects of VC in hybrid corn.

A field study was conducted on an s-triazine–adapted soil to determine the effects of s-triazine exclusion interval (1, 2, 3, or 4 yr), crop production system (continuous corn or continuous soybean), and rhizosphere proximity (bulk or rhizosphere soil) on atrazine degrader populations and activity. Atrazine degrader populations were quantified by a radiological Most Probable Number technique, while degrader activity was assessed via mineralization of ring-labeled 14C-atrazine. As the s-triazine exclusion interval increased, atrazine degrader populations declined exponentially, regardless of crop or rhizosphere proximity. Crop and exclusion interval interacted to affect degrader populations (P = 0.0043). Pooled over rhizosphere and bulk soil, degrader populations were 1.5-fold higher and declined 2.8-fold faster in soybean than corn. An interaction between rhizosphere proximity and exclusion interval was also noted (P = 0.0021), whereby degrader populations were 1.9-fold higher and declined 2.8-fold slower in rhizosphere compared with bulk soil, regardless of crop. The time required for 50% mineralization of ring-labeled 14C-atrazine (DT50) following exclusion of s-triazine herbicides increased linearly at a rate of 2.2 d yr−1. In contrast, the DT50 for this site prior to a known s-triazine application was 85 d and declined exponentially over 5 yr of successive atrazine applications: 24.5 d after 1 yr, 10.8 d after two successive years, and 3.8 d after five successive atrazine applications. Omitting s-triazines can reduce degrader populations and activity in adapted soils, but more than 4 yr is required to return mineralization kinetics to nonadapted levels, regardless of crop or rhizosphere proximity.