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International audience; Ambrosia trifida L. (giant ragweed, Asteraceae) is native to the North American continent and was introduced into Europe and Asia at the end of the 19th century. In its native range, this tall annual species is common in riparian and ruderal habitats and is also a major weed in annual cropping systems. For nearly a century, A. trifida has also been of great concern in the U.S. for its highly allergenic pollen, necessitating targeted control measures to reduce its impact on human populations. Based on the distribution of A. trifida in North America and in its introduced range, riparian systems in the rest of the world may be particularly at risk to invasion, with potential negative consequences for their biodiversity. Currently, A. trifida has invaded Asia more widely than Europe, likely due to the more favourable local conditions in Asia. Throughout its introduced range, A. trifida is host to a limited number of invertebrates and pathogens and only a few biological agents are available for its control. The main impacts of A. trifida at a global level are on crop yield and human health, resulting in significant socio-economic impacts. The success of A. trifida invasion in areas in which it has been introduced is still unclear, but climate change may increase climate suitability, increasing the potential for A. trifida to spread. While effective management in cultivated fields seems potentially possible, the development and control of A. trifida in natural riparian habitats is of great concern due to the difficulty of management in these areas.

Ambrosia artemisiifolia (common ragweed) and A. trifida (giant ragweed) are very important weed species that are invasive in Serbia and are often found in agricultural regions. When these weeds are present at high densities, crop yields can be significantly reduced or even completely destroyed. Unlike A. artemisiifolia, A. trifida is locally present in the Central Backa region (Vojvodina province), and it is expected that its area of distribution will expand in the future. Starting from the assumption that future distribution of A. trifida could take on larger proportions than now, the aim of this study was focused on examining the interaction between these two species. Experiments were conducted using the replacement design model, in which Ambrosia trifida/Ambrosia artemisiifolia per m2, were planted as density ratios of 10/0; 8/2; 4/6; 6/4; 2/8, and 0/10, in a completely randomized block system with four replications. The vegetative parameters (height and dry mass) of A. artemisiifolia were measured in July, August and September over a period of two years (2016 and 2017), and the results were statistically analysed in the Statistical Package for the Social Sciences (SPSS 23). In July 2016, the average height of A. artemisiifolia was in the range between 35.00 and 50.40 cm, in August it was from 68.00 to 95.50 cm, and between 83.75 and 99.80 cm in September. In the following season (2017), the corresponding values ranged from 56.19 to 78.50 (July), 98.38 to 125.50 cm (August) and 111.19 to 148.50 (September). An increase in the number of A. artemisiifolia plants and decrease in A. trifida counts per m2 caused an increase in the dry mass of A. artemisiifolia per plant. The dry mass of A. artemisiifolia ranged from 4.22 to 6.11 g/plant (July), 8.96 to 10.27 g/plant (August) and 7.04 to 19.53 g/plant (September). In the following season, these values ranged from 9.62 to 14.60 g/plant, 14.37 to 28.90 g/plant, and 23.43 to 40.47 g/plant in July, August and September, respectively. Minimum values of vegetative parameters were recorded in the treatment with 2 plants, and maximum in the treatment with 10 A. artemisiifolia plants/m2. This means that interspecific competition is more pronounced in this ragweed species than intraspecific competition.

Introduced seed dispersers can shift seed predation dynamics in native species, impacting native plant establishment and spread. We studied the effect of the non-native earthworm, Lumbricus terrestris, on seed loss dynamics in the large-seeded native annual, Ambrosia trifida (giant ragweed), an expanding agricultural weed whose seeds are heavily predated by mice. Lumbricus terrestris may protect seeds against rodent predation by caching them in its burrows from which seedlings can emerge unharmed; however, little is known about the ability of earthworms to compete with rodents for seeds. We investigated this interaction and how environmental factors affected the competitive outcome. In a 2-year field study, we analysed relative rates of seed removal by earthworms and mice for seeds dispersed at various times in habitats that varied in vegetative cover. Species-specific responses to environmental conditions drove variation in the share of seeds taken by earthworms versus mice, with earthworms gaining relatively more seeds under warmer, wetter conditions and in low plant cover habitats, and mice obtaining more seeds under colder, drier conditions and in high cover habitats. Environmental factors also determined which competitor accessed seeds first, and this conferred a competitive advantage that was compounded over time. Earthworms cached some seeds under all experimental conditions, suggesting that L. terrestris can act mutualistically with giant ragweed in diverse environments. Synthesis and applications. Our results support the hypothesis that the non-native earthworm Lumbricus terrestris behaves as a seed dispersal mutualist for the native annual giant ragweed by caching its seeds in its burrows, thereby reducing their availability to rodent seed predators. The data also support the view that interactions among the environment and competing seed predators determine the fate of seed pools in response to species-specific environmental preferences. In crop fields where L. terrestris is abundant, additional efforts to prevent giant ragweed seed return may be warranted due to the likelihood that earthworms will increase giant ragweed soil seed pools. Lumbricus terrestris is expanding its range throughout temperate regions and may similarly affect secondary seed dispersal and seed predation dynamics of large-seeded species in other plant communities. National Research Initiative of the USDA Cooperative State Research, Education and Extension ServiceUnited States Department of Agriculture (USDA)National Institute of Food and Agriculture [2005-02261]; USDA National Institute of Food and AgricultureUnited States Department of Agriculture (USDA) [102183] Published version The authors thank Renata Harrison for seed tagging methodology; Brian Schutte, Ramarao Venkatesh and Norman Arancon for fieldwork assistance; and Kristin Mercer and two anonymous reviewers for reviewing early drafts. This manuscript is dedicated in the memory of our recently deceased colleague and mentor in earthworm biology, Clive A. Edwards. This project was supported by the National Research Initiative of the USDA Cooperative State Research, Education and Extension Service, grant no. 2005-02261 and by the USDA National Institute of Food and Agriculture, grant no. Hatch Project 102183.

Weeds present important challenges to both conventional farmers who rely on herbicides and organic farmers who rely on cultivation. Data from field experiments indicate that diversifying crop sequences with additional species can improve weed suppression when either herbicides or cultivation serve as primary control tactics. Here, we report the results of modeling analyses that investigated how cropping system diversification would affect the population dynamics of giant ragweed (Ambrosia trifida L.), an annual dicotyledonous species that is problematic in the central U.S. for both conventional and organic farmers. We found that to prevent an increase in giant ragweed density, the minimum control efficacy needed from herbicides or cultivation used in corn (Zea mays L.) and soybean (Glycine max (L.) Merr.) would be 99.0% in a 2-year corn&ndash, soybean system, but 91.4% in a 5-year corn&ndash, soybean&ndash, rye (Secale cereale L.)&ndash, alfalfa (Medicago sativa L.) system. Thus, the diversified rotation would be better buffered against less-than-perfect weed control during corn and soybean phases. Further modeling analyses indicated that the weed suppression effect associated with greater rotation length was attributable not only to increased crop species richness but also to greater temporal variation in planting dates. A planting interval variation index (PIVI), calculated as the coefficient of variation in months between planting activities, was strongly associated with the weed suppressive ability of the rotations we modeled and may be a useful metric for designing other cropping systems. Overall, our results indicate that diversified rotation systems that include both annual and perennial crops are likely to be valuable for managing problematic weed species.

The new herbicide halauxifen-methyl was evaluated for the control of glyphosate-resistant (GR) Canada fleabane and giant ragweed in winter wheat and soybean, and the tolerance of corn and soybean to halauxifen-methyl. Halauxifen-methyl applied alone or in a tankmix controlled GR Canada fleabane in winter wheat, while a tankmix partner was required to provide control in soybean. Halauxifen-methyl applied alone did not control GR giant ragweed in winter wheat or soybean, however, when applied in a tankmix, control was achieved. Halauxifen-methyl applied preplant and pre-emergence at 5 g ai ha-1 and 10 g ai ha-1 was safe in corn however yield was reduced when applied early postemergence at the 10 g ai ha-1 rate. Soybean was extremely sensitive to halauxifen-methyl applied early postemergence at both rates, however minimal injury was observed when either rate was applied preplant and pre-emergence. Halauxifen-methyl could provide an effective solution for the control of GR weeds in corn, soybean and winter wheat.

Giant ragweed is an early emerging and one of the most competitive summer annual species found in many fields throughout North America. Extensive use of glyphosate in glyphosate-tolerant (GT) crops has evolved giant ragweed populations with glyphosate resistance. Field dose-response studies were conducted to determine the influence of growth stage on the level of glyphosate resistance in a suspected giant ragweed population. In addition, efficacy of alternative pre-plant, pre-emergence (PRE) and post-emergence (POST) herbicides were evaluated in corn and soybeans for glyphosate-resistant (GR) giant ragweed control. The field glyphosate dose-response studies confirmed that the suspected giant ragweed population were resistant ranging from 14- to 32-fold resistance depending on the growth stage of glyphosate application. The 10, 20 and 30 cm tall giant ragweed had 14, 17 and 32X resistance level, respectively. The dose-response studies indicated that the 10, 20, and 30 cm tall GR giant ragweed was controlled 90% with 214, 402 and 482 g ae ha-1 of dicamba, respectively, when tank-mixed with glyphosate (1060 g ae ha-1 ) 21 days after treatment (DAT). All evaluated pre-plant herbicides for corn provided ≥ 90% control of the GR giant ragweed at 30 DAT; among which the best control (100%) was achieved with pre-plant application of atrazine (2240 g ai ha-1 ), isoxaflutole (90 g ai ha-1 ), and premix of flumioxazin/pyroxasulfone (315 g ai ha-1 ). Herbicide combinations of different site of action provided greater than 90% control of the GR giant ragweed population in a PRE followed by POST herbicide program in corn and soybean, suggesting that alternative herbicide for giant ragweed control are available.

Le 22 octobre 2014, le Parlement européen et le Conseil ont publié un règlement relatif à la prévention et à la gestion de l'introduction et de la propagation des espèces exotiques envahissantes (EEE). Ce règlement prévoit, en particulier àson article 19, que les États membres mettent en place des mesures efficaces de gestion vis-à-vis d'une liste d'EEE dites préoccupantes pour l'Union Européenne (UE) qui, selon l'article 4, doit être adoptée, par voie d'actes d'exécution, par la Commission européenne début 2016. Cette liste sera régulièrement révisée. Ainsi l'inscription d'une espèce dans cette liste se traduira par la mise en place d'actions de prévention et de lutte coordonnées entre les différents États membres de l'Union européenne, visant à réduire les impacts négatifs de ces espèces en premier lieu sur la biodiversité et les services écosystémiques ainsi que d'autres impacts négatifs éventuels dans le cas de certaines EEE pouvant entraîner des impacts sur la santé humaine et/ou des impacts économiques. Pour toutes les espèces qui seront proposées pour la future liste susmentionnée, la Commission européenne doit disposer d'une analyse de risques respectant 14 normes qu'eIle a fixées dans le rapport « invasive alien species -framework forthe identification of invasive alien species of EU concern. ENV.B.2/ETU/2013/0026 ››, ainsi que 5 critères définis à l'articIe 4 du règlement. Pour un certain nombre d'espèces listées dans le rapport suscité, des analyses de risques sont déjà disponibles. Pour celles n'y figurant pas et qu'un État membre souhaiterait voir proposer dans le cadre de la révision régulière de la liste (cf. Article 4), une analyse de risque est à fournir à la Commission européenne. Parmi les espèces végétales du genre Ambrosiaprésentes en France, plusieurs constituent des espèces exotiques envahissantes à impact potentiel pour la santé puisqu'elles émettent un pollen hautement allergisant pour l'homme. C'est le cas en particulier de l'ambroisie à feuilles d'armoise (Ambrosia artemisiifoliaL.), de l'ambroisie trifide (Ambrosia trifidaL.) et de l'ambroisie à épis lisses (Ambrosia psilostachya DC.). Si pour l'ambroisie à feuilles d'armoise (Ambrosia artemisiifoliaL.), une analyse de risques est disponible dans le rapport susmentionné, tel n'est pas le cas pour l'ambroisie trifide.L'ambroisie trifide est également une adventice des cultures des plus difficiles à gérer dans sa zone d'origine, sa taille et son cycle de végétation induisant une concurrence forte avec la végétation en place. Elle a fait l'objet d'une fiche d'alerte de l'Anses en 2013 et les observations confirment son implantation en Midi-Pyrénées, sur des périmètres permettant encore d'envisager son contrôle.Par ailleurs, cette espèce est visée par plusieurs actions du 3ème plan national santé-environnement (PNSE 3), en particulier l'action n°11 visant à mieux évaluer l'exposition à l'ambroisie et surveiller son expansion géographique, et l'action n°12 qui a notamment pour objectif de renforcer et de coordonner la gestion des espèces végétales et animales dont la prolifération peut être nuisible à la santé publique..

Le 22 octobre 2014, le Parlement européen et le Conseil ont publié un règlement relatif à la prévention et à la gestion de l'introduction et de la propagation des espèces exotiques envahissantes (EEE). Ce règlement prévoit, en particulier àson article 19, que les États membres mettent en place des mesures efficaces de gestion vis-à-vis d'une liste d'EEE dites préoccupantes pour l'Union Européenne (UE) qui, selon l'article 4, doit être adoptée, par voie d'actes d'exécution, par la Commission européenne début 2016. Cette liste sera régulièrement révisée. Ainsi l'inscription d'une espèce dans cette liste se traduira par la mise en place d'actions de prévention et de lutte coordonnées entre les différents États membres de l'Union européenne, visant à réduire les impacts négatifs de ces espèces en premier lieu sur la biodiversité et les services écosystémiques ainsi que d'autres impacts négatifs éventuels dans le cas de certaines EEE pouvant entraîner des impacts sur la santé humaine et/ou des impacts économiques. Pour toutes les espèces qui seront proposées pour la future liste susmentionnée, la Commission européenne doit disposer d'une analyse de risques respectant 14 normes qu'eIle a fixées dans le rapport « invasive alien species -framework forthe identification of invasive alien species of EU concern. ENV.B.2/ETU/2013/0026 ››, ainsi que 5 critères définis à l'articIe 4 du règlement. Pour un certain nombre d'espèces listées dans le rapport suscité, des analyses de risques sont déjà disponibles. Pour celles n'y figurant pas et qu'un État membre souhaiterait voir proposer dans le cadre de la révision régulière de la liste (cf. Article 4), une analyse de risque est à fournir à la Commission européenne. Parmi les espèces végétales du genre Ambrosiaprésentes en France, plusieurs constituent des espèces exotiques envahissantes à impact potentiel pour la santé puisqu'elles émettent un pollen hautement allergisant pour l'homme. C'est le cas en particulier de l'ambroisie à feuilles d'armoise (Ambrosia artemisiifoliaL.), de l'ambroisie trifide (Ambrosia trifidaL.) et de l'ambroisie à épis lisses (Ambrosia psilostachya DC.). Si pour l'ambroisie à feuilles d'armoise (Ambrosia artemisiifoliaL.), une analyse de risques est disponible dans le rapport susmentionné, tel n'est pas le cas pour l'ambroisie trifide.L'ambroisie trifide est également une adventice des cultures des plus difficiles à gérer dans sa zone d'origine, sa taille et son cycle de végétation induisant une concurrence forte avec la végétation en place. Elle a fait l'objet d'une fiche d'alerte de l'Anses en 2013 et les observations confirment son implantation en Midi-Pyrénées, sur des périmètres permettant encore d'envisager son contrôle.Par ailleurs, cette espèce est visée par plusieurs actions du 3ème plan national santé-environnement (PNSE 3), en particulier l'action n°11 visant à mieux évaluer l'exposition à l'ambroisie et surveiller son expansion géographique, et l'action n°12 qui a notamment pour objectif de renforcer et de coordonner la gestion des espèces végétales et animales dont la prolifération peut être nuisible à la santé publique..

Le 22 octobre 2014, le Parlement européen et le Conseil ont publié un règlement relatif à la prévention et à la gestion de l'introduction et de la propagation des espèces exotiques envahissantes (EEE). Ce règlement prévoit, en particulier àson article 19, que les États membres mettent en place des mesures efficaces de gestion vis-à-vis d'une liste d'EEE dites préoccupantes pour l'Union Européenne (UE) qui, selon l'article 4, doit être adoptée, par voie d'actes d'exécution, par la Commission européenne début 2016. Cette liste sera régulièrement révisée. Ainsi l'inscription d'une espèce dans cette liste se traduira par la mise en place d'actions de prévention et de lutte coordonnées entre les différents États membres de l'Union européenne, visant à réduire les impacts négatifs de ces espèces en premier lieu sur la biodiversité et les services écosystémiques ainsi que d'autres impacts négatifs éventuels dans le cas de certaines EEE pouvant entraîner des impacts sur la santé humaine et/ou des impacts économiques. Pour toutes les espèces qui seront proposées pour la future liste susmentionnée, la Commission européenne doit disposer d'une analyse de risques respectant 14 normes qu'eIle a fixées dans le rapport « invasive alien species -framework forthe identification of invasive alien species of EU concern. ENV.B.2/ETU/2013/0026 ››, ainsi que 5 critères définis à l'articIe 4 du règlement. Pour un certain nombre d'espèces listées dans le rapport suscité, des analyses de risques sont déjà disponibles. Pour celles n'y figurant pas et qu'un État membre souhaiterait voir proposer dans le cadre de la révision régulière de la liste (cf. Article 4), une analyse de risque est à fournir à la Commission européenne. Parmi les espèces végétales du genre Ambrosiaprésentes en France, plusieurs constituent des espèces exotiques envahissantes à impact potentiel pour la santé puisqu'elles émettent un pollen hautement allergisant pour l'homme. C'est le cas en particulier de l'ambroisie à feuilles d'armoise (Ambrosia artemisiifoliaL.), de l'ambroisie trifide (Ambrosia trifidaL.) et de l'ambroisie à épis lisses (Ambrosia psilostachya DC.). Si pour l'ambroisie à feuilles d'armoise (Ambrosia artemisiifoliaL.), une analyse de risques est disponible dans le rapport susmentionné, tel n'est pas le cas pour l'ambroisie trifide.L'ambroisie trifide est également une adventice des cultures des plus difficiles à gérer dans sa zone d'origine, sa taille et son cycle de végétation induisant une concurrence forte avec la végétation en place. Elle a fait l'objet d'une fiche d'alerte de l'Anses en 2013 et les observations confirment son implantation en Midi-Pyrénées, sur des périmètres permettant encore d'envisager son contrôle.Par ailleurs, cette espèce est visée par plusieurs actions du 3ème plan national santé-environnement (PNSE 3), en particulier l'action n°11 visant à mieux évaluer l'exposition à l'ambroisie et surveiller son expansion géographique, et l'action n°12 qui a notamment pour objectif de renforcer et de coordonner la gestion des espèces végétales et animales dont la prolifération peut être nuisible à la santé publique..

The overall aim of the project was to understand how plants reacted to environmental stress and evolved to overcome it. The land plants that we see today evolved from a green algal ancestor around 510 million years ago. Plants had to make significant changes to their cellular, morphological, regulatory and physiological processes during their adaptation to the terrestrial environment from an aquatic environment. The first part of the project was to find out how these changes were reflected on the protein makeup of the early land plants. The gene model sequence data of two early land plants, Physcomitrella patens (moss) and Chlamydomonas reinhardtii(green algae). We specifically focused on the protein family expansion of protein kinases due to their roles in various important functions that would affect the transition from water to land. We developed a gene model quality evaluation method to score the gene models of P. patens and C. reinhardtii using well-studied plants such as Arabidopsis thaliana and Oryza sativa (rice) to improve the poor quality gene models that currently exist. The resulting corrected gene models were analyzed using functional annotation methods to understand how the proteomics of the early land plants varied from modern land plants. The second part of the project was to identify the genes responsible for herbicide resistance in Ambrosia trifida (giant ragweed). Giant ragweed is one of the most competitive annual weeds in corn and soybean production across the eastern Corn Belt in the United States. The use of glyphosate (commercial name: Roundup) and glyphosate-ready crop systems managed to keep giant ragweed populations under control. Glyphosate-ready crop systems consist of seeds that are resistant to glyphosate, which enables farmers to use glyphosate to control the population of weeds. But in the last decade, glyphosate-resistant giant ragweed populations have been reported across the world. It is a huge problem to farmers since it results in unusable glyphosate-ready cropping systems and huge yield losses. Glyphosate-resistant and sensitive plants were identified from across the Midwestern United States and a RNA-seq experiment was performed by isolating the total mRNA from leaf material, and obtaining the expressed messenger RNA sequences. The genetic makeup of the sensitive and resistant strains was thus compared based on their transcriptome data, and a list of potential genes that were differentially expressed between them was identified. We also analyzed how much the quality of the transcriptome can be improved by using the transcriptome and genome of sunflower, a closely-related plant.

Mechanisms of resistance to the herbicide glyphosate in Canadian biotypes of giant ragweed are currently unknown. In Ontario, a glyphosate resistant (GR) biotype shows a distinct phenotypic response in the mature leaves characterized by a light-dependent, hydrogen peroxide induced, rapid-necrosing reaction to glyphosate, leaving meristems intact. To gain insight into the subcellular events leading to damage in the mature leaves, transmission electron microscopy was used to compare cellular morphology between GR and glyphosate susceptible (GS) biotypes. Morphological evidence of different programmed cell deaths (PCD) were observed between GR and GS biotypes and a rapid increase in starch accumulation was observed in the GR biotype. This evidence was accompanied by a time lapse quantification of [14C]-glyphosate through biological oxidation and liquid scintillation when application was specific to the apical meristem of GR and GS plants. At 24 hours after application, [14C]-glyphosate was shown to increase in the mature leaves and decrease in the apical meristem of GR plants in comparison to GS plants (P < 0.10). We propose that the translocation impairment of glyphosate in the GR biotype could be due to the combination of two distinct cellular events, PCD and the impairment of an active glyphosate transport system.

Farmers in southern Ontario face a significant threat from glyphosate-resistant weeds such as Canada fleabane and giant ragweed. Left uncontrolled, glyphosate-resistant weed interference in soybean can result in substantial yield loss. The three objectives of this study were: 1) to determine the biologically effective rate of isoxaflutole, metribuzin, and glyphosate on glyphosate-resistant Canada fleabane and giant ragweed biotypes, 2) to assess the response of isoxaflutole and glyphosate applied as a tank-mix on glyphosate-resistant and glyphosate-susceptible biotypes of both species, and 3) to ascertain the response of isoxaflutole, metribuzin and glyphosate tank-mix on the glyphosate-resistant biotypes of both species. In growth room experiments, antagonism is observed at low rates of glyphosate plus isoxaflutole in glyphosate-susceptible giant ragweed and at field rates of the same tank-mix in glyphosate-resistant Canada fleabane. The addition of metribuzin to the tank-mix resulted in an additive or synergistic response across all rates in the glyphosate-resistant biotypes of both species tested. Field experiments confirmed the additive and synergistic response of the three-way tank-mix in both species tested. An estimated 631 (126+504) and 420 (84+336) g a.i. ha-1 of isoxaflutole plus metribuzin in a 1:4 ratio are required for control of glyphosate-resistant giant ragweed and Canada fleabane respectively. Bayer CropScience

Glyphosate resistant giant ragweed poses a significant threat to farmers in southern Ontario as interference from this species can result in massive yield loses. The objective of this study is to investigate two distinct mechanisms of glyphosate resistance present in giant ragweed biotypes. Target site sequencing along with inter-population crosses were performed to obtain insight into the pattern of inheritance. The role of H2O2 was examined using 3,3’-Diaminobenzidine staining and glyoxylate content was measured as a means of investigating glycolate oxidase activity. Sequencing data revealed no target site mutations while F1 survival data showed a decreased resistance level. DAB staining showed evidence of H2O2 accumulation. Glyoxylate levels showed no differences between biotypes. Results from this study will contribute to elucidating the mechanism of glyphosate resistance in giant ragweed. OMAFRA, Monsanto, Agriculture Adaptation Council

The basis of resistance to glyphosate in two giant ragweed populations exhibiting different phenotypic responses to glyphosate is unknown. Population R1 exhibits a rapid necrosis of mature leaves followed by eventual regrowth of young tissue while population R2 shows general chlorosis and cessation of growth followed by growth in 21 to 28 days. The objectives were to determine the resistance index of the two populations and investigate mechanisms of resistance. The resistance index, based on LD50 values, was 16 and 19 for R1 and R2, respectively. Accumulation of shikimate in leaf discs (an indicator of glyphosate target site sensitivity) occurred at lower glyphosate doses for S populations than in R1 and R2 suggesting differential target site inhibition. [14C]-glyphosate was used to measure absorption and translocation. Absorption levels of R1 and susceptible populations were similar. Less [14C]-glyphosate was translocated to the roots and above the treated leaf in the R1 population.

Continuous glyphosate use has contributed to an increasing number of problematic glyphosate-resistant (GR) weeds. The mechanism of resistance in many GR weeds is poorly understood, in part, due to a poor understanding of how exactly glyphosate kills a plant. In previous research, the efficacy of glyphosate was demonstrated to be strongly influenced by root invading soil-borne microorganisms. However, this interaction among plants, glyphosate, and soil microorganisms has only been studied in a number of crop plants, but not in weed species. This is surprising since the soil biotic environment has a strong impact on the activity of this important herbicide. Gaining a better understanding of these interactions may shed more light on the performance of glyphosate in the field and the evolution of glyphosate resistance. The objective of this research was to determine the role of soil microbes in the resistance to glyphosate of three problematic weeds of the midwestern United States: giant ragweed (Ambrosia trifida L.), horseweed [(Conyza canadensis (L.) Cronq.], and common lambsquarters (Chenopodium album L.). Through a series of greenhouse and lab experiments we determined that root colonization by soil microorganisms increased the activity of glyphosate in glyphosate-resistant (GR) and -susceptible (GS) biotypes of giant ragweed and a GS common lambsquarters biotype, but not in horseweed biotypes. The GS biotypes of each weed species were colonized by a greater number of soil microorganisms, specifically oomycete (e.g. Pythium spp. and Phytophthora spp.) pathogens, when treated with glyphosate, compared to the GR biotypes. These data suggested that the ability of giant ragweed to tolerate a glyphosate application may involve differences in the susceptibility to soil microbial colonization, specifically Pythium spp. However, the degree of giant ragweed susceptibility to two Pythium species, P. ultimum and P. aphanidermatum, did not differ between the GR and GS biotypes grown in disease conducive conditions, in the absence of glyphosate. Utilizing next-generation sequencing revealed that the rhizosphere microbial community of giant ragweed is different between the GR and GS biotypes, and the microbial community is perturbed by a glyphosate treatment. Glyphosate does cause changes to the diversity of the rhizosphere microbial community 3 days after treatment, and this needs to be investigated further. Overall, the results of this research demonstrate that rhizosphere interactions are important to the mode of action of glyphosate, in the weed species selected. These findings suggest that the range of tolerance to glyphosate observed in weeds and the evolution of resistance in weed biotypes may also be influenced by rhizosphere interactions. Understanding these interactions are crucial in understanding the biology of these weed species and the resistance to this herbicide.

Giant ragweed (Ambrosia trifidia) was the first glyphosate resistant weed in Canada. Giant ragweed interference in soybean has resulted in yield losses of greater than 90%; therefore, control of this competitive weed is essential. In 2011 and 2012, glyphosate resistant giant ragweed was confirmed at 34 additional sites Ontario. In addition, there are 11 locations confirmed with cloransulam-methyl resistant giant ragweed and 5 locations with multiple resistance to both glyphosate and cloransulam-methyl. Glyphosate resistant giant ragweed has been found in four additional counties. Field trials were conducted at 5 locations in 2011 and 2012 to determine the most effective control options. Based on these experiments, glyphosate tankmixes with 2,4-D or amitrole provide the most effective control. These two tankmixes provided greater than 90% control. The minimum dose of 2,4-D required for acceptable control of glyphosate resistant giant ragweed is 500 g a.e. ha-1. Monsanto Canada Inc.; Grain Farmers of Ontario; Agricultural Adaptation Council

Giant ragweed (Ambrosia trifida L.) is a competitive annual plant found in disturbed landscapes and is the most troublesome weed in Indiana and the US Corn Belt. It is one of the most common and problematic weeds in corn and soybean production. The introduction of herbicide glyphosate, N-(phosphonomethyl) glycine in early 1970's provided farmers with a better and low-cost tool to control weeds. The use of glyphosate drastically increased after the development of glyphosate resistant agronomic crops in 1996 and was use as a post-emergence selective herbicide. This led to overreliance and repeated use of glyphosate for weed control especially in roundup ready corn and soybean cropping systems and resulted in tremendous selection pressure for evolution of glyphosate resistant weeds and specifically giant ragweed in Indiana. Our research investigated the response of glyphosate-resistant (GR) and glyphosate-susceptible (GS) giant ragweed to glyphosate treatment under varying environmental conditions and the fitness and competitiveness of glyphosate resistant giant ragweed. Greenhouse studies investigated response of GS and GR giant ragweed biotypes to various doses of glyphosate and to light and temperature. GR plants had a unique response when treated with glyphosate, exhibiting initial rapid necrosis of mature leaves within 12 hours of treatment. GR plants do not die from a glyphosate treatment but resumed normal-growth from axillary meristems and reproduce. The progression of the response and symptoms resemble a typical hypersensitive response similar to that observed on some plants after pathogen attack. GS plants do not exhibit rapid leaf necrosis but their leaves become chlorotic, then necrotic and plants die over a 2-3 week period. The GR50 for GS and GR biotypes were 426.49 g ae ha-1 and 860.87 g ae ha-1 and the estimated GR90 for GS and GR biotypes were 515.28 g ae ha-1 and 3338. 39 g ae ha-1 respectively indicating that the dose required to achieve 90% of GS biotype was lower than the recommended field rate of 700 g ae ha-1 and amount to achieve 90% control of GR biotypes was almost 5 times greater than the recommended glyphosate rate. The GR50 was two times greater for GR than GS and GR90 was 6.5 times greater for GR than GS. The results also show that short period dark treatment and different temperatures did not improve glyphosate activity on giant ragweed but delayed the injury caused by the herbicide. Field study results showed that in absence of glyphosate, when GR and GS plants were grown independently, the GS plants grow taller than GR plants and GR plants were wider than GS plants. However, no differences occur in leaf area, seed production and total plant biomass. Competition studies showed that when grown in mixtures, the two biotypes were similar in total biomass and seed production. The results show that resistance trait in GR plants is not associated with growth penalty and do not incur ecological cost in field. Results of this research confirm the presence of glyphosate resistant giant ragweed in Indiana and show that giant ragweed can survive high doses of glyphosate. Results also show that manipulating environmental conditions specifically during the short-term, post spraying period have no major influence in glyphosate performance on giant ragweed.

Glyphosate-resistant giant ragweed can be found in agricultural fields throughout the mid-western United States and southwestern Ontario. Environmental factors can influence growth and herbicide efficacy in C3 plant species. I measured the aboveground injury to resistant and susceptible seedlings for 28 d following glyphosate treatment to test the hypothesis that young leaf stages would be more susceptible to glyphosate under warm, dry, low-CO2 conditions. Glyphosate-resistance was not affected by environmental factors, leaf stage, or glyphosate dose, but plants grown at the highest temperature after spray had the least glyphosate injury. Resistant alleles may be associated with fitness penalties because they are rare in populations without herbicide selection pressures; however, in a greenhouse trial, resistant biotypes recovered from glyphosate injury and produced seeds. My results suggest that potentially stressful growth conditions and treatment at young growth stages will not improve the control of resistant giant ragweed biotypes.

Giant ragweed is an extremely competitive weed and poor control in soybean could lead to significant yield losses for Ontario producers. In 2008, a giant ragweed biotype near Windsor, ON was not controlled with glyphosate and further testing confirmed it as the first glyphosate-resistant (GR) weed in Canada. Giant ragweed seed was collected from 102 locations in Essex (70), Kent (21), Lambton (10) and Waterloo (1) counties to document the occurrence and distribution of GR giant ragweed in Ontario. Giant ragweed seedlings were sprayed with glyphosate at 1800 g a.e. ha-1, and evaluated 1, 7, 14 and 28 days after application (DAA). Results from the survey concluded that there are 47 additional locations in southwestern Ontario with GR giant ragweed. The majority of the sites were found in Essex county, but there was one location in both Chatham-Kent and Lambton counties. Field trials were established at six sites with GR giant ragweed during the 2010 and 2011 growing seasons. The objectives were to determine the level of giant ragweed control with increasing doses of glyphosate, and glyphosate tank mixes applied either preplant or postemergence. Control of giant ragweed increased with higher doses of glyphosate, but only at doses that are not economical for producers. The most effective glyphosate tank mixes were 2, 4-D ester, saflufenacil, linuron, and cloransulam-methyl providing up to 98, 94, 99 and 97% control 4 weeks after application (WAA), respectively. Glyphosate plus dicamba in dicamba-tolerant soybean provided up to 100% giant ragweed control, 4 WAA at the three confined field trial locations. Monsanto Canada Inc.; Agricultural Adaptation Council; Grain Farmers of Ontario