Impact of soil microorganisms on weed biology and ecology

Quoique les populations de mauvaises herbes aient été traditionnellement réprimées par des méthodes chimiques et culturales, la lutte biologique par inondation du milieu avec des agents microbiens représente une stratégie supplémentaire de gestion des mauvaises herbes. Les agents pathogènes foliaires ont longtemps fait l'objet de recherches comme agents de lutte biologique potentiels, mais les microorganismes de la rhizosphere et leur influence sur la croissance et le développement des mauvaises herbes ont été ignorés jusqu'à tout récemment. Le sol de la rhizosphere contient une multitude de microorganismes, tels les rhizobactéries, les champignons phytopathogènes présents dans le sol et les champignons mycorhiziens à arbuscules, qui ont tous un impact direct ou indirect sur les mauvaises herbes et sur leur aptitude à la compétition. Dans certains cas, des microorganismes spécifiques ont un effet nuisible sur les mauvaises herbes et ils peuvent être exploités comme agents de lutte biologique. Les champignons mycorhiziens, omniprésents, sont des symbiotes bénéfiques qui peuvent transmetttre un avantage compétitif à leurs plantes-hôtes, tout particulièrement si la dépendance mycorhizienne s'exprime chez les mauvaises herbes plutôt que chez les cultures. Il peut être possible de tirer profit des divers microorganismes du sol en réduisant directement ou directement la compétition des mauvaises herbes et en faisant pencher l'avantage compétitif en faveur de la culture. Cependant les connaissances actuelles sur les relations entre les micro-organismes, les mauvaises herbes et les cultures sont limitées. Des efforts de recherche sont requis afin d'explorer l'utilisation des microorganismes du sol comme autre outil de lutte contre les mauvaises herbes.While weed populations have traditionally been controlled by chemical and cultural methods, inundative biological control with microbial agents offers an additional strategy for managing weeds. Foliar pathogens have long been sought after as potential biocontrol agents, but rhizosphere microorganisms and their influence on weed growth and development have been ignored until recently. Rhizosphere soil is replete with a variety of microorganisms such as rhizobacteria, pathogenic soil-borne fungi, and arbuscular mycorrhizal fungi, all of which have a direct or indirect impact on weeds and their competitive ability. In some cases, specific microbes have a detrimental effect on the weeds and can be exploited as biological control agents. The ubiquitous mycorrhizal fungi are beneficial symbionts that can impart a competitive ad vantage to their plant hosts, particularly if mycorrhizal dependency is exhibited in weeds as opposed to crops. It may be possible to exploit various soil microbes by directly or indirectly reducing weed competition and tipping the competitive advantage in favor of the crop. However, information available on microbial/weed/crop relationships is limited and research efforts are required to explore the use of soil microorganisms as another weed management tool

Parasitism of spores of the vesicular-arbuscular mycorrhizal fungus, Glomus dimorphicum

Nous avons examiné des spores parasitées du Glomus dimorphicum. Les microscopies photonique et électronique ont révélé des perforations d'environ 0,25 à 1,0 µm de diamètre dans la paroi de la spore. La présence de papilles, réponse dynamique de l'hôte, suggère que le parasitisme se produit pendant que le champignon mycorhizien à vésicule et arbuscule est encore vivant. Aucune structure filamenteuse n'a été détectée dans les spores; cependant, des kystes d'organismes ressemblant à des amibes ont été trouvés dans les spores et ont aussi été observés en milieu gélose sur lequel ont été placées des spores du G. dimorphicum stérilisées en surface mais contenant de tels organismes. Il est postulé qu’un organisme ressemblant à une amibe était le parasite, puisque les perforations de la paroi de la spore étaient minuscules et qu'aucun champignon ou bactérie n'a été détecté dans les spores.Spores of Glomus dimorphicum were examined for parasitism. Light and scanning electron microscopy revealed perforations, approximately 0.25 to 1.0 µm in diameter, in the spore wall. The presence of papillae, a dynamic host response, suggested that the parasitism occurred while the vesicular-arbuscular mycorrhizal fungus was still alive. No filamentous structures were detected in the spores; however, cysts of amoeba-like organisms were found in the spores and were also observed on agar plates on which surface-sterilized spores of G. dimorphicum containing such organisms were placed. It is postulated that an amoeba-like organism was the parasite, since the perforations on the spore wall were minute and no bacteria or fungi were seen inside the spores

Canadian biopesticides and bioherbicides

Non-Peer ReviewedBiopesticide technology is emerging as a viable and environmentally-friendly pest management tool in agriculture. Although the current global biopesticide market is small in comparison to the synthetic pesticide market, biopesticides are expected to exceed $1 billion in annual sales. The Canadian public's demand for safer foods and concern for the environment have encouraged initiatives to develop alternatives to conventional pesticides. Biopesticides are classed by Health Canada as reduced risk products that are less hazardous to human health and the environment and they represent the next generation of pest control products with novel modes of action. Agriculture and Agri-Food Canada (AAFC) has invested in a strategic priority to promote the development and commercialization of this technology. This paper presents a summary of new and emerging Canadian biopesticides and bioherbicides being developed by AAFC researchers

Use of rhizobacteria as biological control agents of downy brome

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Competitiveness-shifts from weeds to crops using arbuscular fungi

Non-Peer ReviewedArbuscular fungi (AF) colonize ca. 80% of terrestrial plant roots and typically improve their growth by enhancing nutrient uptake, reducing disease severity and/or imparting resistance to abiotic stress. Therefore, AF-colonized crop cultivars exhibit early vigor and superior growth compared to cultivars not colonized by AF. However, routine AF inoculation of crops is limited because of their obligate nature. Nevertheless, there are strategies that can be readily adopted by growers to exploit the indigenous AF community. Our project aimed at determining the relationship between the mycorrhizal dependency of crops and crop competitiveness against weeds. The mycorrhizal dependency of eight barley and eight wheat cultivars on an AF mixture resembling that of an indigenous AF community was evaluated in natural soil. There were significant differences between the cultivars in their response to AF. The most and least mycorrhizal cultivar (based on biomass) were evaluated for their competitiveness against wild oat at four crop:weed density levels (1:0.5; 1:1; 1:2; 1:4) in soil with and without the AF mixture. Results showed that the most AF dependent cultivar in both crops exhibited superior growth and competitiveness against wild oat up to a level of 1:1 crop:weed density. This confirms that mycorrhizal dependency is partially linked to crop competitiveness against weeds and that the choice of cultivar may be a strategy which can enhance crop competitiveness while reducing chemical herbicide use

Arbuscular mycorrhizal fungi influence competition between barley and wild oat

Non-Peer ReviewedArbuscular mycorrhizal fungi (AMF) are soil fungi that intimately associate with most crops and influence their productivity. This study determined (i) the mycorrhizal dependency of eight barley cultivars and (ii) whether barley competitiveness against wild oat was linked to its mycorrhizal dependency. Of the eight cultivars tested, Virden was the most dependent on AMF whereas CDC Earl was the least dependent; Earl and Virden were therefore evaluated for competitiveness against wild oat at weed density ratios of 1:0.5, 1;1, 1:2 and 1:4 with or without AMF. Regardless of the AMF treatment, the total shoot dry weight of both barley varieties decreased with increasing crop:weed ratio. Earl derived 32% less benefit than Virden at a crop:weed ratio of 1: 0.5, and at a crop:weed ratio of 1:1, the total shoot dry weight of wild oat competing against Virden was significantly lower than that of wild oat competing against Earl. Regardless of the crop:weed ratio, (i) both barley varieties responded positively to the AMF mixture, (ii) the total shoot dry weight of AMF-inoculated Virden was 13% higher than that of AMF-inoculated Earl, and (iii) the shoot dry matter ratio of barley:wild oat was greater for AMF-inoculated Virden than Earl. At crop: weed ratios of up to 1:1, AMF-inoculated Virden plants had significantly more total shoot biomass than uninoculated Virden, whereas this was not the case with Earl. In general, wild oat competing against AMF-inoculated Virden had the least shoot dry matter at all the crop:weed ratios compared to all other treatments. These results suggest that the highly mycorrhizal Virden appeared to be more competitive than Earl and indicates that barley competitiveness may be partially linked to its mycorrhizal dependency

Interactions between wild oat and a weed-competitive and non-competitive wheat cultivar as influenced by arbuscular mycorrhizal fungi

Non-Peer ReviewedThe response of a weed-competitive (Columbus) and non-competitive (Oslo) wheat (Triticum aestivum L.) cultivar, alone and in competition with wild oat (Avena fatua L.) to arbuscular mycorrhizal fungi (AMF) was assessed in a greenhouse study using four AMF species. Plants were inoculated with 300 spores of Glomus clarum, G. etunicatum, G. intraradices or G. mosseae and grown for 77 d in field soil containing low levels of indigenous AMF populations. The AMF species had no significant (P<0.05) effect on the shoot fresh or dry weight of single stands of Oslo or Columbus compared to the uninoculated controls. However, G. etunicatum significantly (P<0.05) enhanced the shoot fresh weight of single stands of wild oat, and G. intraradices significantly (P<0.05) increased the shoot fresh and dry weight of wild oat compared to the uninoculated control. The competitiveness of wild oat in competition with Oslo was significantly (P<0.05) enhanced by inoculation with G. mosseae, whereas the other AMF species had no effect. In contrast, inoculation of Oslo with G. clarum significantly (P<0.05) increased the ability of Oslo to withstand wild oat competition. On the other hand, there were no differences in the ability of any of the AMF species to impact on wild oat growth in competition with Columbus. However, G. intraradices significantly (P<0.05) increased the shoot dry weight of Columbus in competition with wild oat. These preliminary results indicate that different AMF species interact differently with various hosts, and that these interactions may be specific. In addition, it is apparent that these specific interactions may enhance the competitiveness of a non-competitive host against weeds

Formulation development and delivery of biopesticides

Non-Peer ReviewedBiopesticide formulation development is integral for end product development and risk reduction associated with commercialization and acceptance by the end user. Development of robust formulations for biopesticides is a key step towards advancing this technology into integrated pest management systems. A granular formulation protocol using extrusion-spheronization-fluidized bed drying for biopesticidal bacteria and fungal hypha and spores is described. Establishing low granule water activity (aw, 0.2-0.3) is a key factor in extending the shelf-life of the product. Starch type and amount provided controlled release attributes to the biopesticide granules. Microencapsulation of bioherbicide, Colletotrichum truncatum 00-003B1 (Ct), conidia and bioinsecticide nucleopolyhedrovirus (NPV), by complex coacervation is described for foliar application of biocontrol agents

Can Arbuscular Mycorrhizal Fungi Reduce the Growth of Agricultural Weeds?

BACKGROUND: Arbuscular mycorrhizal fungi (AMF) are known for their beneficial effects on plants. However, there is increasing evidence that some ruderal plants, including several agricultural weeds, respond negatively to AMF colonization. Here, we investigated the effect of AMF on the growth of individual weed species and on weed-crop interactions. METHODOLOGY/PRINCIPAL FINDINGS: First, under controlled glasshouse conditions, we screened growth responses of nine weed species and three crops to a widespread AMF, Glomus intraradices. None of the weeds screened showed a significant positive mycorrhizal growth response and four weed species were significantly reduced by the AMF (growth responses between -22 and -35%). In a subsequent experiment, we selected three of the negatively responding weed species--Echinochloa crus-galli, Setaria viridis and Solanum nigrum--and analyzed their responses to a combination of three AMF (Glomus intraradices, Glomus mosseae and Glomus claroideum). Finally, we tested whether the presence of a crop (maize) enhanced the suppressive effect of AMF on weeds. We found that the growth of the three selected weed species was also reduced by a combination of AMF and that the presence of maize amplified the negative effect of AMF on the growth of E. crus-galli. CONCLUSIONS/SIGNIFICANCE: Our results show that AMF can negatively influence the growth of some weed species indicating that AMF have the potential to act as determinants of weed community structure. Furthermore, mycorrhizal weed growth reductions can be amplified in the presence of a crop. Previous studies have shown that AMF provide a number of beneficial ecosystem services. Taken together with our current results, the maintenance and promotion of AMF activity may thereby contribute to sustainable management of agroecosystems. However, in order to further the practical and ecological relevance of our findings, additional experiments should be performed under field conditions