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

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

Non-Peer Reviewe

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

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

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

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

Efficacy of Rhizobacteria as biological control agents of grassy weeds

Non-Peer ReviewedDeleterious rhizobacteria (DRB) associated with the roots and rhizosphere of weeds have tremendous potential as biological weed control agents. Screening and evaluation of the efficacy of DRB for biological control of downy brome (Bromus tectorum), green foxtail (Setaria viridis), and wild oats (Avena fatua) was conducted and methods for optimizing biocontrol activity were determined. Several hundred isolates of DRB were screened, with more than 100 isolates with 280% suppression of root growth in laboratory bioassays showing weed inhibitory properties. An increase in root growth suppression was demonstrated when unfiltered bacterial broth was incorporated into agar, compared to cell-free culture filtrate alone. Nutritional factors also played a role in enhancing biocontrol activity. Increases in root and shoot suppression were demonstrated when selected DRB isolates were grown in a nutrient broth, compared to a minimal salts broth. Specific requirements in the nutritional environment of the bacterial culture medium and the effect these changes have on propagule yield, efficacy and stability will have a significant impact on the potential of a biological control agent and its success as a commercial product

Biological control of weeds using plant pathogens

Non-Peer ReviewedChanges in program directions at the Saskatoon Research Centre have resulted in the organization of a research team equipped to develop microorganisms for the biological control of important agricultural weeds. This paper presents a brief introduction to the science of inundative biological control and an overview of the research progress made on controlling Canada thistle, wild oats, and green foxtail using fungi and bacteria

Effect of manure application on soil microbial activity

Non-Peer ReviewedSoils from field trials treated with hog and cattle manure in spring of 1997 were sampled in the fall after harvest and soil microbial activity assessed. CO2 evolution measurements indicated that 6 months after addition, hog and cattle manure had little influence on measured microbial activity. In contrast, manure additions greatly stimulated soil microbial activity immediately after application when manure was freshly applied to the soils in the laboratory. For example, the highest level of microbial activity was observed at 3 days after treating the soil in the lab with 400 kg N-ha-l of cattle or hog manure. These initial results indicate that time of sampling is critical when assessing the effect of manure on soil microorganisms, with measurements required immediately after application as well as at longer time intervals. The fresh application of manure stimulated microbial activity by lo-fold compared to that measured in samples taken 6 months after application. However, microbial activity decreased with time and amounts of C evolved leveled out at about 20 p.g Cg-l soil after 38 days, similar to activity observed in the samples after 6 months. Preliminary results on the effects of manure application on the incidence of soil entero-pathogens (fecal coliforms) and wheat root rot incidence are discussed