Biological control of plant diseases by AMF under field conditions is the effect of interactions between AMF and various groups of organisms in the rhizosphere. Sikora (1997) proposed "biological system management," a holistic approach for improving plant root systems that adopts specific cultural practices that promote plant defense mechanisms such as tolerance and/or resistance to pathogens, and the use of organisms that are antagonistic towards pathogens and that target sensitive developmental stages of pathogens. This approach offers a viable alternative to integrated pest management and inundative approaches such as the application of high levels of microbial inocula to the nonrhizosphere soil for biological control purposes, and underscores the significance of mycorrhizae in root health (Sikora 1997).
A diverse AMF community contains a mycorrhizal assemblage and species abundance that naturally aid the host to endure adverse conditions to ultimately enhance plant growth. Research shows that inclusion of host crops (Bever et al. 1996; Johnson et al. 1992) and/or cultivars that exhibit high mycorrhizal responsiveness can significantly improve AMF functioning (Boyetchko and Tewari 1995; Xavier and Germida 1998). Therefore, rotation of crops that are dependent on mycorrhizae will ensure early AMF root colonization and high sporulation of even the most sensitive AMF species in soil. Minimal disturbance to the soil also guarantees early contact between an emerging seedling and the AMF hyphal network in soil that distributes nutrients and initiate early colonization of AMF propagules in soil. Excessive fertilizer and pesticide use can alter plant chemistry and cause changes in AMF assemblage and abundance, resulting in a poor AMF community that does not benefit the host (Gazey et al. 1992; Jasper et al. 1979; Johnson et al. 1992). Caution in the choice of cultural practices that potentially alter AMF diversity would prove to be fruitful.
4.2.2 Improved Understanding of Microbial (AMF) Ecology and Ecosystem Functioning
It is common knowledge that AMF functioning in natural ecosystems can be altered by various factors including interactions with other organisms. However, specifics on the topic are lacking. Knowledge generated from studies addressing AMF efficacy in a typical rhizosphere community, under moisture and salinity stress and soil disturbance, in soils containing extreme indigenous AMF levels, and in the presence of antagonizers is required for the development of effective AMF biocontrol agents.
Bagyaraj (1984) suggested that AMF species selection for a desired activity must be based on their ability for survival, aggressive colonization of host roots, and efficacy. Use of AMF species originally isolated from test host roots has proven advantageous for many plant species including agricultural crops, forest tree species, and orchard crops
(Reena and Bagyaraj 1990a,b; Talukdar and Germida 1994; Vinayak and Bagyaraj 1990). Screening procedures must include selection pressure similar to that in which the AMF will be applied.
Research shows that plant pathogens can be controlled not only by the use of biocontrol agents, but also by the induction of resistance responses in plants. Inoculating plants with AMF has been shown to induce resistance in plants. Such plant immunizations are a viable approach for transplant crops because of the ease of AMF inoculation, while more innovative methods are required with direct-sown crops.
Research shows that "priming" plants against pathogens using selective AMF inocula (or plant immunization) helps protect plants by inducing a SIR response (Cordier et al. 1998). The inoculum may be applied to seeds, transplanted crops, or plantlets produced through tissue culture before being transplanted into pathogen-infested fields. Application of the agent prior to transplanting eliminates the need for complex formulations and application techniques, guarantees "targeted placement," and greater biocontrol activity, reduces costs associated with application and has a minimal impact on the environment (Boyetchko 1996; Glass 1993). Inoculum may include one or more AMF species or other organisms such as bacteria or fungi that exhibit sustained and coordinated biocontrol activity. The application of a multiple agent mixture may concurrently confer control for more than one plant disease by more than one mechanism rather than single inoculants targeted for control of only one plant disease or pathogen. Application of two or more biocontrol agents targeting different life stages of a pathogen may also be more effective than sequential application of the biocontrol agents. In some instances, augmenting soil with organic amendments such as forest humus and charcoal compost has enabled a significant reduction in disease severity (Kobayashi 1993; Wei et al. 1987).
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