īiocontrol microbes that are applied to seeds or soil prior to planting may colonize the spermosphere and/or rhizosphere of seedlings and thus may be present at or near infection courts of soilborne pathogens. Nonetheless, in an alternate taxonomy of interspecific relationships, mutualism and parasitism are often considered to be part of a continuum. The distinction between trophic and nontrophic relationships is somewhat arbitrary, since it focuses only on “what an organism eats and what eats it.” Parasitism typically is categorized as a trophic interaction, since a parasite derives nutrition from its host, whereas mutualism is generally considered a nontrophic interaction. Unlike gnotobiotic systems, soil is characterized by a multitude of both trophic (food webs) and nontrophic (e.g., mutualism, commensalism, neutralism, amensalism, antagonism, and competition) interspecific relationships. A wide range of fungi have shown potential as agents for biological control of soilborne phytopathogens, and this review will focus on these fungal biocontrol agents. Application of fungi and bacteria as microbial antagonists of plant pathogens offers prospects of environmentally benign pest control. Many traditional chemical means of disease control (e.g., methyl bromide and certain fungicides) are being or have been phased out due to economic considerations and/or mandate. Reduction of chemical pesticide usage, including chemicals for control of soilborne plant pathogens, is widely recognized as a desirable goal for agriculture and forestry. Several of these tools are described here, including the use of molecular biology to generate biocontrol agents with useful marker genes and then to quantify these agents in natural systems, epifluorescence and confocal laser scanning microscopy to observe their presence and activity in situ, and spatial statistics and computer simulation modeling to evaluate and predict these activities in heterogeneous soil habitats. Characterization of these ecologically complex systems is challenging, but a number of tools are available to help unravel this complexity. However, the biocontrol agents themselves are also subject to the same types of interactions, which may reduce or in some cases enhance their efficacy against target plant pathogens. Greater application of PGPR is possible in agriculture for biocontrol of plant pathogens and biofertilization.Fungal biological control agents against plant pathogens, especially those in soil, operate within physically, biologically, and spatially complex systems by means of a variety of trophic and nontrophic interspecific interactions. Some of these rhizobacteria may also be used in integrated pest management programmes. Recent progress in our understanding of their diversity, colonizing ability, mechanisms of action, formulation and application should facilitate their development as reliable biocontrol agents against plant pathogens. Although significant control of plant pathogens has been demonstrated by PGPR in laboratory and greenhouse studies, results in the field have been inconsistent. The use of PGPR has become a common practice in many regions of the world. PGPR can also provide protection against viral diseases.
They can suppress a broad spectrum of bacterial, fungal and nematode diseases. These rhizobacteria can stimulate plant growth directly by producing growth hormones and improving nutrient uptake or indirectly by changing microbial balance in the rhizosphere in favour of beneficial microorganisms. PGPR can profoundly improve seed germination, root development and water utilization by plants. Plant growth promoting rhizobacteria (PGPR) are indigenous to soil and the plant rhizosphere and play a major role in the biocontrol of plant pathogens.
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