Plant root-bacterial interactions in biological control of soilborne diseases and potential extension to systemic and foliar diseases
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Plant-associated bacteria reside in the rhizosphere, phyllosphere, and inside tissues of healthy plants. This chapter discusses concepts and examples of how naturally occurring and introduced bacteria may contribute to management of soilborne and foliar diseases. Introduced bacteria which have demonstrated biological control activity against soil-borne pathogenic fungi and nematodes include rhizobacteria (root-colonising bacteria) and endophytic bacteria (bacteria isolated from within healthy plant tissues). Recently, some introduced rhizobacteria have been found to enhance plant defences, leading to systemic protection against foliar pathogens upon seed or root-treatments with the rhizobacteria. In these cases, introduction of the rhizobacteria results in reduced damage to multiple pathogens, including viruses, fungi and bacteria. An alternative strategy to the introduction of specific antagonists is the augmentation of existing antagonists in the root environment. This augmentation may result from the use of specific organic amendments, such as chitin, which stimulate populations of antagonists, thereby inducing suppressiveness. Inter-cropping or crop rotation with some tropical legumes, including velvetbean (Mucuna deeringiana), lead to management of phytoparasitic nematodes, partly through stimulation of antagonistic microorganisms. Some biorational nematicides, such as specific botanical aromatic compounds, also appear to induce suppressiveness through alterations in the soil microbial community.
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