Plant root-bacterial interactions in biological control of soilborne diseases and potential extension to systemic and foliar diseases

Plant-associated bacteria reside in the rhizosphere, phyllosphere, and insi de tissues of healthy plants. This chapter discusses concepts and examples of how naturally occurring and introduced bacteria may contribute to manage ment of soilborne and foliar diseases. Introduced bacteria which have demon strated biological control activity against soilborne pathogenic fungi and nematodes include rhizobacteria (root-colonising bacteria) and endophytic b acteria (bacteria isolated from within healthy plant tissues). Recently, so me introduced rhizobacteria have been found to enhance plant defences, lead ing to systemic protection against foliar pathogens upon seed or root-treat ments with the rhizobacteria. In these cases, introduction of the rhizobact eria results in reduced damage to multiple pathogens, including viruses, fu ngi and bacteria. An alternative strategy to the introduction of specific a ntagonists is the augmentation of existing antagonists in the root environm ent. This augmentation may result from the use of specific organic, amendme nts, such as chitin, which stimulate populations of antagonists, thereby in ducing suppressiveness; Intercropping or crop rotation with some tropical l egumes, including velvetbean (Mucuna deeringiana), lead to management of ph ytoparasitic nematodes, partly through stimulation of antagonistic microorg anisms. Some biorational nematicides, such as specific botanical aromatic c ompounds, also appear to induce suppressiveness through alterations in the soil microbial community.