Allelochemicals: Biological Control of Plant Pathogens and Diseases, 123 -142. © 2006 Springer. Printed in the Netherlands.

capable of regulating ecosystem health and biodiversity (Wardle et al., 1997; Mallik, 2000).

Many secondary metabolites can act at both the trophic and the informational level, becoming attractants or repellents, toxins or growth stimulants, depending upon the microbial partnerships involved (Dusenbery, 1992). Accordingly, microbially produced allelochemicals have been reported to express themselves as lytic agents or enzymes (Fridlender et al., 1993; Jacobson et al., 1994; Glick et al., 1999), antibiotics (Lynch, 1976; Bender et al., 1999), siderophores (Buysens et al., 1994; Marschener and Crowley, 1997) auxins (Patten and Glick, 1996; Glickman et al., 1998; Glick et al., 1999), volatile compounds (Claydon et al., 1987; Bakker and Schippers, 1987) and phytotoxic substances (Hoagland and Cutler, 2000).

An intimacy is seen to extend between plants and microbes, in as much as plants appear able to influence the composition of the microbial community around their root systems by leaking specific carbohydrates, carboxylic and amino acids (Grayston et al., 1998) into the root zone, as well as through the 'carbon-loading' that occurs as root cell material is sloughed off during root growth (Hawes and Brigham, 1992). Hawes et al., 1998). In turn, rhizobacteria appear able to induce root exudation responses in plants (Bolton et al., 1993; Merharg and Killham, 1995). The result is a circular allelopathic cascade initiated by plant root exudates that trigger a positive microbial 'allelopathic feedback' in which the final receptor organism is also the initiator.

It is this allelochemical interaction amongst soil microbial communities and the way in which their relationships subsequently influence plant health and disease development that will form the emphasis of the present chapter.

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