Introduction

Biological control of plant pathogens presents a compelling method of increasing plant yields by suppressing or destroying pathogens, enhancing the ability of plants to resist pathogens, and/or protecting plants against pathogens. Micro-organisms antagonistic to plant pathogens may be derived from the resident microbial community or may be of foreign origin. Although there are concerns towards the release of an organism of foreign extraction, in general, biological control presents a myriad of benefits such as being a component of the environment, resistant to development of chemical pesticide resistance, being relatively safe and risk free, and by being compatible with sustainable agriculture. Arbuscular mycorrhizal fungi (AMF) form one such group of organisms that can act as bioprotectors of plants. These zygomycetous fungi that form specialized structures such as arbuscules and/or vesicles are obligate biotrophs and utilize host photosynthates for their growth. They are ubiquitous and co-exist with over 80% of terrestrial plants including agricultural or horticultural crops. Their interactions with rhizosphere flora and fauna influence the growth and fitness of the associated plants (Azcon-Aguilar and Barea 1992; Fitter and Sanders 1992). An incompatible association between the host plant and the indigenous AMF community can lead to serious losses in crop yields, indicating the significance of AMF in crop production. In contrast, a compatible association can result in enhanced plant productivity, through enhanced host P nutrition (Ravnskov and Jakobsen 1995), prevention or control of plant diseases caused by soil-borne pathogens (Caron 1989a; St-Arnaud et al. 1995), and/or enhancement of plant hormonal activity (Frankenberger and Arshad 1995).

The rhizosphere, a zone of soil loosely surrounding the roots, is a dynamic environment wherein complex chemical and microbiological activities occur (Lynch 1990). The mycorrhizosphere is the region of the rhizosphere that is subjected to modifications following AMF colonization of the host plant (Linderman 1988). Induced biochemical changes in the plant as a result of AMF root colonization is collectively termed the "mycorrhizosphere effect." The mycorrhizosphere effect typically results in a transient or permanent shift in the resident microbial community that may favor the elimination or proliferation of pathogens (Edwards et al. 1998; Meyer and Linderman 1986; Nemec 1994; Paulitz and Linderman 1989). In general, these changes are mediated by modifications in host root membrane permeability that subsequently leads to modifications in root exudate composition (Graham et al. 1981; Ratnayake et al. 1978). Meticulous management of the mycorrhizosphere may serve as an effective, safe, and environmentally friendly alternative to conventional methods of plant disease control.

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