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Allelochemicals: Biological Control of Plant Pathogens and Diseases, 181 -192. © 2006 Springer. Printed in the Netherlands.

heavy reliance on pesticides. Enhancing crop resistance against diseases and herbivores is an ideal approach to reduce pesticide application.

Plants possess both constitutive and inducible chemical defense mechanisms. Before pathogen infection, plants may contain significant amounts of constitutive secondary metabolites including phenolics, terpenoids, and steroids, which are toxic to invading organisms (Levin, 1976; Mauch-Mani and Metraux, 1998). Plants may also activate their production of certain defensive chemicals after pathogen infection. These inducible defense compounds are usually only produced and accumulated after specific recognition of the invading organism, and are known as phytoalexins (Dixon, 1986; Hammerschmidt, 1999). Plants can acquire induced resistance to pathogens after infection with necrotrophic attackers, nonpathogenic root-colonizing pseudomonads, salicylic acid, beta-aminobutyric acid and many other natural or synthetic compounds (Conrath et al., 2002; Benhamou, 1996).

Mycorrhizal fungi provide an effective alternative method of disease control especially for those pathogens which effect the below ground plant parts. In mycorrhizal fungi lies enormous potential for use as biological control agent for soil-borne diseases as the root diseases are of the most difficult to manage and lead to losses in disturbing proportions.

The mycorrhizal symbiosis substantially influence plant growth under a variety of stressful conditions and their role in biological control of soil/root - borne pathogens is of immense importance both in the agricultural system as well as in the forestry (Linderman, 1994).

Mycorrhizal associations increase growth and yield of many crop plants by enhancing nutrient uptake, resistance to drought and salinity and increases tolerance to pathogens (Gianinazzi-Pearson, 1996; Mukerji, 1999; Singh et al., 2000; LudwigMüller, 2004). Of the seven types of mycorrhiza known (Srivastava et al., 1996; Mukerji et al., 1997; Raina et al., 2000; Redecker et al., 2000), ectomycorrhiza and vesicular-arbuscular mycorrhiza (VAM) are more important in agriculture and forestry. Ecotmyocrrhizal associations are more prevalent in temperate and sub-temperate regions, while VAM/AM associations are common features of tropical and substropical regions of the world. During colonization, distinct structures are formed by the arbuscular mycorrhizal (AM) fungi, with in the host roots - internal hyphae, arbuscules and vesicles (Walker, 1992). The complex cellular relationship between host roots and AM fungi requires a continuous exchange of signals, for proper development of mycorrhiza in the roots of a host plant (Gianinazzi-Pearson, 1996). Plant hormones may be a suitable candidate for the regulation of such a symbiosis. There is little information about the function of plant harmones during the colonization process although there is evidence that they are involved in signaling events between AM fungi and host plants (Barker and Tagu, 2000; Ludwig - Müller, 2000a,b). In addition, it has been suggested that phytohormones, such as IAA and cytokinins, released by mycorrhizal fungi may also contribute to the enhancement of plant growth. (Frankenberger Jr. and Arsad, 1995).

This review describes the role of mycorrhizal associations in disease control.

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