Mycorriza In Disease Resistance

Safir (1968) found that inoculation of onion with Glomus mosseae could significantly reduce pink root disease due to Pyrenochaeta terrestris. Later studies indicate that AM fungi can induce resistance or increase tolerance to some root-borne pathogens (Azcon - Aguilar and Barea, 1996, 1997; Caron et al., 1986; Caron, 1989; Cordier et al., 1997; Dehne, 1982; Hooker et al., 1994; Trotta et al., 1996). Glomus mosseae protected peanut plants from infection by pod rot fungal pathogens Fusarium solani and Rhizoctonia solani (Abdalla and Abdel-Fateh, 2000).

The Glomus intraradices increased P uptake and reduced disease development of Aphanomyces euteiches in pea roots (Bodker et al., 1998) Mycorrhization with Glomus mosseae and G. intraradices induced local or systemic resistance to Phytophthora parasitica in tomato roots (Cordier et al., 1996, 1998; Pozo et al., 2002). Decreased pathogen development in mycorrhizal and non-mycorrhizal parts of inoculated roots is associated with accumulation of phenolics and plant cell defense response. The protective effects induced by AM fungi against a phytoplasma is reported in tomato (Lingua et al., 2002).

AM protects an annual grass from root pathogenic fungi in the field (Newsham et al., 1995). Inoculation of onion with Glomus sp. Zac-19 delayed onion white rot epidemics caused by Sclerotium cepivorum Berk by two weeks and increased the yield by 22% under field conditions (Torres-Barragan et al., 1996). Diospyros lotus inoculated with Glomus mosseae, Glomus intraradices, Glomus versiforme significantly increased the plant growth and decreased the disease caused by Cercospora kaki under field conditions. The AM fungal inoculum even suppressed postharvest disease of potato dry rot (Fusarium sambucinum) in prenuclear minitubers (Niemira et al., 1996).

Root rot caused by Fusarium solani significantly contributes to crop yield decline, up to 50%. The inoculation of common bean (Phaseolus vulgaris) with Glomus mosseae, besides decreasing propagule number of F. solani in the rhizosphere, decreased root rot by 34 to 77% (Dar et al., 1997). In the presence of the root nodulating symbiont Rhizobium leguminosarum, mycorrhizal inoculated plants were more tolerant to the fungal root pathogen. This indicates that interactions between mycorrhizal fungi and other rhizosphere microbes might have greater effects on soil-borne pathogens than mycorrhizal fungi alone. Davis and Menge (1980) found that Glomus fasciculatum reduced Phytophthora root rot of citrus at low level of soil phosphorus but had no effect in high phosphorus soil. The VAM fungi has also been employed as biocontrol agents for Macrophomina root rot of cowpea and Fusarium wilt of tomato (Ramaraj et al., 1988). The understanding of the mechanisms of plant disease resistance in mycorrhizal plants would provide better directions towards the development of efficient crop production and sustainable agriculture.

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