Initial evidence for the role of ectomycorrhizal fungi in disease suppression was provided by a number of field observations that showed mycorrhizal-associated seedlings or trees of both angiosperms and gymnosperms were more resistant to root pathogens than their nonmycorrhizal counterparts (Marx 1973). Ectomycorrhizal roots of various Pinus spp. and Sitka spruce (Picea sitchensis) seedlings were resistant to infection by a Rhizoctonia sp. that could readily infected nonmycorrhizal feeder roots (Levisohn 1954). Richard et al. (1971) suggested that the presence of ectomycorrhizal fungus, Suillus granulatus in the substratum completely prevented any negative effect of endophytic Mycelium radicis-atrovirens on Picea mariana seedlings. Hashimoto and Hyakumachi (2001) also suggested that the ectomycorrhizal fungi suppressed the deleterious effect of endophytic M. radicis-atrovirens on Betula platyphylla var. japonica seedlings.

Arbuscular mycorrhizae associations have been shown to reduce damage caused by soil-borne plant pathogens. Although few AM isolates have been fully studied, some appear to be more effective than others. Furthermore, the degree of protection varies with the pathogen involved, and can be modified by soil types and other environmental conditions. Trotta et al. (1996) reported that the AM fungus, Glomus mosseae, reduced adventitious root necrosis and necrotic root apices caused by Phytophthora nicotianae var. parasitica by 63 -89%. The AM associations are also known to limit the damage by bacterial pathogens and pathogenic root nematodes (Garcia-Garrido and Ocampo 1989; Hussey and Roncadori 1982), however, the results are not consistent.

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