Suppressive Influence ofLaccaria laccataonFusarium oxysporumand on Douglas-fir Seedlings

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Abstract
Some strains of the ectomycorrhizal fungus L. laccata can invade primary roots of seedlings of Douglas-fir (P. menziesii) and protect them from lethal attack by F. oxysporum. The potential of L. laccata to inhibit F. oxysporum and to influence the development of young seedlings was appraised. In dual cultures on agar media with pH relatively less favorable for growth of F. oxysporum than L. laccata, diffusible metabolites of a mycorrhizal strain of L. laccata inhibited growth and caused distortion of hyphae of the pathogen. Cell-free fluid from cultures of this strain of L. laccata delayed germination of microconidia and chlamydospores of F. oxysporum. Only slight suppression occurred under conditions that favored rapid growth of F. oxysporum. A nonmycorrhizal strain of L. laccata inhibited growth of F. oxysporum over a wide range of conditions on agar media, but culture filtrates did not affect spore germination. Antibiosis may be a partial basis for root protection by the mycorrhizal strain of L. laccata. On agar plates and in gnotobiotic systems without soil, L. laccata or its cell-free metabolites suppressed root growth of Douglas-fir. In sterile or pasteurized soil, however, no suppression occurred.