Role of Laccaria laccata in protecting primary roots of Douglas-fir from root rot

The influence of Laccaria laccata on Fusarium oxysporum, and its ability to induce resistance to the pathogen in primary roots of Douglas-fir were investigated. Extracellular metabolites of L. laccata inhibited F. oxysporum hyphae, and delayed germination of microconidia and chlamydospores. Laccaria laccata or its cell-free metabolites also induced accumulation of osmiophilic materials in cortical cells of the primary root. The osmiophilic materials were primarily phenolic in nature. One-month-old primary roots, incubated with L. laccata or killed inoculum, were challenged with F. oxysporum. Rate of growth of the pathogen toward the root and intensity of surface colonization were not influenced by L. laccata, thus antibiosis was judged unimportant in root protection. However, cortical infection of the primary root by the pathogen was significantly less in seedlings affected by L. laccata than in controls. The frequency of hyphae was inversely proportional to the concentration of osmiophilic materials. Phenolics induced by L. laccata in the primary root are thus associated with resistance and may be the chemical basis for root protection.