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Cytology and Histology

Phenolic Compounds and Resistance to Fungal Pathogens Induced in Primary Roots of Douglas-Fir Seedlings by the Ectomycorrhizal Fungus Laccaria laccata. D. M. Sylvia, Former graduate research assistant, Department of Plant Pathology, Cornell University, Ithaca, NY 14853-0331, Present address of senior author: Department of Plant Pathology, University of Florida, Gainesville 32611; W. A. Sinclair, professor, Department of Plant Pathology, Cornell University, Ithaca, NY 14853-0331. Phytopathology 73:390-397. Accepted for publication 23 August 1982. Copyright 1983 The American Phytopathological Society. DOI: 10.1094/Phyto-73-390.

A mycorrhizal strain of Laccaria laccata, which can protect primary roots of seedlings of Douglas-fir (Pseudotsuga menziesii) from lethal root rot by Fusarium oxysporum, induced an accumulation of osmiophilic materials in cortical cells of primary roots. Histochemical tests demonstrated that the osmiophilic materials were primarily phenolic. One-month-old primary roots, incubated with L. laccata, its cell-free metabolites, or killed inoculum, were challenged with F. oxysporum or Pythium irregulare. Rate of advance of hyphae toward roots, extent of surface colonization and cortical infection by the pathogens, and amount of phenolic material in the cortex were estimated by optical techniques. Cortical infection by each pathogen was significantly less in seedlings affected by L. laccata or its metabolites than in controls. Hyphal frequency was inversely proportional to the concentration of osmiophilic materials. Rates of growth of the pathogens toward the root and intensity of rhizoplane colonization were not influenced by L. laccata. Thus, we judged antibiosis unimportant in root protection although L. laccata had been shown capable of inhibiting F. oxysporum in vitro. Phenolics induced by L. laccata in the primary root may be the basis for root protection.