Culture of surface-sterilized needles from a stand of old-growth Douglas fir trees on the western slope of the Oregon Cascades revealed the presence of latent fungal infections in all needles examined older than 3 years. Scanning electron micrographs of cut needle surfaces seen in transverse section showed occasional intercellular fungal hyphae in the needle parenchyma. Two species of fungi,Phyllostictasp. and a suspected member of the Hemiphacidiaceae, appeared as the major colonizers of the needle blade; a third species, as yet unidentified, commonly grew out from the needle petiole. Endophytes were widespread in needles throughout the watershed regardless of the elevation and exposure of the individual trees sampled. Similarly, height of the needles in the tree canopy showed little correlation with the frequency of internal infections. Individual needles were cut into 10 segments before culture, and patterns of endophyte distribution within the needles were observed. The data are consistent with the hypothesis that needles are multiply infected during their 1st year. The initial rise in endophyte frequency occurs between November and March on 1st-year needles, a pattern suggesting waterborne spores as agents of infection. BothPhyllostictaand the hemiphacidiaceous fungus are known to produce gloeoid conidia, which are thought to be dispersed by rain. With increasing age, all needle segments become infected with endophytes.
展开▼
