Extracellular Sheaths on Hyphae of Two Species of Helminthosporium

SUMMARYTransmission-electron micrographs indicate the presence of a well-defined extracellular sheath, approximately 0.5 μm thick, on more than 90% of hyphae of Helminthosporium victoriae on and in oat leaves 24 h after inoculation. Similar, but less well-developed sheaths were found on hyphae of H. maydis race T on and in corn leaves 48 h after inoculation and these findings confirmed previous scanning-electron-microscope observations. On inoculated oat leaves, hyphal sheaths on nonpathogenic isolates of H. victoriae were entirely similar to those on pathogenic isolates. Hyphal sheaths are primarily products of fungal metabolism since they formed on hyphae of H. victoriae grown in distilled water.