CONIDIOGENESIS IN OIDIODENDRON PERICONIOIDES AND ULTRASTRUCTURE OF ERICOID MYCORRHIZAS FORMED WITH RHODODENDRON BRACHYCARPUM

Oidiodendron periconioides was isolated from hair roots of seedlings o f Rhododendron brachycarpum growing in pots containing a commercial pe at moss in Japan. The unusual process of conidiogenesis in O. periconi oides involved the formation of conidia from vesicle-like swellings th at developed from vegetative hyphae or from the apex of a mononematous conidiophore. Scanning electron microscope examinations of mature con idia revealed that they were regularly dimpled, giving them the charac teristic and previously reported radiate pattern of sculpturing visibl e with the light microscope. Oidiodendron periconioides formed ericoid mycorrhizas that were morphologically and ultrastructurally similar t o previously studied ericoid systems involving Oidiodendron mains and Hymenoscyphus ericae, and a few other species of Rhododendron. Rough-w alled hyphae grew appressed to the epidermal cells of young hair roots , penetrated the external walls via narrow penetration tubes, and inva ginated the plasma membrane of the host cell during the process of for ming a complex system of hyphal loops. Infection was restricted to the zone proximal to the active region of the meristem. Hyphal complexes were short-lived and degenerated as the epidermal cells matured and di ed. The physiological role of O. periconioides as a mycorrhizal endoph yte is unknown.