Abstract
Morphological studies on Peronospora parasitica Pers. ex Fr. was carried out in order to understand its behavior in tissues of Japanese radish. Fungal growth in leaf, petiole, hypocotyl and root of a susceptible cultivar, Shirokubi-miyashige and a resistant cultivar, Heian-tokinashi was investigated. In leaf, the hypha turned and twisted irregularly in the intercellular spaces of spongy parenchyma, and usually formed one haustorium in one host cell. However in root tissue where the parenchyma cells were large and much closer together, the hypha was smooth and formed one to several haustoria in one host cell. Hyphal growth pattern in petiole and hypocotyl was similar to that in root tissue. Fungal growth rate in root slices of resistant and susceptible cultivar was already significantly different on the 3rd day after inoculation, while the shape of its intercellular hypha and haustorium was almost the same. Prior to haustorium formation, a hitherto undescribed structure, leaf-like structure (LL structure), was formed from the intercellular hypha in narrow space between root parenchyma cells. It is flat in shape of ca. 6μm in thickness, covering the surface of host cell. Size of the LL structure was proportional to that of host cells. The LL structure formed various type of haustoria, ranging 0 to 25 numbers in one cell. The number of the LL structures depended on the hyphal density in the tissue.
