Surface ultrastructure of conidia, germ tubes, appressoria and conidiophores of Peronospora parasitica, and of the diseased leaves (Raphnanus sativus) was observed by the scanning electron microscopy. Mature conidia were approximately 7×10μm in length. Most of appressoria were connected laterally with the germ tube. Appressoria and germ tubes had smooth surfaced structures. Penetration pegs can usually be found on the surface of the diseased region when materials were fixed 4 days after inoculation. Usually, cuticular penetration was observed, and stomatal infection was sometimes found. Conidiophore was formed through the stoma. In the early stages of conidiophore formation, no concave image was found in the basal region of the conidiophore, and then a contracted image was found in the region when the conidiophore developed. This seems to be caused by the mechanical pressure of stomatal guard cells. A wrinkled surface of young conidiophore was also observed. Conidia were formed directly from the swelling tips of the conidiophore, and they had the same surface structure as the conidiophore. Old conidia, however, had many wart-like structures, although the mature conidiophore had a smooth surface. Stomata on naturally infected leaves seem to expand more remarkably as compared with those of healthy ones. The surface of the diseased region also showed a crumpled image by exfoliation of wax and cuticular substance.
