Abstract
1. In the present paper the writers deal with the results of a study on the oospore germination of Sclerophthora macrospora (Sacc.) Thirum., Shaw et Naras., the causal fungus of downy mildew of rice plant, and inoculation tests by oospores. 2. When the infected leaves were kept on soil flooded with water in autumn, the oospores became germinable at the end of January, and remained viable at least until May. The percentage of germination ranged from 4.08 to 0.14 during January to May. However, oospores of the infected leaves left on dry soil showed no germination. If the infected leaves were burried 2cm deep in the soil, slight germination of oosporores was obtained irrespective of the water condition in soil. 3. Germinability of the oospores was noticed after about 2 months when the infected leaves were kept on soil with flooded water in early March, May, and at the end of August, respectively. The mature oospores in the rotted leaves can germinate within 48 hours in distilled water. 4. Crushing of rotted leaves is not a necessary measure for the germination of the oospores with high germinability. 5. Distilled water may be a favorable medium for oospore germination. Soil extract did not promote germination. Tap water used seems to check the germination. Addition of 5×10-2-10-3 percent peptone in distilled water inhibited the germination. 2% sucrose solution rather accelerated the germination, but it retarded slightly the formation of zoosporangia at the tip of germ tubes. 6. The optimum temperature for the oospore germination seemed to lie between 18-20°C, the highest 25-26°C, and the minimum somewhat lower than 10-12°C. 7. In inoculation experiments to rice seedlings using oospores, a consideraby high perce-ntage of the disease incidence may sometimes be obtained. 8. Owing to the present results, the writers consider the oospores to be fairly important infection source to wild grasses in autumn and to the rice seedlings in the nursery stage.
