Germ Tube Tip Research Articles

Studies on the downy mildew of rice plant.

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.

On the Relation of Atmospheric Humidity to the Infection of the Rice Plant by Ophiobolus Miyabeanus ITO et KURIBAYASHI and to the Germination of its Conidia

1. This paper deals with the results of the writer's experiments on the relation of relative humidity to the infection of the rice plant by Ophiobolus Miyabeanus ITO et KURIBAYASHI and also to the germination of its conidia.2. After the drops of conidial suspension of the causal fungus sprayed on rice seedlings had dried, these seedlings were kept in desiccators, in which the air had been adjusted to the desirable constant relative humidities by means of using sulphuric acid in various concentrations. Those desiccators containing the inoculated rice seedlings were placed for 18 hours in a room controlled at 25°C., and then the seedlings were removed to a green-house bench. After 5 days, the seedlings inoculated and kept in the air of 100, 97.5, 95 and 92% in relative humidities for 18 hours showed typical lesions of the disease, while the seedlings kept at 89% for the same period remained quite healthy.3. Small drops of conidial suspension of the fungus and pure water as controls were placed on clean slide-glasses and dried naturally at room temperature. After the drops had dried completely, these slides were placed on supports in PETRI dishes, in which the air humidity had been controlled in various degrees by means of using sulphuric acid. These dishes were kept at 25°C. for 18 hours and then the germinability of the spores in the different air-humidities was tested. The conidia on the slides kept at 92% relative humidity germinated slightly, while those kept at 89% showed no signs of germination.4. The limitation of the relative humidity in which the disease occurs is, therefore, almost the same as in the case of the conidial-germination of its causal fungus on slide-glasses. It is quite reasonable to assume that no infection of rice seedlings by the fungus takes place in relative humidities lower than 89%, since the conidia are not able to germinate under such conditions.5. The drying of the conidial suspensions of the causal fungus caused to reduce or retard remarkably the germinability of its conidia.6. The new conidia were formed on the tips of germ-tubes from the conidia of the causal fungus kept at 25°C. for 18 hours in the relative humidities higher than 97.5%.