1989 Volume 55 Issue 5 Pages 561-566
A method for propagation of Japanese radish-downy mildew fungus (Peronospora parasitica Pers.: Fr.) was developed using the seedlings of Japanese radish (Raphanus sativus Linn. var. horteinsis). Since the cotyledon and the true leaf of the seedling are resistant to the fungus, they were subjected to hot water (50C) treatment or the root was cut off to weaken resistance. Each of the cotyledons of cv. Awa-ichigo, Sarakamuri and Daimaru-shogoin, which had been grown for 7 to 11 days in the field and treated with hot water were put in a test tube (2.8×19cm) containing 15ml of distilled water after wrapping root with cotton. The upper surfaces of these cotyledons were inoculated by dropping a conidial suspension of the fungus. The percentage of the conidiophore formation on the cotyledons grown for one week at 20C, 1, 000lux after inoculation was highest on the 11-day-old cotyledon, treated with hot water for 60sec, of cv. Daimaru-shogoin. On the true leaf of 3-week-old seedlings, the percentage was highest on cv. Sarakamuri treated with hot water for 30sec. Since the cotyledons of cv. Shirokubi-miyashige, Heian-tokinashi and Daimaru-shogoin, which had been grown for 4 to 6 days in a growth chamber (25C, 5, 000lux), were very susceptible to hot water treatment, their roots were cut off instead. Cut surfaces of hypocotyls were wrapped with cotton wetted with sterilized distilled water or a modified Knop solution in order to keep the cotyledons from withering. The lower surfaces of cotyledons were inoculated with the suspension of conidium. The percentage of the conidiophore formation was highest on the 6-day-old cotyledon of cv. Shirokubi-miyashige. In the case of cv. Daimaru-shogoin, the 4-day-old cotyledon was best suited. A dark treatment (18hr) of the infected cotyledon on the 6th day after inoculation stimulated conidiophore formation following synchronized formation of the conidia. Also, many conidiophores were formed on the infected cotyledon when moved into an incubator (20C) afte: being stored in a refrigerator (5C) for two weeks from the 3rd day after inoculation.