Further studies on the rice dwarf virus.-I

Abstract

1. The incubation periods of the rice dwarf virus were investigated both in the insect vectors and rice plants. In the leafhoppers, the shortest incubation period was found to be 8∼12 days and the longest to be 33∼37 days at a constant temperature of 18°C., whereas no insect became infective at 13°C. It appears that the virus does not multiply in the insect body at 13°C. In young rice plants the shortest incubation period was 11 days at 18°C.2. The ability to transmit the virus seems to vary in leafhoppers captured at different localities, the leafhoppers from Chiba Prefecture having transmitted the virus more readily than those from Shiga and Akita Prefectures. The leafhoppers from Shiga and Akita Prefectures became infective more readily when virus preparations were injected with a glass capillary into their bodies through a puncture made in the abdomen.3. The virus concentration in the leafhopper was shown to increase most rapidly during a period of 15∼20 days after the virus acquisition. In an infective leafhopper, the virus concentration appears to be almost equal in the cephalothorax and in the abdomen, the extracts from these parts having the same dilution end point, 2×10-4.4. The virus concentration in the infected rice plant attained its maximum 40 days after inoculation. At this time, extracts from leaves and stems of diseased rice plants were infective at a dilution of 10-3 and those from roots at a dilution of 10-2. Extracts from the yellow-green portions of affected leaves were infective when diluted to 10-4, while those from the green portions were remarkably less infective.5. The virus was carried in the vectors through 4 serial passages. It was calculated that the quantity of the virus in the starting leafhopper would have undergone a dilution of 10-11 in the last group of the series, if the virus had not multiplied. Since the dilution end-point of the juice from infective insects was 10-4, the author was led to the conclusion that the virus multiplies in the insect vector.