Abstract
A mosaic disease was found in potato cultivars of Irish Cobbler, Benimaru, Tarumae, and Eniwa at the Potato Foundation Stock Seed Farm, Obihiro, Hokkaido, in 1967 and succeeding two or three years. The diseased plants showed pale-green small stipples along leaf veins, followed by general mottling, on leaves at their flowering stage, and later showed necrotic spots on lower leaves. In Irish Cobbler blonzing of upper surface of leaves was also observed at the later stage of growth. Potato virus X was not detected in the diseased plants by the serological method, but the virus which would be potato virus S was isolated from diseased plants of Irish Cobbler. When inoculated with the virus by sap inoculation, similar symptoms to potato virus infected plants developed on virus-free plants of Irish Cobbler. Positive reaction was obtained between the virus and the antiserum against potato virus S. By sap inoculation, the virus caused veinclearing and mosaic symptoms on Nicotiana debneyi, chlorotic small local lesions and systemic veinclearing on Chenopodium murale. Chenopodium amaranticolor C. quinoa, Solanum villosum, Cyamopsis tetragonoloba (Texsel), Comphrena globosa, and Tetragonia expansa were also susceptible to the virus. The virus was inactivated in ten minutes between 55 and 60C, and after 3-4 days at room temperature (ca. 20C). Dilution end point was between 1:1, 000 and 1:10, 000. By the direct negative staining method, elongated particles of about 650nm in length were frequently observed in the diseased plants. The virus could not be transmitted by green peach aphid, Myzus persicae (Sulz.). In cross protection tests, potato virus S isolated from apparently healthy plants of Irish Cobbler completely protectedthe this virus infection. Mild mosaic symptoms appeared on virusfree plants of Irish Cobbler by sap inoculation with potato virus S. From these results it is concluded that the mosaic disease examined is caused by a strain of potato virus S, designated PVSM by the present authors. Symptoms caused by the virus on several potato cultivars are described. The symptoms on Irihs Cobbler were very clear at 20C, but not at 25C.
