A new strain of cucumber mosaic virus (CMV-YM) was isolated from tobacco plant (cv. Bright Yellow) collected in Kagawa Prefecture, Shikoku Island, Japan, in 1960. In 1969 the occurrence of the same strain was also observed in some districts in Okayama Prefecture. The diseased tobacco plants showed only yellow mottle symptoms without malformation and necrotic lesions, and little retard of plant growth.
By the sap inoculation, the virus induced brilliant yellow mottle on seedlings of some cultivars of tobacco, Nicotiana glutinosa, N. sylvestris and red pepper (Capsicum annuum). The symptoms on these inoculated host plants are characterized by the lack of malformation, distortion and necrosis. The virus caused mild green mottle in Zinnia elegans, local and systemic necrotic lesions in sesame, mild yellow mottle in cucumber and necrotic local lesions on inoculated leaves of cowpea, Chenopodium amaranticolor and C. quinoa. Physalis floridana and spinach showed green mottle and a little malformation. The strain can be differenciated from other strains of CMV, viz. ordinary (CMV-O), yellow (CMV-Y), mild (CMV-C) and legume (CMV-LE) strains, which were obtained from field tobacco in Japan, by the reactions of some differencial hosts, such as tobacco, N. glutinosa, sesame, cowpea and zinnia.
Thermal inactivation point of crude juice extracted from infected tobacco leaves was 55-60°C for 10 minutes. Dilution end point was around 1:10, 000 with phosphate buffer (0.1M, pH 7.0). Longevity in vitro was 2-3 days at 25°C. Phenol extracts from infected tobacco leaves were more infectious than control buffer-homogenate, similarly to those of the other strains. Cross protection against other strains including necrotic strain (CMV-N), in tobacco, cowpea and zinnia was completely positive. CMV-C strain gave incomplete protection against the other strains (Tomaru et al., 1967), but it gave complete protection against the present virus, suggesting that the latter is more closely related to CMV-C. It needs, however, further investigations to determine the relationship between these two strains. Serological positive reactions were observed between the present virus and CMV-Y antiserum, and antiserum of the present virus and other 5 strains in agar-gel diffusion and microdroplet-precipitin tests. Electron microscopy of the purified virus showed spherical or polyhedral particles with a diameter of about 30mμ. The periodic change in the virus concentration in top leaves of infected tobacco plant was almost similar to the case in CMV-O.
For the strain described here the authors propose the name yellow mild mottle strain of CMV (CMV-YM).