Abstract
Cucumber mosaic virus (CMV) and watermelon mosaic virus (WMV) were inoculated alone or together to three cucumber cultivars having different degrees of resistance to the viruses, and the concentration of the virus in the infected leaf and the rate of transmission of the viruses by the Aphis gossypii were examined. The virus concentration in leaves infected with either CMV or WMV showed a highly positive correlation with the rate of transmission by the aphid. The concentration of the CMV was low in the cultivar showing milder symptoms. On the other hand, the concentration of WMV was high in any cultivar, and it was easily transmitted by the aphid from the torelant cultivar, Aonaga-suyo, showing no symptoms. The disease symptoms were severer in leaves infected with both viruses in all the cultivars tested, than in the leaves infected with either one of the viruses. The transmission rates of both viruses when leaves were doubly infected with both viruses was much different from that when the leaves were infected with only one virus. The transmission rate of CMV from doubly infected leaves was higher in all the cultivars tested than that from the singly infected leaves. The transmission rate of CMV was increased 7 fold in the cultivar Aonaga-suyo which is resistant to CMV. Conversely, the transmission rate of WMV from doubly infected leaves in all the cultivars tested was markedly decreased than that from the singly infected leaves. These results are thought to reflect the differerence in the concentration of the two viruses in the leaves infected with both viruses together.
