PROBLEM TO BE SOLVED: To treat a plant infected with cucumber mosaic virus, by inoculation or administration of a satellite RNA, an attenuated virus or the like to the plant so as to mitigate disease symptoms.
SOLUTION: A plant infected with cucumber mosaic virus (CMV) is treated by inoculating or administering it with a satellite RNA, an attenuated virus containing the same or an artifact producing the satellite RNA in cells so as to migitate disease symptoms. The satellite RNA is an RNA which exists in a CMV particle and is other than those essential to CMV infection and proliferation and is preferably an RNA molecle with 300-400 bases. Furthermore the satellite RNA preferably suppresses proliferation of CMV and mitigates disease symptoms. The attenuated virus is preferably produced by combination of the attenuated satellite RNA with mild strain CMV.