A biological control agent for plants and a method of inoculating the plants with the biological control agent in order protect against plant disease are provided. The treatment method consists of inoculating the plant with a strain of cucumber mosaic virus (CMV), referred to as the KU1 strain, having an associated viral satellite RNA of SEQ ID NO: 1. Particularly, the KU1 strain may be used to protect plants against a particular viral strain of CMV, referred to as the KU2 strain, characterized by the associated viral satellite RNA of SEQ ID NO: 2, which causes tomato necrosis. Additionally, the KU1 strain may be used to protect against potato spindle tuber viroid (PSTV), which causes tomato stunting, fusarium wilt disease in the tomato (caused by Fusarium oxysporum f. sp. Lycopersicae), and leaf spotting disease in the tomato (caused by Alternaria alternate).