Identification of a 334-ribonucleotide viral satellite as principal aetiological agent in a tomato necrosis epidemic.

Cucumber mosaic virus (CMV), a widespread and economically important virus of vegetable crops, often contains a satellite RNA, here designated CARNA-5 (for CMV-associated RNA 5). Viral satellites are small nucleic acids that are sequence-unrelated to, but replicatively dependent upon, the viral genome. They essentially are molecular parasites of their helper viruses, and thereby frequently modulate viral symptom expression. Some isolates of CARNA-5 change normally moderate CMV symptoms in tomato into a lethal disease named tomato necrosis; others ameliorate CMV symptoms in tomato and other important crop plants. Here we report on the identification and molecular characterization of a 334-nucleotide necrogenic CARNA-5 isolated from tomato fields in southern Italy, where a massive outbreak of lethal necrosis occurred in the summer of 1988. This is the first time that direct evidence is given for the involvement of a viral satellite in a crop disease of epidemic scale. The possible molecular interrelationships between plant, virus, satellite and other factors that influence the satellite-induced symptom modulation underlying such a catastrophe are discussed.