Modulation of Viral Plant Diseases by Secondary RNA Agents *

This chapter considers secondary RNA agents, capable of modulating viral plant disease symptoms, only viral satellites and defective interfering RNAs. The disease turned out to be of viral etiology and probably represents the first clearcut, documented example of viral plant disease modulation by a secondary RNA agent, specifically, a viral satellite. The parallel correlations of the high or low incidence of cucumber mosaic viral-induced tomato necrosis in the French work and the high and low proportions of CARNA 5 in cucumber mosaic viral-S, both depending on host passage history of the virus, were strikingly obvious. Cucumber mosaic viral-associated and other viral satellites modulate viral disease expression in a number of different ways, but very often in the form of symptom attenuation. The fact that this latter disease attenuation was in tomato showed that there existed "strains" of CARNA 5 capable of exerting different disease-modulating properties in the same host species.