Yang, G., S. Tolkowsky Laboratory, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel
Che, X., S. Tolkowsky Laboratory, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel
Gofman, R., S. Tolkowsky Laboratory, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel
Ben-Shalom, Y., S. Tolkowsky Laboratory, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel
Piestun, D., S. Tolkowsky Laboratory, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel
Gafny, R., S. Tolkowsky Laboratory, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel
Mawassi, M., S. Tolkowsky Laboratory, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel
Bar-Joseph, M., S. Tolkowsky Laboratory, Agricultural Research Organization, Volcani Center, Bet Dagan, Israel

Citrus tristeza virus (CTV) strains were previously catalogued as seedling-yellows (SY) and non-SY (nSY) types, according to their yellowing and stunting effects on indicator seedlings. Among subisolates of the VT strain, which were selected from chronically infected Alemow plants, there was a correlation between the presence of 2.4-, 2.7- and 4.5-kb D-RNAs, and SY and nSY reactions, respectively. Similarly, plants infected with Mor-T subisolates, which cause SY, contained D-RNAs of 2.6 to 2.8 kb, while nSY subisolates from recovered sour orange tissue contained a major D-RNA of 5.1 kb. Plants harboring the 2.7-kb D-RNA were protected against challenge inoculation with a subisolate harboring the 4.5-kb D-RNA. This study suggests that the nSY reaction results either from the absence of SY gene(s) in the genomes of certain CTV strains or through the suppression of the effects of SY gene(s) by D-RNAs with 5' parts larger than 4000 nt.