APPLICATION OF SELECTED VIROIDS FOR DWARFING AND ENHANCEMENT OF PRODUCTION OF VALENCIA ORANGE

Citrus viroids, CVd-Ia, CVd-IIa, and CVd-IIIb, representative of three distinct Citrus Viroid Groups, resulted in a statistically significan t reduction of 20-50% in tree size of Valencia orange, Citrus sinensis (L.) Osb., on Poncirus trifoliata (L.) Raf, rootstock. Reduction in c anopy volume was accompanied by an increase in yield per tree in the p resence of CVd-IIa. whereas a net decreases in yield resulted with eit her CVd-Ia or CVd-IIIb. Nevertheless, the yield per canopy volume was significantly enhanced in all viroid-containing trees when compared wi th the viroid-free trees. Analyses of ''dwarfing factors'' from differ ent citrus growing countries indicated that single viroid isolates wit h nucleotide sequences similar to CVd-IIIb have been independently sel ected.:Although pathogenic agents in some hosts, selected viroids may also be viewed as transmissible, small, nuclear ribonucleic acids (Tsn -RNAs) in those specific hosts which do not express a disease syndrome .