EFFICIENT SEARCH FOR NEW RESISTANT GENOTYPES TO THE CITRUS TRISTEZA-CLOSTEROVIRUS IN THE ORANGE SUBFAMILY AURANTIOIDEAE

Virulent isolates of the citrus tristeza virus (CTV) are continuously arising and their spread threatens the world citrus industry. Methods for effective utilization of material conserved in germplasm banks are needed in plant improvement. Two objectives are pursued in the presen t paper: a search for new CTV-resistant genotypes and tests of two str ategies for this search. One of these tests is based on a study of gen etic relationships among genera and species of the orange subfamily an d the other on scores of molecular markers known to be linked to the C TV-resistant locus. Sampled plants were graft-inoculated with a mild C TV isolate (T-346) and two virulent ones (T-388 and T-305). Susceptibl e plants were those where CTV multiplication was detected beyond 4 mon ths after inoculation. All cultivars of Poncirus trifoliata tested, as well as Severinia buxifolia and Atalantia ceylanica, were resistant t o the three CTV isolates; Fortunella crassifolia (Meiwa kumquat) resis ts two of them. The finding of CTV resistance in this species, closely related to cultivated Citrus species, opens a new arena for CTV-resis tance improvement of oranges and mandarines by sexual hybridization. \ The searching strategy based on phylogenetic data has been successful , whereas the other one may be worthwhile only when the search is rest ricted to the species where linkage analysis is available. A good docu mentation system that allows quick sampling of accessions to build up core collections and where the location of new and useful genes could be easily worked out, is suggested to enhance germplasm utilization.