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 present paper: a search for new CTV-resistant genotypes and tests of two strategies for this search. One of these tests is based on a study of genetic relationships among genera and species of the orange subfamily and the other on scores of molecular markers known to be linked to the CTV-resistant locus. Sampled plants were graft-inoculated with a mild CTV isolate (T-346) and two virulent ones (T-388 and T-305). Susceptible plants were those where CTV multiplication was detected beyond 4 months after inoculation. All cultivars of Poncirus trifoliata tested, as well as Severinia buxifolia and Atalantia ceylanica, were resistant to the three CTV isolates; Fortunella crassifolia (Meiwa kumquat) resists two of them. The finding of CTV resistance in this species, closely related to cultivated Citrus species, opens a new arena for CTV-resistance 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 restricted to the species where linkage analysis is available. A good documentation 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.