Strategies to search for new Citrus Tristeza Virus resistant genotypes in a germplasm bank

Abstract

Virulent isolates of citrus tristeza virus (CTV) are continuously arising and their spread threatens world citrus industry. Methods for effective utilisation of material deposited in germplasm banks are needed. Two objectives are pursued: the search of new CTV resistant genotypes within a citrus germplasm bank and to test two sampling strategies for this search. One is based on a study of genetic relationships among species of the orange subfamily and the other based on scoring genotypes at molecular marker loci known to be linked to the CTV resistant locus Ctr. Sampled plants were graft inoculated with a mild and two virulent CTV isolates. Susceptibility, i. e. multiplication of CTV in plants, was checked by DAS-ELISA and DTBIA at four, six and ten months after inoculation. All cultivars of Poncirus trifoliata tested, Severinia buxifolia, Atalantia ceylanica and Fortunella crassifolia behave as CTV resistant. Given that F. crassifolia yields edible fruits, the finding of CTV resistance in this species makes the improvement for CTV resistance of scion varieties, now feasible. It is shown that the sampling strategy based on phylogenetic data can be very useful to search new genotypes expressing an agronomically important trait. The marker-assisted criterion may only be worthwhile when the search is restricted to accessions of the species where linkage analysis was carried out. Therefore, a good documentation system that allows quick and easy sampling of accessions to form core collections in order to carry out the search of new and useful genes, is suggested to enhance genetic resources.