'Plant Breeding: Sustaining the Future' Abstracts of the XVIth EUCARPIA Congress, Edinburgh, Scotland, 10-14 September 2001 GENETIC ANALYSIS OF THE INTERACTION OF CTV AND CITRUS M.J. ASINS, C. RUIZ, G.P. BERNET, M. CAMBRA, J. GUERRI, E.A. CARBONNELL Instituto Valenciano de Investigaciones Agrarias, 46113 Moncada (Valencia), Spain

Citrus Tristeza Virus (CTV) has caused the death of millions of trees grafted on sour orange (C. aurantium). However, this rootstock is very well adapted the Mediterranean, semi-arid conditions.  The aim of the present research is to analyse genetically CTV multiplication on a progeny derived from the cross between C. aurantium and the CTV resistant Poncirus trifoliata var "Flying Dragon", over a four year period.  Graft propagations of 66 hybrids were done on healthy sweet orange as rootstock.  Three months later each rootstock was graft inoculated with two patches of infected tissue (isolate T-346). One, two and three years after inoculation, hybrids and infected patches were used to detect CTV by DTBIA.  Additionally, CTV multiplication was evaluated every year as the optical density of DAS-ELISA reactions.  The genetic control of CTV multiplication was studied using more than 150 molecular markers.  Most of them were microsatellites, IRAPs, SCARs and resistance gene candidates.  A major QTL was located where Ctr had been previously mapped on the P. Trifoliata genome.  Additionally, a putative QTL, Ctx-1 was found on the same homeologous linkage group of C. aurantium, linked to a resistance gene candidate (a NBS-LRR resistance gene).  SSCP analysis of CTV genes P20 and P25 (Coat Protein) using CTV specific primers in susceptible hybrids were carried out to test whether or not this QTL from sour orange is a defeated resistance gene.  Since only a haplotype of the virus was visualised in susceptible plants, differences in multiplication are not related to selection of CTV genotypes within the isolate by the host but, to the amount id virions present in the flush.