Citrus rootstock breeding: response of four allotetraploid somatic hybrids to Citrus tristeza virus induced infections

Four allotetraploid somatic hybrids of citrus, with potential for rootstock improvement, have been evaluated for their response to Citrus tristeza virus (CTV) infection. CTV is the most important viral pathogen affecting citrus production worldwide. Somatic combinations of 'Milam' lemon (Citrus jambhiri Lush.) + Sour orange (C. aurantium L Osb.), Calamondin (C. madurensis Lour.) + 'Keen' sour orange (C. aurantium L.), Calamondin + 'Femminello' lemon (C. limon L. Burm. F.) and Cleopatra mandarin (C. reshni Hort. ex Tan.) + 'Femminello' lemon, were studied. Plants were grafted with CTV-infected "Valencia" sweet orange budwood. Two different CTV strains collected in Sicily, considered as "mild" and "severe", were used to inoculate candidate rootstocks. The goal of this work is to select a rootstock alternative to CTV susceptible Sour orange, still the prevalent rootstock in the Mediterranean basin. DAS-ELISA and real-time PCR assays confirmed a decreased level of viral replication in tested somatic hybrids, as compared to the susceptible genotypes sour orange and Citrus alemow. The Calamondin+'Keen' sour orange genotype did not support any replication in either CTV strain. Somatic hybridization is confirmed to be an effective tool to obtain functionally new rootstocks. Our results can be considered a starting point to open new approaches for the Mediterranean citrus industry. Indeed, the four somatic hybrids have been propagated for agronomical multisite evaluation trials to further assess if their horticultural performance, fruit holding capacity, and soil adaptation are adequate to replace sour orange.