Characterization of p20 and p23 genes in Corsican isolates of Citrus tristeza virus.

Tristeza disease, caused by the Citrus tristeza virus (CTV), is the most important viral disease for almost all the citrus species. Main syndromes are quick decline, stem pitting, vein clearing and seedlings yellows but expression and intensity of symptomatology depends on the interactions of the factors involved in the pathosystem such as host-strain-rootstock-climate-vector. Several isolates of CTV have been studied all over the world. Two Corsican isolates have been previously characterized, the K strain from Marumi kumquat, known to be symptomless on Mexican lime, and the CaL -1 from calamondin inducing stem pitting, but no sequences of p20 gene are published or available in GenBank. Two additional CTV strains were recently found in Corsica. The isolates LA5 and CO3 were detected in two different orchards of 40 year old Clementine on sour orange trees that didn't show any specific symptoms ascribable to tristeza. Results of the molecular characterization of these isolates are reported. CTV presence was ascertained in young shoots collected in field by RT-PCR using specific primers for the gene p20. The amplicons obtained were analyzed by cloning and sequencing. Sequences analysis pointed out high similarity between the two isolates (98%). Multiple alignment analysis with all the p20 CTV gene sequences deposited in GenBank evidenced the identity of the virus detected in the two orchards. Respect to the already described mild, severe and atypical groups of CTV isolates, phylogenetic analysis revealed a new cluster including both LA5 and CO3.