EFFECTS OF APHID TRANSMISSION ON THE STRUCTURE OF A MILD AND A SEVERE CITRUS TRISTEZA VIRUS POPULATION FROM ITALY

Two homogeneous Italian Citrus tristeza virus (CTV) populations associated with quick decline and seedling yellows were behind the heavy losses on Navelina and Tarocco sweet oranges along the Apulia region and the South-Eastern of Sicily, respectively. The genetic divergence between selected isolates was assessed by sequencing the p25 major coat protein gene. For a better understanding of their spread dynamics and the effect of aphid transmission on their genetic structure, representative isolates from each CTV population were experimentally transmitted by local aphid biotypes and molecularly characterized by single strand conformation polymorphism of the p18, p20, p23 and p25 genes, amplification with the POL markers and p23 sequencing. A high nucleotide identity was observed between the Apulian isolates and the Spanish mild strain T385, and also between the Sicilian isolate and the Spanish strain T318A. Moreover, the CTV isolates studied appeared to be highly transmissible by A. gossypii (50% of efficiency), with only minor variations being detected in the population structure of some genomic segments of the aphid-derived sub-isolates. These results provide a plausible explanation of the rapid and homogeneous dispersion of the two CTV types within the Italian groves.