Severe <i>Citrus tristeza virus</i>Isolates from Eastern Mexico Are Related to the T36 Genotype Group

Citrus tristeza virus (CTV) outbreaks have been reported in the main citrus growing region of Mexico in the past four years. Recently, in eastern Mexico (the major citrus-growing region in the country), severe CTV isolates have been detected. However, the molecular identity of observed isolates remains unestablished. This research was undertaken to elucidate the molecular characterization of CTV populations spreading in this region and to compare it with phylogeny of existing isolates. Genotyping of 32 collected isolates was performed using reverse-transcription polymerase chain reaction (RT-PCR) with sequence analysis of the coat protein (CP) gene, putatively associated with pathogenicity. This protein is a 25 kDa major capsid protein, which forms a long virion body coating 95% of the particle length. A comparative sequence analysis was performed using CTV sequences from different geographical origins already published and deposited in the GenBank databases. Phylogenetic analysis showed that the degree of sequence divergence among isolates correlated with their pathogenicity. Based on the sequencing results, the collected isolates were categorizedn as mild or severe phylogenetic clusters, each being genetically distinct. The severe group was associated with either a-like or with a T36-like genotype. The latter group matched with the quick decline and stem pitting drastic symptoms observed in the field. This study identified the presence of severe CTV isolates related to the T36-like genotype and to the cause of quick decline and stem pitting in sweet orange propagated on sour orange rootstock. Knowledge derived from these analyses could serve to design management strategies for this disease and to understand the current epidemic outbreak in scenarios where the most efficient vector is present.