Citrus tristeza virus in Florida: A Synthesis of Historical and Contemporary Biological, Serological, and Genetic Data

The decline of trees on sour orange rootstock is the only commercially important disease currently caused by Citrus tristeza virus (CTV) in Florida. Virus isolates from commercial citrus plantings in Florida have previously been characterized biologically as mild (do not cause decline on sour orange rootstock) and severe (do cause decline). CTV-induced stem-pitting is not currently a commercially important disease in Florida. Discrimination of severe and mild isolates in Florida has been accomplished previously by biological indexing and by the monoclonal antibody MC13, which does not react to Florida mild isolates. The Florida T30 and T36 isolates are type isolates for the mild and severe groups respectively, and both genomes have been fully sequenced. We developed a PCR-based assay to characterize CTV isolates by amplification of sequence specific molecular markers using primers derived from the nucleotide sequence of the VT, T3, T30 and T36 isolates. The pattern of amplified markers is used to assign to a CTV isolate a specific genotype. When we applied this marker analysis to a collection of over 400 CTV accessions from Florida the T30 and T36 genotypes, either singly or together, were the primary genotypes in commercial citrus in Florida. The VT genotype was found in some Meyer lemon trees, not in commercial sweet orange or grapefruit plantings. The T3 genotype, which is common in many other countries, was not found in commercial trees. Biological indexing of select isolates supported the association of the T36 genotype with induction of a graft incompatibility on sour orange rootstock. The widespread distribution in Florida of two predominant genotypes may reflect the sources of CTV germplasm that have been introduced and subsequently spread via propagation and aphid vectors. The limited genetic variability of CTV currently present in Florida may facilitate the use of genetically based assays to regulate future ingress of other CTV genotypes into Florida citrus budwood.