Abstract:
Citrus tristeza virus (CTV) is a RNA plant virus that infects the phloem cells of members of the family Rutaceae. CTV has a very important impact on the citrus industry worldwide and in South Africa especially so on grapefruit. CTV isolates can cause differing levels of severity of Tristeza disease, which can lead to quick decline as well as stem pitting and seedling yellows. Mild strain cross-protection is commonly used in South Africa to control the negative effects of the virus. This control mechanism is based on the super-infection exclusion principle where the presence of one specific genotype of CTV prevents the secondary infection of strains of the same genotype. This necessitates the characterization of CTV sources occurring within given citrus producing areas to know which genotypes to protect against, as well as the thorough characterization of potential cross-protection sources to ensure the specific genotypes that need to be protected against are present and to ensure that there are no strains within the source that would cause severe symptoms. The aim of this study was to characterize several sources of CTV which could potentially be used for cross-protection and at the same time to use and evaluate several methods for this. By doing next generation sequencing on an overlapping amplicon template of the 3’ half of the genome it was found that the three Grape Fruit Mild Strain 12 sub isolates, GFMS 12-7, 12-8 and 12-9 mostly exists of a T68 genotype previously identified as CT-ZA3. Using immuno-captured virus particles as template, followed by the production of cDNA through the use of degenerate primers and random amplification of the DNA as well as a p33 gene amplicon for next generation sequencing, it was found that the New Venture 41/2 candidate mild source is a mixed source containing at least the VT, RB, B165 and HA16-5 genotypes. The B390/3 candidate mild source was characterized through biological indexing and was found to only produce mild symptoms on the hosts used in the trial. The virus population was also characterized through Sanger and Illumina sequencing of the p33 gene as well as using genotype specific RT-PCRs. The source is dominated by a Taiwan-Pum/SP/T1–like isolate which belongs to the RB genotype. Additionally a comprehensive phylogenetic analysis was performed on 45 published complete genomes of CTV where it was shown that 9 genotypes exist, namely VT, T36, RB, T30, B165, T68, HA16-5, T3 and A18. The best method for genotyping, as found to produce the phylograms most similar to the complete genome phylograms, was found to be by doing a Bayesian analysis on a concatenated dataset of three segments of the genome, namely ORF 1b, ORF 2 and ORF 5.
