Genotypic diversity of citrus tristeza virus within red grapefruit, in a field trial site in South Africa

Abstract

Grapefruit cultivars (Citrus paradisi Macfad.) are extremely sensitive to Citrus tristeza virus (CTV) infections and are pre-immunized with mild-strain crossprotecting sources not containing components that elicit symptoms such as stempitting and decline, to ensure longer periods of productivity. However, preimmunizing sources often lose their efficiency and for this reason the previously commercially applied grapefruit cross-protecting source GFMS (grapefruit mildstrain) 12 has been replaced by GFMS 35. This study was undertaken to determine the diversity of CTV genotypes within trees that were inoculated with either GFMS 12 or GFMS 35. Samples were collected from a number of different trees of two red grapefruit cultivars (cv. Star Ruby and cv. Flame), planted 10 years prior to sampling in the Malelane production area of South Africa. Reverse transcription-polymerase chain reaction amplification of a 5‘ variable region (A-region) and a 3‘ conserved region (p23 gene) was followed by cloning, sequencing of multiple clones and phylogenetic analyses. The genotypic identities of clones were determined based on their relatedness to reference CTV strains. Sequence types within the VT genotypic group dominated in all of the samples, with T30-like sequence types being a minor component in some populations of the field collected samples. The original preimmunising populations of GFMS 12 and GFMS 35 were characterised on greenhouse maintained plants and compared with the populations exposed to field infections by aphids. While the methodology employed only allows a coarse representation of the genotype composition of the CTV population, this study provides insight into which genotypes of CTV must be incorporated within a mildstrain cross-protecting source within the South African Citrus Improvement Scheme (SACIS).