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12 has been replaced by GFMS 35.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).
