Citrus tristeza virus (CTV) causes severe losses in grapefruit production in South Africa and requires mild
strain cross-protection to maintain production. Unfortunately cross-protection breakdown of the pre4
immunizing CTV grapefruit mild source GFMS12 is prevalent in grapefruit in South Africa. The CTV
genotype composition of the GFMS12 population inoculated onto different hosts was determined by sequencing
part of ORF1a and the p23 gene of multiple clones from each plant. Analysis of the GFMS12 population in
Mexican lime, Marsh and Star Ruby grapefruit varieties, revealed that at least four genotypes occur in the
GFMS12 population, and that genotype compositions differed amongst the populations in different host plants.
Single aphid transmitted sub-isolates derived from the GFMS12 mother population on Mexican lime appeared
to contain three populations of a mixture of VT-like and recombinant B165/VT-like genotypes; a mixture of
recombinant RB/VT- and B165/VT-like genotypes; and a single recombinant B165/VT-like genotype. This
study underlines the importance of determining the genotype composition of a potential CTV pre-immunizing
source on a range of inoculated host species before utilization.