Abstract:
Citrus tristeza virus (CTV) is responsible for the most devastating viral
disease of citrus crops and affects the profitability of citriculture on a
worldwide scale. In South Africa, where CTV is endemic, the devastating
effects of CTV severe-strain infections are reduced through the intentional
inoculation of virus-free budwood with a mild-strain cross-protecting source.
Cross-protection breakdown however occurs in grapefruit trees in South
Africa and results in a reduction of yield and quality in grapefruit producing
areas. Grapefruit cultivars are particularly sensitive to the stem-pitting effects
of some severe CTV strains and despite the rigorous cross-protection
scheme, severe stem-pitting and decline symptoms are still observed in the
field. The GFMS 12 cross-protecting isolate was used as the cross-protecting
source for grapefruit plantings in South Africa until its use was terminated in
2007. As early as 1993 however, severe stem-pitting was observed in the
field. GFMS 35 has been employed as an alternate cross-protecting source,
due to the apparent poor cross-protection ability of GFMS 12. Using
nucleotide sequence analysis of a 5’ variable region (A-region) and a 3’
conserved region (p23 gene region), CTV population studies were performed
on GFMS 12 and GFMS 35 pre-immunised Star Ruby and Flame trees in the
field, in the hot production area of Malelane. The viral population of GFMS 12
and GFMS 35 sources, maintained on Mexican Lime and Star Ruby, plants
were characterized as references. Severe VT-like strains dominated (up to
100% of clones) in all of the field samples. Of the two grapefruit cultivars
sampled, no clear differences in their strain assemblage was obtained,
suggesting that relative to each other, these cultivars played no role in
shaping the CTV population strain composition that were observed. VT-like
strain dominance was also observed in most of the sources maintained on
Star Ruby. A shift in population strain composition was observed in the
sources maintained on Mexican lime suggesting the important role that hosts
may play in strain selection. A number of anomalous results were obtained for
the sources that appeared not to follow the “general trend” of population strain
composition observed in the other samples. These anomalies may have been
caused by when the samples were collected, the age of the tissue material collected, PCR bias during amplification or some other factor resulting in a
population shift in the sources. From these results it is near impossible to
pinpoint the exact mechanism responsible for CTV cross-protection
breakdown. However, the scale of the complexity of CTV population dynamics
in South Africa has become clear. From this it will be possible to begin
changing the way that cross-protection candidates are evaluated in South
Africa, from an entirely empirical approach to one that incorporates much
needed molecular methods.
