Citrus tristeza virus populations in Grapefruit trees, pre-immunized with two different cross-protecting sources

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.