Doctoral Degrees (Genetics)

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Browsing Doctoral Degrees (Genetics) by Author "Cook, Glynnis"

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    Characterization of citrus tristeza virus variants and their influence on symptom expression in grapefruit
    (Stellenbosch : Stellenbosch University, 2019-04) Cook, Glynnis; Maree, H. J.; Burger, Johan T.; Stellenbosch University. Faculty of Agrisciences. Dept. of Genetics.
    ENGLISH ABSTRACT: Citrus tristeza virus (CTV), a member of the family Closteroviridae, was responsible for large scale destruction of citrus, especially in the Americas, due to tristeza disease and necessitated a production switch to less sensitive rootstocks. CTV however continues to affect citrus through the stem-pitting disease phenotype which is especially problematic in sweet orange, grapefruit and lime cultivars. In South Africa, the productive lifespan of grapefruit trees was severely affected by stem-pitting, requiring early tree replacement with an associated lag in production. This affect was later mitigated by applying cross-protection, a management strategy using non-stem-pitting sources of CTV, but without prior knowledge of which CTV strains were responsible for stempitting or which strains were present in the cross-protection sources. To understand the disease and unravel mechanisms underlying cross-protection, it is necessary to characterise CTV strains to investigate both virus-host- and strain-interactions. The aim of this study was firstly to identify single-strain isolates belonging to different strains, to characterise them biologically and to determine full-genome sequences. These characterised CTV isolates were further used in a complementation study to investigate possible synergistic interactions affecting stem-pitting. Complete viral genomes of eight single-strain isolates were determined during the study. Two commercial grapefruit cultivars, ‘Star Ruby’ and ‘Marsh’, were used in a glasshouse trial to evaluate the ability of specific strains to induce stem-pitting in single or mixed infections. Evaluation over four years showed that symptom expression of mild strains did not result in altered symptom expression when in combination with each other. Importantly demonstrating that there was no additive effect on stem-pitting expression with multiple isolates. Relative quantitation of the strains in ‘Marsh’ and ‘Star Ruby’ plants indicated that the individual strain concentrations were not significantly altered when in combination with the other strains. A valuable discovery made within this project was the characterisation of two variants of the T68 strain, derived from the same GFMS12 source, but displaying differences in stem-pitting severity in grapefruit. This finding demonstrates the co-existence of severe and mild variants of the same strain in one source and provides an explanation for the presumed strain segregation event observed for the GFMS12 cross-protection source that resulted in the discontinuation of the source for use in cross-protection of grapefruit. The characterisation of these variants will further assist in the identification of the sequence determinants for stem-pitting in grapefruit.