Faculty of Natural and Agricultural Sciences
Department of Microbiology and Plant Pathology
Plant viruses
The primary research of the plant virology program is directed at support of the South African Citrus Improvement Program (CIP) as well as the Wine Grape Certification scheme. Winetech, the wine industry research co-ordinating body, supports the grapevine research component financially, while Citrus Research International supports the Citrus component. In both certification schemes virus control plays a central role, with citrus tristeza virus (CTV) being the most important virus in the citrus scheme, and Grapevine leafroll assocated virus type 3 (GLRaV-3), the most important one for the wine grape scheme. Both of these viruses belong to the Closteroviridae family but CTV is in the Closterovirus genus and is aphid transmissible, while GLRaV-3 is in the genus Ampelovirus and is mealybug-transmissible. The differences in mode of transmission require that the respective certification schemes employ different strategies for control of these viruses. As CTV is easily and rapidly transmissible by highly mobile aphid vectors, citrus material from which viruses have been eliminated are protected against CTV infection in the field through the pre-inoculation of planting material with mild CTV strains. In contrast with this, wine grape material is subjected to virus elimination techniques and then propagated under conditions to minimize re-infection. This approach is possible as the re-infection takes place relatively slowly as mealybugs are generally sessile and it is possible to provide essentially “virus-free” propagation grapevine material. Neither strategy is foolproof. In citrus the cross protection by mild CTV strains is not always durable and severe CTV symptoms may occur with time. In grapevines on the other hand, the certified planting material is often re-infected by GLRAV-3 when healthy planting material is established in the field.
Research projects at the Plant virology program are directed at doing basic and applied studies to understand and control 1) the occasional lack of CTV cross protection durability, 2) GLRaV-3 re-infection of certified material, and 3) determining the apparently virus-like epidemiology of a fastidious bacteria, Candidatus Liberibacter africanus (Laf) vectored by Trioza erytreae (a psylla), the cause of citrus greening in South Africa. With these objectives in mind postgraduate students are involved in studying the aspects such as a) characterisation on a molecular level of CTV populations within pre-immunising mild sources, b) the population dynamics of CTV strains within the mild strain cross protecting populations under different environmental conditions, c) determination of CTV strain composition within South African citrus orchards, d) development of more effective CTV pre-immunizing populations, e) control of grapevine leafroll disease in white cultivars by early detection methods, f) the alternate hosts, distribution, reciprocal transmission and variability of Laf and the related bacteria Candidatus Liberibacter africanus spp. Capensis (LafC) of the indigenous Rutaceous tree, Calodendrum capense, and g) Sequencing the genome of the two bacteria.
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