Plant Virology Research Projects

 

• Grapevine leafroll disease

In the South African Vine Certification Scheme field infection by grapevine leafroll disease of Vitis clones from which virus had been eliminated, is a major problem. A number of sources and means of spread have been identified in South Africa through spatio-temporal analyses of grapevine leafroll disease in foundation- and mother-blocks as part of this research program. Various spread mechanisms have been inferred through the disease spread patterns caused and research is being conducted at confirming the mechanisms and to quantify parameters within them. Furthermore, control of these various spread mechanisms through various interventions are being applied, modified and evaluated at Vergelegen wine estate, and serves as a model for control of grapevine leafroll worldwide. Research is being conducted to expand the success in controlling leafroll in red grape cultivars also to white cultivars by the development of efficient detection mechanisms in these cultivars. Because of the simultaneous occurrence of multiple spread mechanisms in vineyards, infrequent or less easily identifiable mechanisms, are often obscured by the more dominant or obvious mechanisms. As control strategies become increasingly better at controlling known spread mechanisms, these previously obscured dispersal mechanisms are expected to become more obvious and may require specific additional control interventions. This possibility is also being addressed as part of this program. Contact person: Gerhard Pietersen.

The variability of a virus closely related to Grapevine leafroll disease, Grapevine leafroll associated virus-3 (GLRaV-3), has been studied. Knowledge of the spread of GLRaV-3 variants in vineyards plays an important role in understanding the virus-vector interactions that leads to the spread of LR in vineyards. In this project the focus was on the virus and its population structure and mainly the characterisation of South African GLRaV-3 variants. The other aspect studied was the distribution and spread of the variants in selected vineyards in the Western Cape.

• Viral Aetiology of Shiraz disease, Shiraz decline and Corky bark disease on grapevine

Shiraz disease (SD) is of great concern to the South African grapevine industry since it kills affected grapevines of noble cultivars such as Shiraz and Merlot, and is spreading naturally in vineyards. Once infected plants show symptoms of SD, they never recover, and die within 3 - 5 years. Shiraz disease is latent in grapevines of other cultivars and rootstocks, but it can be transmitted easily from these to SD-susceptible grapevines by grafting with infected tissues. The importance of this highly destructive disease is emphasized by fact that Shiraz (Syn. Syrah) cultivar is widely planted in countries like Australia, France, and the USA. The disease, which is similar (or identical) to SD, was reported from Australia, and has been temporarily named Australian Shiraz disease (AuSD).

Shiraz disease was first described in South Africa in 1985. Since then, its suspected viral aetiology has remained a mystery. In our laboratory (PPRI, Virology), we showed that the disease is transmitted by the mealybug, Planococcus ficus, and found that a vitivirus, Grapevine virus A (GVA), is associated with the disease. Our results revealed extensive molecular variability in this virus in South African vineyards. Three divergent molecular groups of the virus (I, II, III) were identified. Results showed that the variants of molecular group II are closely associated with Shiraz disease, and variants of molecular group III are commonly present in GVA-infected, SD-susceptible grapevines that consistently do not express symptoms of the disease. Genomes of eight variants of GVA were sequenced and the data deposited in the GenBank/EMBL database. Further study revealed that variants associated with SD in South Africa are also present in grapevines affected by Australian Shiraz disease, and in USA vineyards. Recently, a biologically viable and stable DNA clone of GVA variant associated with SD was constructed for use in the further study of the aetiology of this disease (fulfillment of 3^rd Koch's postulate). Contact person: Darius Goszczynski

• Citrus tristeza virus (CTV) and Citrus greening

Within the South African Citrus Improvement Programme (CIP), all citrus is pre-immunized with one of a few mild strain populations of citrus tristeza virus (CTV). While this protects the plant from infection by severe forms of CTV in the majority of instances, on some occasions severe symptoms still are found. It is unknown whether this is due to 1) super-infection of the plant with wild-type severe forms of the virus, 2) mutations within the mild population to severe forms, 3) selection, under specific environmental conditions for severe forms of CTV, inherently present in the population, or 4) combinations of the above or other mechanisms. Research is conducted to gain insight into the cause of this phenomenon, specifically on grapefruit in South Africa where this problem is most severe.

Citrus greening is a destructive disease of citrus and is caused in South Africa by a fastidious bacterium Candidatus Liberibacter africanus. The disease has been reduced to manageable levels through stringent vector control strategies, but remains a problem in cooler citrus production areas of South Africa. The perpetuation of the disease may be due to the presence of hosts other than citrus, which may serve as reservoirs of the disease. Research is conducted currently to determine the possibility that other hosts of the bacteria exist as well as to study the epidemiology of the disease.