As there are no products which can be applied to plants in the field to control virus diseases in a manner equivalent to fungicides to control fungal diseases, control of virus diseases generally is achieved by the use of resistant plants or by controlling the spread of the virus between plants. In order to do this the epidemiology of the disease must be understood and control interventions applied at strategic points in the disease cycle. The research program of the Plant Virology Group focuses on grapevine leafroll disease of wine grapes, and the Citrus disease caused by citrus tristeza virus (CTV). The group also studies citrus greening disease, associated with a fastidious bacterium “Candidatus” Liberibacter africanus (Laf), which has an epidemiology not unlike that of plant viruses. These three diseases are amongst the most important diseases of Wine Grapes and Citrus, two high value crops in South Africa
Grapevine leafroll disease is has been controlled for decades within the South African Vine Certification Scheme, where virus-free nuclear stocks of vine cultivars are produced following heat-therapy and meristem tip culture. However such virus-free material remains susceptible to viral infection and often progeny from the nuclear plants, planted in the field become infected again. Surveys by the members of the plant virology group in recent years have shown that grapevine leafroll-associated virus type 3 (GLRaV-3) is the primary cause of these infections. A number of sources and means of spread have been identified by members of the Plant Virology Group in South Africa through spatio-temporal analyses of grapevine leafroll disease in foundation- and mother-blocks. Following these studies a successful control strategy based on planting certified planting material, rouging of infected vines, control of mealybug numbers and dispersal has been developed by us. Assessments of the efficacy of these control strategies have been conducted at a model wine estate, Vergelegen Wine Estate in Somerset West, where the success of the control strategy has been unequivocally demonstrated. Current research by the research group focuses on refinements of the control strategies, for example 1) the development of large scale detection of GLRaV-3 in white cultivars, as disease symptoms are not obvious in these and rouging cannot be based on visual symptom identification, as in red cultivars, but must be on tests for the virus and 2) trials to confirm the spread of leafroll between two successive vineyard blocks and assessment of control methods such as fallow periods, and systemic insecticide use with or without herbicide to prevent the perpetuation of grapevine leafroll infection from a preceding vineyard. Research is also conducted on developing a next generation sequencing-based diagnostic protocol for the viruses of grapevine, in support of the South African Wine Grape Certification Scheme.
As with the wine grapes virus-free Citrus planting material is produced within the South African Citrus Improvement Programme (CIP). However because of the extremely rapid rate of re-infection of such material in the field by CTV, all citrus planting is pre-immunized with mild strain populations of CTV prior to release from the CIP. While this protects the plant from infection by severe forms of CTV in the majority of instances, on some occasions severe symptoms are still found, especially in the case of Grapefruit. 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 pre-immunizing mild population to severe forms or 3) selection, under specific environmental conditions for severe forms of CTV, inherently present in the pre-immunizing population. The plant Virology group are busy with studies where the strains present in greenhouse maintained pre-immunizing CTV populations and field planted, pre-immunized Citrus trees with various symptoms are being characterised, pure sources of CTV strains are isolated, and factors affecting strain dynamics are assessed.
Citrus greening disease has been reduced to manageable levels through stringent vector control strategies and rouging, but remains a problem in the cooler citrus production areas of South Africa. The perpetuation of the disease in spite of stringent rouging may be due to the presence of hosts other than citrus, which may serve as reservoirs of the disease. The Plant Virology group are investigating the possibility that other hosts of the bacteria exist with the initial emphasis is being placed on evaluating indigenous Rutaceous species