Project title
Analysis of spatial distribution patterns indicative of spread of leafroll disease

Project leader
Pietersen, G

Institution
ARC Plant Protection Research Institute

Team members
Kasdorf, G G F

Project description
Grapevine leafroll disease is a serious disease of grapevines worldwide, and has a number of viruses associated with it. In the South African Plant Certification Scheme for Wine Grapes, valuable Vitis scion and rootstock clones are subjected to a procedure to eliminate any potential viruses occurring in them. Such virus-free material remain susceptible to re-infection by viruses and are protected against this using various procedures (e.g. isolation distances, regular indexing, rouging etc.) through a number of generations of multiplication in a certification scheme. The ultimate aim being to supply nurseries and producers with the best available planting material. In practice however grapevine leafroll disease symptoms are encountered in the field in these protected generations as well as in production vineyards within relatively short periods of time.

Yearly, large scale screening of nuclear material (1st generation after virus-elimination) using various techniques, have shown that known viruses are completely eliminated by the procedures used, and these plants, kept in tunnels, remain virus-free. The major problem is the re-infection of Vitis material once planted in open fields.

This study aims to: 1) identify the major sources of virus causing re-infection, and the means of spread of the viruliferous vectors in the field by determining the spatial distribution of grapevine leafroll disease infected plants within young (one to five year old) mother block vineyards and foundation blocks, and 2) to test the maxim that the only virus important in the spread of leafroll disease in South Africa is grapevine leafroll-associated closterovirus 3 (GLRaV-3).

The spatial distribution of the disease is determined by recording the disease status of plants, plotting this in two-dimensional representations and then analysing these by various statistical models for spread patterns. Where feasible, such patterns were then correlated with on-ground features (e.g. proximity to severely infected vineyards, prevailing winds etc.). Samples of infected material from each block representing re-infection from different possible sources are analysed by immuno-electron microscopy (IEM).

Based on these results the most important reservoir of virus and means of spread will be identified and strategies developed to minimise this spread. This approach is likely to have a larger impact over a shorter term than to attempt control of the vector directly without this information.

Presentation(s)
1. Pietersen, G. 2001. Winetech Grapevine Virus Workshop, 3 May.
2. Pietersen, G. 2001. Winetech Grapevine Virus Workshop, 30 August.
3. Pietersen, G. 2001. 25th South African Society of Enology and Viticulture National Congress, November, Somerset West, 15 November.
4. Pietersen, G. 2002. Winetech Grapevine Virus Workshop, 6 May.
5. Pietersen, G. 2002. Winetech Roadshow, 8 May, Worcester.
6. Pietersen, G. 2002. Winetech Roadshow, 9 May, Paarl.

Final report
http://www.sawislibrary.co.za/dbtextimages/finalreport51.pdf

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