WINETECH Technical Yearbook 2020

Poor detection of grapevine leafroll-associated virus type 3 in rootstocks

Adding complexity to certification schemes, scions of V. vinifera are grafted (Photo 4) onto a number of American Vitis species rootstocks in countries, such as South Africa, where phylloxera ( Daktulosphaira vitifoliae ) occurs. This is required as V. vinifera vines on their own roots are attacked by this aphid-like insect. Many rootstocks are asymptomatic hosts of GLD (photo 5) and require virus spe- cific laboratory-based detection methods to test for virus infection. The reliability of detection of GLRaV-3 in rootstocks in certification schemes by PCR has, however, not been fully assessed anywhere. In this study, we assessed the detection by the very sensitive laboratory technique, reverse-transcriptase polymerase chain react ion (RT-PCR) of GLRaV-3 in individual vines of the most widely utilised rootstocks in South Africa compared with their corresponding scions (photo 6). Most of the initial work was done on Richter 99 ( Vitis berlandieri X Vitis rupestris) and most of the information presented here refers to this rootstock. We also compare the variants of GLRaV-3 found in both Richter 99 rootstocks and scions of selected individual vines and we determine the presence of other leafroll-associated viruses in Richter 99.

A number of viruses have been reported from vines with grapevine leafroll disease (GLD) and are known as grapevine leafroll-associated viruses. Amongst these, grapevine leafroll-associated virus 3 (GLRaV-3) appears to be the most prevalent and widespread virus and this certainly is the case in South Africa. Commercial grapes are primarily cultivars of Vitis vinifera , V. labrusca , Muscadinia rotundifolia , V. amurensis and several interspecific hybrids. Worldwide, grape­ vines are prone to various virus diseases of which GLD is the most common and widespread (photo 1). It is economically important as it reduces plant vigour and longevity, fruit yield and quality. GLD can be successfully controlled using an integrated control strategy which includes producing and planting certified, virus- tested propagation material, vector number and dispersal management, and roguing of infected vines. Detection of GLD for these purposes is easily achieved in a number of red cultivars where symptoms of GLD are obvious in autumn and infected plants easily identified (photo 2). This is more difficult in white cultivars, where symptoms are not obvious (photo 3) and where laboratory- based virus detection techniques have to be employed to detect the viruses associated with the disease.

APRIL 2020

GERHARD PIETERSEN 2 & MEGAN HARRIS 1 : 1, FABI, Department of Microbiology and Plant Pathology, University of Pretoria, Pretoria; 2 Department of Genetics, University of Stellenbosch, Stellenbosch KEYWORDS: Grapevine leafroll virus (GLD) detection, reverse- transcriptase polymerase chain reaction (RT-PCR)

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