Identification of factors conferring virus resistance in grapevine cultivars.
Maree, H J
University of Stellenbosch. Faculty of AgriSciences. Department of Genetics
Maree, H J
Burger, J T
Objectives & Rationale
Anecdotal reports of symptomless plants appearing among symptomatic plants in severely grapevine leafroll-diseased (GLD) vineyards of the Western Cape have been doing the rounds for many years (pers. comm. J. Wiid, G. Kriel, G. Pietersen, E. Archer). Recently, more cases of apparently leafroll resistant (ALR) plants in older (>20 years) grapevine leafroll virus-infected vineyards have been observed. These include individual grapevine plants of the cultivars (cvs) Pinot gris, Pinot noir and Shiraz, which have been growing within severely diseased vineyards for decades, but that do not display typical GLD symptoms. Subsequent testing failed to detect any of the common viral pathogens in these plants. The reason for this apparent natural resistance or tolerance, is not known, but can possibly be due to viral cross protection by unknown viral variants, by genetic mutation(s) in the plant, or a combination of these. Thus, the susceptibility/resistance to viruses in plants of the same cv in the same location present an ideal opportunity to identify the basis for this apparent increased resistance.
We, in collaboration with Westcape Biotech and E. Archer (consultant), propose a two-pronged approach to determine the nature of this apparent resistance. To investigate if the observed ALR is the result of cross protection by avirulent variants of known or novel viruses/viroids, we will use our experience and competency in conventional, as well as NGS-based diagnosis of grapevine viruses to unequivocally compare the viromes of GLD and ALR vines from these vineyards. The second hypothesis, i.e. that of a genetic basis for the observed ALR, will be investigated by a series of reciprocal grafting experiments of the respective tissues onto GLD and ALR host plants of the different cultivars.
Pinot gris plant material that was identified by E. Archer was collected and supplied to WestCape Biotech. This material was in vitro propagated and are now held in green houses at WestCape Biotech. All other vineyards identified as potential sources of resistance were visited and individual vines identified for subsequent virus diagnostics in the new (2018) growing season. Since the project was terminated at the end of the 2017 season, these diagnostics were not done.
The project was granted a one-year extension in order to renew attempts to identify grapevine plants of the cultivars Pinot gris, Pinot noir and Shiraz, which have been growing within severely diseased vineyards for decades, but that do not display typical GLD symptoms. Pinot gris plants have since been in vitro propagated at WestCape Biotech and are being held in their green houses. Subsequent discussions with key stakeholders (both Industry and Winetech) resulted in a decision to terminate the project for the lack of reliable source material required to execute the second phase of the project