WINETECH Technical Yearbook 2021

the most sensitive method of detection of grapevine viruses is RT-PCR. The technique requires a profound knowledge of targeted virus sequences to ensure rel iable detection. Equally valuable as sequence data is biological data on grapevine viruses. We must bear in mind that from a strictly scientific perspective, the viral aetiology of GLRD and GRWD has not been firmly confirmed. The third Koch’s postulate, which states that isolation of a pathogen and re-infection of a host with a pure culture of that pathogen, and which must be followed with the development of disease symptoms, has not been fulfilled. The reason for this lapse in knowledge is the fact that grapevines are usually infected with a mixture of various virus species and different genetic variants of a single virus species, and separation of them is not possible. Although some viruses, like GVA, GVB and GLRaV-2, were isolated in alternative herbaceous host N. benthamiana , transmission of them back to Vitis is impossible using current techniques. The only way to obtain a pure culture of grapevine viruses is to construct biologically active cDNA clones of these viruses in the laboratory (Goszczynski, 2018). The cDNA clones for the following GLRD- and GRWD-associated viruses were constructed: GVA, GVB, GRSPaV, GLRaV-2 and -3 (Galiakparov et al ., 1999; Sardarelli,

2000; Meng et al ., 2013; Kurth et al ., 2012; Jarugula et al ., 2012). In the ARC- PHP laboratory, biologically active cDNA clones were constructed to GVA and GVB (Goszczynski, 2015). Currently, the full data on virus infection of grapevines using cDNA clones has only been established for GLRaV-2 by a laboratory in the USA (Kurth et al ., 2012). Soon we can expect other research papers to confirm the successful infection of grapevines using cDNA clones of viruses. Recently an Italian laboratory published a paper on successful fulfilment of Koch’s third postulate for a member of the family Betaflexiviridae , Grapevine Pinot gris virus (GPGV), which is known to be associated with stunting, chlorotic mottling and leaf deformation of some grapevines (Tarquini et al ., 2018). SUMMARY About 40 years of intensive research of grapevine leafroll (GLRD) and grapevine rugose wood (GRWD) diseases has revealed that many virus species of the two virus families, Closteroviridae and Betaflexiviridae , are associated with these diseases. Results have also revealed that each virus species is composed of a large number of divergent genetic variants, some of them with putative different pathogenicity to grapevines. Accumulated data is crucial for the accurate detection

and further study of these important viruses. Two South African laboratories, ARC- Plant Health and Vitis Laboratory of Stellenbosch University, have played a significant role in creating this worldwide knowledge. ACKNOWLEDGEMENTS All the research described in this article was made possible by the financial support of Winetech, South Africa, and with the aid of institutions such as KWV, nurseries Vititec and Ernita, and with further help from people like J.H. Booysen, A. Andrag, R. Carstens, G. Kriel, N. Spreeth, T. Oosthuizen, J. Wiid and the late Prof. P. Goussard. REFERENCES Martelli, 2017. In: Meng B, Martelli GP, Go- lino DE, Fuchs M, (eds). Grapevine viruses: molecular biology, diagnostics and manage- ment. Springer, Cham, 31-46. Namba et al ., 1979. Annals of the Phyto­ pathological Society of Japan 45, 497-502. Gugerli et al . 1984. Revue Suisse de Viti­ culture, Arboriculture et Horticulture . 16, 299-304. Hu et al ., 1990. Journal of Phytopathology . 128, 1-14 Zimmermann et al., 1990. Journal of Phyto­ pathology . 130, 205-218

Conti et al ., 1980. Phytopathology 70, 394- 399. Rosciglione et al ., 1983. Vitis 22, 331-347. Boscia et al ., 1993. Archives of Virology 130, 109-120. Goszczynski et al ., 1996. Vitis 35, 133-135. Monette & Godkin, 1993. Plant Pathology ( Trends in Agriculture Science ) 1, 7-12. Meng et al ., 2005. Virus Genes 31, 31-41. Angelini et al ., 2017. In: Meng B, Martelli GP, Golino DE, Fuchs M, eds. Grapevine viruses: molecular biology, diagnostics and manage- ment. Springer, Cham, 141-165. Goszczynski & Jooste, 2003. European Jour­ nal of Plant Pathology 109, 397-403. Goszczynski, 2007. Plant Pathology 56, 755- 762. Goszczynski et al ., 2008. Virus Research 138, 105-110. Goszczynski, 2018. Journal of Plant Pathol­ ogy 100, 105-109. Shi et al ., 2004. Virus Genes 29, 279-285. Goszczynski, 2010. Virus Genes 41, 273- 281. Goszczynski, 1996. Journal of Phytopathol­ ogy 144, 581-583. Goszczynski, 2019. Wineland , November 2019. Ling et al ., 2004. Journal of General Virolo­ gy 85, 2099-2102.

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