WINETECH Technical Yearbook 2019

ILANI MOSTERT 1 , JOHAN BURGER 1 & HANO MAREE 1,2 1 Department of Genetics, Stellenbosch University, Stellenbosch; 2 Citrus Research International, Stellenbosch University, Stellenbosch KEYWORDS: Grapevine rupestris stem pitting-associated virus, GRSPaV. MARCH 2019 DETECTION AND CLASSIFICATION OF GRAPEVINE STEM PITTING- ASSOCIATED VIRUS

The aim of this study was to investigate the genetic diversity of GRSPaV in South Africa and to evaluate the success of biological indexing as a detection tool for this virus. Grapevine rupestris stem pitting-associated virus (GRSPaV) is a ubiquitous grapevine virus commonly detected in cultivated vines. It has been reported to be associated with rupestris stem pitting (RSP) and possibly Syrah decline (SD) (Photo 1) (Meng & Gonsalves, 2007). Its only known means of transmission is through vegetative propagation or grafting, and there have been no reports of an insect vector capable

of spreading GRSPaV (Meng & Gonsalves, 2007). Hardwood indexing is the primary method of diagnosis used to monitor the GRSPaV status for the fulfilment of sanitary requirements for candidate mother plants. Currently, six different variant groups, also referred to as strains, of GRSPaV are recognised (groups I, IIa, IIb, IIc, III and IV). Each variant group is represented by the complete genome sequence of one or more isolates of which 18 are available on an online database of nucleotide sequences (GenBank). Sequences of two parts of the virus genome, the coat protein and the replicase polyprotein, are used to detect and classify GRSPaV variants (Glasa, et al ., 2017). The symptoms caused by GRSPaV vary greatly among virus variants and depend largely on the cultivar infected. Infection of indicator ‘St. George’ grapevines ( V. rupestris Scheele) with group IIa variants does not elicit symptoms associated with RSP, but is strongly associated with grapevine vein necrosis (GVN), while variants of group IIc cause RSP. Furthermore, infection with group IIb causes mild or no symptoms, whereas group I is closely associated with Syrah decline (Habili, et al ., 2006; Lima, et al ., 2006; Meng, et al ., 2005). Inconsistencies in symptom expression may be explained by simultaneous infection by multiple viruses or multiple sequence variants of GRSPaV (Nakaune, et al ., 2008).

(IMAGE: VITIS LAB, DEPARTMENT OF GENETICS, STELLENBOSCH UNIVERSITY.) PHOTO 1. Pitting and grooves on the stem of V. rupestris St. George, the indicator for rupestris stem pitting.