WINETECH Technical Yearbook 2020

high percentage of GRSPaV infections in vineyards is confusing the grapevine industry. If the GRSPaV infections are common, the intriguing question is, what is the pathogenicity of this virus to grapevines? Should the industry be concerned about this virus? GRSPaV is regarded as being associated with rupestris stem pitting disease (RSPD), which is one of the rugose wood complex diseases (RWD) of grapevines (Meng & Rowhani, 2017). When tissue of a grapevine infected with GRSPaV is grafted to Vitis rupestris cv. St. George plant, symptoms of modified wood (pitting), like that shown in Figure 1, may appear on the woody cylinder of this grapevine. The problem is that the modification of wood is clearly visible only in grapevine V. rupestris cv. St. George, which is especially sensitive to RSPD and is used as an indicator of this disease. In other grapevines GRSPaV infections are latent. So what does this mean? Does it mean that the virus does not induce any pathogenic effect in grapevines of these cultivars, but co-exist in peaceful relation with these plants? There are only two papers in which authors address this important question. An Italian laboratory reports that GRSPaV infection triggers various physiological changes in V. vinifera , but it seems these changes would have no negative influence on the productivity of a vineyard (Gambino et al .,

2012). The authors suggest that GRSPaV evolved to peacefully co-exist in grapevines. Similar conclusion regarding influence of GRSPaV on grapevine productivity has been reported from Canada (Reynolds et al ., 1997). However, in both studies little or no attention is given to a detailed characterisation of population of genetic variants of GRSPaV infecting grapevines used in the study. It is well known that not all GRSPaV-infected grapevines grafted to St. George induce RSPD symptoms in this indicator (Meng & Rowhani, 2017). It is believed that the lack of inducing RSPD symptoms in some cases is because of the presence of mild or not pathogenic strains of GRSPaV. Presently eight groups of genetically divergent variants of GRSPaV have been identified (Meng & Rowhani, 2017). The groups are named as GRSPaV-1, -ML, -JF, -PN, -SY, -BS, -SG1 and -SLS. Clear divergence of genome sequences and encoded proteins between members of different groups of GRSPaV suggest that these are different biological strains of this virus, with different pathogenicity to the grapevine host. And this may be true. The laboratory led by Dr. Meng, soon after the discovery of GRSPaV, also discovered that the St. George they used as an indicator of RSPD was already infected with GRSPaV (Meng & Rowhani, 2017). Later they determined that this was a genetic variant GRSPaV-

industries. These grapevines have precise data from RT-PCR testing and woody indexing, and would be excellent for the study. Of special value is the biological data. Genetic heterogeneity of GRSPaV population present in these grapevines could be determined in just a few weeks. Regarding the second option, although the construction of the cDNA clone of GRSPaV is a relatively easy and quick way to obtain a pure culture of any genetic variant of GRSPaV, knowledge of precise events occurring during infection of grapevines by viruses is, in general, still insufficient. To induce the disease, the virus has to successfully infect grapevine cells, increase its titer and systemically spread in a plant. The construction of the cDNA clone of GRSPaV was reported six years ago, in 2013 (Meng & Rowhani, 2017). Although it was shown that the clone is infectious to grapevines, the virus was hardly detectable in successfully infected plants. We are still awaiting the report that the GRSPaV originating from such infection is increasing its titer and spreading in grapevine. It may merely be a matter of time since for woody plants, unlike herbaceous hosts, it takes months or years to develop virus infections. Of concern, however, is the fact that the clone is a DNA copy of genetic variant GRSPaV- GG closely related to -SG1 variant, which is putative not pathogenic to grapevines (Meng & Rowhani, 2017). Presently a

SG1, classified as being not pathogenic to grapevines. Surprisingly, this is the only solid information on pathogenicity of GRSPaV. There is no data on putative severe genetic variants of GRSPaV. In addition, somewhat also surprising is that 22 years after the discovery of GRSPaV, and numerous statements that this virus is associated with RSPD, very little is known about populations of GRSPaV variants inducing clear, severe RSPD symptoms in St. George indicator. Such data, generated in different laboratories, could point to putative severe strains of GRSPaV, which are urgently needed for progress to be made in the study of pathogenicity of this virus in grapevines. Basically, there are two ways to investigate GRSPaV pathogenicity in controlled laboratory conditions: 1. Using various natural single infection of grapevines with GRSPaV; or 2. produce cDNA clone of this ssRNA virus and infect virus-free grapevines using the clone. Regarding the first option, finding a grapevine infected only with GRSPaV in vineyards may be a serious challenge. However, as the virus is relatively very difficult to eliminate from grapevines, the author believes that single GRSPaV infections can be quickly found in collections of nucleus material of grapevine

WINETECH TECHNICAL YEARBOOK 2020 8