WINETECH Technical Yearbook 2019

Grafting, however, that combines two different Vitis species, is not without its problems. Perfect grafting requires full connection of vascular tissues (phloem and xylem) of grafted grapevine species (Melnyk, 2017). Failure of this connection, which may go unnoticed by the farmer, results in poor growth of grapevines, delayed bud burst, low fruit production, or decline of grafted plant after initial good growth. The problem with graft union compatibility is sometimes exhibited by external symptoms like “swelling” of scion above the graft union in some rootstock/ V. vinifera combinations. Although the abnormally developed with graft unions can be environment-, rootstock- or scion-related (Hunter & Volschenk, 2015), investigations revealed that virus infection of grapevines are also associated with these symptoms (Bonfiglioli et al ., 1998; Goussard, 2013). The term “rugose wood diseases” (RWD) of grapevines refers to the modified grapevine woody cylinder (grooving and pitting), and exceedingly thick bark of a scion, which are putatively induced by virus infections (Bonfiglioli et al ., 1998; Goussard, 2013). A comprehensive progress review in the study of RWD, between 1962 to 2013, which provided a solid foundation for further investigation of this disease, was published in the Journal of Plant Pathology in 2014 (Anonym, 2014). Briefly, a breakthrough in the clear identification of RWD was the

discovery of grapevines especially sensitive to RWD. This has led to the identification of four diseases of rugose wood complex: Rupestris stem pitting (RSPD), Kober stem grooving (KSGD), corky bark (CBD) and LN33 stem grooving (LNGD). The discovery was followed by descriptions of virus species associated with RWD, grapevine virus A (GVA) and grapevine virus B (GVB), and transmission of these viruses between grapevines by insect pests common in vineyards, such as mealybugs. The full molecular characterisation of genomes of these viruses has led to establishment of a new taxonomic group, the genus Vitivirus (Anonym, 2014 & 2018B) . In addition to the members of this group, viruses of the genus Foveavirus , like grapevine rupestris stem pitting-associated virus (GRSPaV), discovered in 1998 (Meng et al ., 1998), were also found to be associated with RWD. The number of recorded members of the genus Vitivirus , rapidly expanded after the introduction of high-through sequencing (HTS). This modern technique generates sequence data for any genetic material, including viruses, present in the investigated sample. Currently 11 virus species of the Vitivirus genus, named with sequential letters of the alphabet: GVA, GVB, GVD, GVE, GVF, GVG, GVH, GVI, GVJ, GVK and GVL, were discovered in grapevines in Italy, France, Japan, South Africa, New Zealand, USA, Argentina and South Korea (Anonym, 2014; Abou-Ghanem et al ., 2007; Coetzee et al ., 2010; Al Rwahnih et al ., 2012; Blouin et

al ., 2018A&B; Candresse et al ., 2018; Diaz- Lara et al ., 2018; Jo et al ., 2017; Debat et al ., 2019). It indicates that the genus Vitivirus comprises a large number of virus species and suggests that these viruses are common in vineyards worldwide. This is highly intriguing considering that accumulated research data strongly suggest that two of them, GVA and GVB, induce two diseases of RWD complex, KSGD and CBD, respectively (Anonym, 2014). Thus, should we consider that the remaining nine vitiviruses, GVD-GVL, are also potentially able to cause abnormal development of graft unions? If yes, the industry should be concerned. There are reports that the vitiviruses, unlike members of the family Closteroviridae , associated with grapevine leafroll disease (GLRD), are relatively difficult to eliminate from grapevines using commonly used virus elimination techniques (Gribaudo et al ., 1997). In addition, these viruses are easily transmitted between grapevines by common vineyard pests like mealybugs (Anonym, 2014). There are many quest ions regarding vitiviruses that should be answered. If these viruses are really widely present in vineyards worldwide and are able to induce RWD, what prevents them from inducing pathological changes in graft unions? Perhaps only a small number of grapevine cultivars are susceptible to RWD? Or, perhaps only a small percentage of populations of genetic variants

of each member of the Vitivirus genus are pathogenic to grapevines. It is possible that a high titre of vitiviruses is needed to visualise the pathogenic effect of these viruses in grapevines and to reach this hypothetical threshold titre they might need support from other viruses. I n South Af r i ca , CBD- a f fec ted LN33 grapevines are always infected with GVB and a closterovirus, grapevine leafroll-associated virus 3 (GLRaV-3). This virus and the other members of the family Closteroviridae encode strong suppressors of anti-virus grapevine defence system (Gouveia et al ., 2012). A recent article published by the USA laboratory strongly suggests that the synergy between grapevine vitiviruses and grapevine leafroll-associated viruses is a reality. It was found that the titres of GVA and GVB in co- infection with the closteroviruses, GLRaV-2 and GLRaV-3, were clearly higher than the titres were in the absence of these co- infections (Rowhani et al ., 2018). In addition, despite the association of vitiviruses with RWD, the pathogenicity of these viruses to grapevines is still not well known. An anatomical study of CBD-affected grapevine revealed abnormal development of xylem and phloem tissues (Beukman & Gifford, 1969). Thus, GVB, which is the suspected cause of this disease, is able to deregulate differentiation of cambium cells to xylem and phloem. How does GVB do it? What grapevine genes are targeted? Does GVB,