Winetech Technical Yearbook 2022

FIGURE 1. Illustration of results to determine the GLRaV-3 variant status of four GLRaV-3 variant groups.

study, the transmission dynamics of vari ants from group I and VI occurring in the Napa Valley in the USA, were examined by Blaisdell and other authors. 3 These au thors concluded that vector transmission of the group VI variant alone was more frequent, followed by transmission with mixed infections of the two, while trans mission with the group I variant alone was the least common. This was the first evi dence that GLRaV-3 variants are biolog ically distinct. The result of the group VI variant being transmitted more frequently in the USA study was not confirmed in the South African study. A possible explana tion could be the more frequent presence of the group VI variant in the USA vines, different genotypes studied or interactions with other viruses. Results from a previous survey study showed that grapevine leaf roll-associated virus 3 (GLRaV-3) variants of group II and VI occurred predominantly in South African vineyards (refer to Wine tech-funded project GenUS11-1). 1 A reduced frequency of transmission of a single variant in mixed transmissions was observed, possibly due to competition be tween variants. In two of the source plants (14/12/4 and 10/20/6), a combination of variants, I + III and I + II, respectively, were transmitted more effectively than the single variants. In a previous study (PPRI 11/18), the four GLRaV-3 variants, representing

group I, II, III and VI, in single infected vines, or when occurring in combination with GVA, were transmitted equally well under controlled conditions. Statistically, no indication of variant-vector specificity was detected in the transmission experiments under controlled conditions. However, the Vitivirus GVA, was transmitted at higher frequencies from GLRaV-3/GVA infected plants and was transmitted without the si multaneous transmission of GLRaV-3. CONCLUSION The study demonstrated that GLRaV-3 variants from group I and II established at a higher frequency in recipient plants during transmission either as single trans missions or in combination with each oth er. The exact interaction between variants in a plant is still unknown, but it is clear that competition between variants exists, either in the mealybug vector or in the recipient plant after transmission. The interaction between GVA and the group II variant needs to be investigated in more detail. Not all grapevine farmers have the fi nancial means to remove and replace infected vines and therefore the problem of leafroll will continue to have a serious economic impact in certain grape-produc ing regions. This study aimed to support the management strategies of the industry

through gaining knowledge of GLRaV-3 variants and other grapevine infecting viruses and their frequency of transmis sion. The study concludes that complex interactions between the mealybug vector, the viruses themselves and grapevine exist and establishing vineyards with certified material and maintaining frequent scout ing initiatives for the mealybug vector will assist to manage the disease. The use of newer technologies, like next generation sequencing, can provide more in-depth answers to the pathogen status of a vine and this will assist the industry to ensure that all certified material is disease-free. REFERENCES https://www.wineland.co.za/glrav-3 leafroll-variants-transmission-dynamics/

For more information, contact Elize Jooste at joostee@arc.agric.za.

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WINETECH TECHNICAL YEARBOOK 2022