Winetech Technical Yearbook 2022

JULY

Transmission dynamics of grapevine leafroll associated virus 3 (GLRaV-3) variants THE TRANSMISSION FROM SOURCE PLANTS INFECTED WITH DIFFERENT COMBINATIONS OF MIXED VARIANT INFECTIONS WAS STUDIED WITH THE AIM TO UNDERSTAND THE BIOLOGICAL DIFFERENCES BETWEEN GLRAV-3 VARIANTS RESPONSIBLE FOR LEAFROLL SPREAD IN VINEYARDS. BY ELIZE JOOSTE & KERSTIN KRÜGER I INTRODUCTION might be more easily transmitted by the mealybug vectors, or that a combination of these variants in a plant can cause a more aggressive spread of the virus. METHODOLOGY A transmission experiment using Plano coccus ficus , the mealybug associated with GLRaV-3 spread, was carried out using source plants that were infected with a mixture of GLRaV-3 variants and viruses (table 1). A 24-hour Inoculation Access Period (IAP) and Acquisition Access Pe riod (AAP) were used in the experiment where 20 first-instar nymphs fed on infect ed source plants (IAP) and the first-instar nymphs were then transmitted to healthy ‘Cabernet franc’ recipient plants (AAP). A RT-PCR HRM analysis 2 was used to detect the transmission frequency of indi vidual GLRaV-3 variants from the mixed variant infections in recipient plants after transmission. Detection of the presence of other grapevine infecting viruses i.e., grapevine virus A (GVA), grapevine virus B (GVB) and grapevine rupestris stem pitting virus (GRSPaV), was carried out using virus-specific primers. RESULTS AND DISCUSSION Grapevine leafroll-associated virus 3 (ge nus Ampelovirus ; family Closteroviridae ) is prevalent and economically important world-wide and associated with grapevine leafroll disease (GLD). Phylogenetic stud ies up to 2018, have grouped GLRaV-3 isolates into nine different monophyletic groups and four supergroups, making GLRaV-3 a genetically highly diverse virus species. In addition, new divergent vari ants have been discovered from around the world. In South Africa, results from a previous survey study showed that grape vine leafroll-associated virus 3 (GLRaV-3) variants of group II and VI occurred pre dominantly (refer to Winetech-funded project GenUS11-1). 1 This is an indica tion that these two variants of the virus Figure 1 provides illustrations of the out comes from the RT-PCR HRM analysis which allowed for the determination of the GLRaV-3 variant groups. As can be seen from Figure 1A, there was an overlap in the melting curves of group I and II vari

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WINETECH TECHNICAL YEARBOOK 2022