Thesis Defence: Mitigating the effects of Grapevine leafroll-associated virus 3 and Grapevine red blotch virus in ‘Merlot’ and ‘Cabernet Sauvignon’ using crop thinning, and investigating the role of virus titer in disease severity in the Okanagan Valley, British Columbia

April Mahovlic, supervised by Dr. Miranda Hart, will defend their thesis titled “Mitigating the effects of Grapevine leafroll-associated virus 3 and Grapevine red blotch virus in ‘Merlot’ and ‘Cabernet Sauvignon’ using crop thinning, and investigating the role of virus titer in disease severity in the Okanagan Valley, British Columbia” in partial fulfillment of the requirements for the degree of Master of Science in Biology.

An abstract for April Mahovlic’s thesis is included below.

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ABSTRACT

Grapevine leafroll-associated virus 3 (GLRaV-3) and Grapevine red blotch virus (GRBV) are economically important viruses in British Columbia that decrease grapevine health and fruit quality. This study aimed to investigate the use of crop thinning to mitigate negative impacts of GLRaV-3 and GRBV on vine health and fruit quality of ‘Merlot’ and ‘Cabernet Sauvignon’ vines in the Okanagan Valley, British Columbia, as well as understand the role of virus titer in disease severity. A commercial vineyard site with adjacent ‘Merlot’ and ‘Cabernet Sauvignon’ blocks was selected in a complete-randomized trial conducted in 2021 and 2022 growing seasons. GLRaV-3 and GRBV vines were paired 1:1 with healthy vines, and each pair was designated a cropping treatment, applied at pea-sized berries stage. “Full crop” vines had 1.5 clusters per shoot, and “half-crop” vines had 1.0 cluster per shoot. Vine health parameters and virus titer measurements were taken four times in the season and fruit quality measurements were taken once at harvest. GLRaV-3 infection increased crop load and titratable acidity (TA), and decreased Brix. Crop thinning decreased TA and anthocyanins and increased pH of healthy vines. “Half crop” GLRaV-3 vines had significantly higher pH than “full crop” GLRaV-3 vines. However, all these impacts were year and cultivar dependent, suggesting that crop thinning is not sufficient to mitigate GLRaV-3 impacts in ‘Merlot’ and ‘Cabernet Sauvignon’. GRBV had more consistent impacts on vine health and fruit quality, as infection decreased leaf greenness (SPAD), increased berry weight, cluster weight, pruning weight, crop load, TA and pH, and decreased Brix, yeast assimilable nitrogen (YAN). Crop thinning did not significantly impact GRBV infected vines, thus was not sufficient to mitigate virus effects. GLRaV-3 and GRBV titer was highest at pea-sized berries and harvest. We suggest viral load may follow a seasonal pattern corresponding to temperature changes. Viral titer was negatively correlated with photosynthesis (Anet), stomatal conductance (gs), SPAD, berry weight, cluster weight, yield, TA and YAN. However results differed by virus, cultivar and year, and weren’t indicative of negative impacts to vine health or fruit quality.