Source-sink manipulation does not mitigate the effects of grapevine red blotch virus (GRBV) infection on fruit sugar and flavonoid accumulation in Cabernet-Sauvignon

Grapevine red blotch virus (GRBV) negatively affects the composition of grapevine (Vitis vinifera L.) berries by reducing total soluble solids and anthocyanins, leading to economic losses for grape producers. Negative effects of GRBV were suspected to be due to impeded carbon translocation from leaves to fruit which limits sugar and flavonoid accumulation in berries. A two-year trial was conducted to determine whether an increase in source: sink ratio may affect sugar allocation and mitigate the effects of GRBV on Cabernet-Sauvignon plants. Experimental design was factorial (2 by 2) with healthy plants that did not have the virus (GRBV (-)) and plants having GRBV (GRBV (+)) and plants were subjected either untreated (UNT) or cluster thinned down to 10 clusters (CT). Effects of cluster thinning and virus status on leaf and shoot total soluble sugars (TSS), plant water status, leaf gas exchange, berry primary and secondary metabolites, and yield components were measured. The TSS in leaves began to accumulate around véraison. In shoot sap, GRBV(-) plants had greater concentration in TSS than GRBV(+) plants. The presence of disease improved plant water status increasing the stem water potential and increasing berry mass. However, juice total soluble solids were consistently lower in GRBV(+) plants despite increasing source: sink ratio by 3× with cluster removal. Likewise, GRBV(+) plants produced berries with lower anthocyanin content at harvest regardless of CT in both years. Our results suggest that GRBV infection severally impeded carbohydrate translocation out of the leaves, and in contrast to healthy plants reducing the number of clusters does not induce a reconcentration of sugars in the remaining clusters.