Influence of mixed and single infection of grapevine leafroll-associated viruses and viral load on berry quality.

Grapevine leafroll disease (GLD) is a viral disease that affects grapevines (Vitis vinifera L.) and has a severe economic impact on viticulture. In this study, the effect of grapevine leafroll-associated viruses (GLRaV) on berry quality was investigated in clones of cultivar cv. Crimson Seedless table grapes infected with GLRaV. RT-PCR confirmed the identity of the clones: clone 3236, infected only with GLRaV-3 (termed Single); clone 3215, infected with GLRaV-3, GLRaV-4 strain 9 and grapevine virus A (termed Mixed), and a viral free clone of the same genetic background of the infected clones (termed Control). The berry quality indices of size, sugar, acidity, and anthocyanin content were measured at harvest maturity. RT-qPCR was used to determine viral load. The study was repeated over two years. A two-way, multivariate analysis of variance (MANOVA) was applied with clone and year as independent variables and the measured berry quality parameters as a dependent variable. All dependent variables were significantly affected by viral infection (Wilks, λ, [2,33] = 0.033895, p-value < 0.001), while only titratable acidity (TA) was affected by year. Average berry dry mass decreased (p-value < 0.001). The water content of both infected clones was greater than that of the control (p-value < 0.001). Both infected clones displayed reduced sugar content as a fraction of the berry dry mass (p-value < 0.001). The anthocyanin and the phenol content of the infected clones were significantly reduced compared to the control clone (P < 0.001, P < 0.05, clone 3236 and clone 3215, respectively). Finally, the viral load was highly variable, and no quantitative relationship between viral load and berry composition was found.