The potential impact of Red Blotch on wine quality will likely depend upon incidence in the vineyard. At low incidence, the impact may be negligible; however, at higher incidence the impact may be substantial. The objective of this study was to determine the threshold incidence of Red Blotch in Cabernet Sauvignon vineyards at which separation of harvest is required to avoid negative impacts on wine quality. Small-lot wines were made from Cabernet Sauvignon grapes from the same vineyard block with 0, 25, 50, or 100% of fruit from Red Blotch-infected vines. Must composition (Brix, pH, and TA) correlated significantly with percent Red Blotch fruit. Wine composition was also impacted significantly by percentage Red Blotch fruit. Ethanol correlated negatively (r² = 0.97) with percent Red Blotch fruit. Wine color was impacted by percent Red Blotch fruit. Color density (A₄₂₀ + A₅₂₀) correlated negatively (r² = 0.92). Total iron-reactive phenols (r² = 0.60) and tannins (r² = 0.67) in finished wines correlated negatively with percent Red Blotch fruit in must. The results show that the potential impacts of Red Blotch on must and wine composition depend upon incidence in the vineyard and degree of fruit maturity delay in virus-infected vines. Duo-trio difference tests were performed on wines. There was a significant difference between wines made with 0 and 50% Red Blotch fruit (p < 0.05). The comparison of wines with 0 and 25% Red Blotch fruit was inconclusive (p = 0.059). There was no significant difference between wines with 25 and 50% Red Blotch fruit (p = 0.40). Incidence of Red Blotch from 25 to 50% are sufficient to have an significant impact on wine sensory properties and, therefore, quality. Separate harvest of virus free and Red Blotch vines is likely warranted when vineyard incidence of Red Blotch is above 25%.

Funding Support: Treasury Wine Estates