Predicting the time course of grape ripening

Background and Aim:  To predict harvest time for logistic applications in vineyards and wineries, measurements of total soluble solids (TSS) are typically combined with projections assuming TSS increases at 1°Be/week. Here, three questions were asked: (i) what is the magnitude and direction of the bias in estimates of ripening assuming 1°Be/week across varieties, regions and seasons? (ii) what are the climatic drivers of this bias? and (iii) could thermal rates improve predictions? Methods and Results:  Actual TSS of Chardonnay, Shiraz and Cabernet Sauvignon was measured in commercial vineyards in Coonawarra, Barossa Valley and Riverland. Predictions used (i) the default 1°Be/week; (ii) variety- and location-specific chronological rates between 0.8 and 1.2°Be/week; and (iii) variety- and location-specific thermal rates between 0.009 and 0.019°Be/Cd. In eight of the nine cases, 1°Be/week biased ripening predictions. Seasonal bias correlated with evaporative demand in the Riverland and Coonawarra and with minimum temperature in the Barossa Valley. Thermal rates were not superior to calibrated chronological rates to predict ripening. Conclusion:  Locally calibrated rates significantly improved ripening predictions for major grapevine varieties in regions with contrasting climates and viticultural profiles. Significance of the Study:  Improved prediction of ripeness time would allow for better allocation of key resources in vineyards and wineries – labour, time, machinery, transport, analytical services, chemicals and storage.