Determination of Cultivar-specific Threshold Temperatures and Heat Requirements for Prediction of Budbreak, Full Bloom, and Veraison in Wine Grape

Weather conditions have a significant impact on crop growth and development. The main controlling factor is temperature which affects plant development and many other plant processes. Phenological models such as thermal time or Growing Degree Day model have been widely applied to predict the development of many species. These concepts can also be applied for grapevine. In addition to being used as a tool for decision making for grapevine management and site selection, phenological models can be used to optimize vineyard production systems for cost, quality, and environment to improve the overall long-term sustainability of the grape production system. So far the use of a Degree Day model that has the same based temperature for all cultivars is common. The goal of this study was to evaluate the threshold temperature and heat requirements for the prediction of three growing stages for four Vitis vinifera L. cultivars. Historical phenological data for 20-years was used to estimate heat requirements for individual consecutive growing stages were used to develop a model for budbreak, blooming and veraison prediction in Prosser (WA). Model performance was evaluated for the interaction of different threshold temperatures and heat requirements. Significant differences were found in heat demand among cultivars and developmental stages for early and late cultivars. The threshold temperature increased along wine grape development from budbreak to veraison. The threshold temperatures for budbreak ranged from 6.5°C for the early cultivars such as Chardonnay to 8.3°C for the late cultivars such as Cabernet Sauvignon. The highest requirements were obtained for the growth stages of bud burst to first bloom and full bloom to veraison where major changes in phenology occurred during the transition period from vegetative to reproductive. The preliminary results obtained provide valuable information to determine the start of grapevine development based on local weather data. The initial results are now being evaluated with new data from an independent data set. It is expected that the final model will be implement as a decision support tool on the AgWeatherNet web portal (www.weather.wsu.edu).