Validation of a model for the growth and development of the Thompson Seedless grapevine. II: Phenology

This study compares the prediction of phenology models for the grapevine cultivar Thompson Seedless that are based on degree days (°D above 10°C) with patterns observed in the field. Budbreak, bloom, and the development of total soluble solids were monitored in six vineyards over a latitudinal range of 300 km in the San Joaquin Valley of California in 1983. The models predicted dates of 50% budbreak and 50% bloom within a few days of the observed events. Relative calendar timing followed the expected latitudinal trends, with earliest budbreak and bloom dates observed at the southernmost sites. The model predicted maturity (20° to 21.5°Brix) reasonably at four vineyards and underestimated it at the other two, one of which had vines that were water stressed and lightly cropped relative to the others. Predictions of °Brix differed substantially from observations in the range 14° to 18° at most sites. The factors influencing vine phenology, including temperature, solar radiation, water availability, and vine management practices, are discussed.