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.