Prediction of ‘Concord’ Grape Maturation and Sources of Error1

Fruit samples of grape (Vitis labrusca L., cv. Concord) from 6 vineyard locations were collected at 7 to 10-day intervals beginning prior to veraison and continuing through development of 16% soluble solids for a period of 19 years. The 19-year average date for peak bloom in these vineyards was May 19, for 8% soluble solids development was July 27 (69 days from peak bloom), and for 16% soluble solids development was August 23 (96 days from peak bloom). Heat unit summations were more closely related to development of soluble solids than to changes in either titratable acidity or color. Using degree-day accumulations and effective heat unit summations did not prove to be methods superior to use of the number of calendar days for predicting grape maturation. Predictions from 8 to 16% soluble solids development were more accurate than predicting from peak bloom (when 50% of clusters showed bloom). Variations between years and between vineyard locations within a given year prevented accurate predictions from the 3 methods. Other deterrents observed in predicting development of soluble solids included the cultural variables of fruit load and soil moisture.