Predicting Peach Maturity Dates in South Carolina with a Growing Degree Day Model

Forecasting maturity date for weekly ripening fruit crops is important for retail marketing. A model based on growing degree days calculated from a base temperature of 7 degrees C has been used to predict harvest dates in California with positive results. However, this model has not been applied to other climates, specifically the southeastern U.S. The objective of this research was to predict harvest dates for peach cultivars using a Growing Degree Day formula based on temperatures for the first 30 days after flowering. Three formulas were used to calculate Growing Degree Days so that a comparison could be made to find the model that best suited South Carolina. Two of these models, the BE (or Baskerville-Emin method) and the Averaging Method, require average daily temperatures, and the third requires hourly temperatures to complete a prediction. The method commonly used in California was the BE method. We found this method to be the simplest and most effective model to use for our conditions. This method was applied to two locations in South Carolina that experience different temperature patterns to further test its capability to predict fruit maturity under diverse climatic conditions. Five cultivars were tested in the South Carolina Piedmont near Clemson University, and eleven cultivars were tested near Ridge Spring, South Carolina, which borders the Piedmont and Sandhills geographic regions. It was hypothesized that high temperatures during the first 30 days after bloom would result in an earlier harvest date. The results from these two regions were in accordance with the expected outcome with highly significant regression coefficients of r=.979, r=.984 and r=.984 for early, mid, and late season cultivars, respectively. However, the early and mid-season cultivars in the cooler Piedmont test location had a more negative association (earlier maturity with high post-bloom temperatures) than those at the warmer Ridge Spring location.