Predicting Peach Fruit Size Potential from GDD 30 Days Post-Bloom

Early flower bud or fruitlet removal in peach is known to increase peach fruit size at harvest. However, early spring temperatures during and after bloom are not actively tracked to see if they affect future growth potential of peach fruit. Growing degree days or hours (GDD/GDH) have been used previously with a Relative Growth Rate (RGR) Fruit Model to estimate fruit growth potential and to provide a decision making tool for cultural practices such as timing of thinning for specific cultivars. Weather data were converted via the "Averaging Method" to growing degree days base 7 degrees C for 30 days post-bloom for years 2002-2010 at Ridge Spring, South Carolina. Percent packout for number one fruit, 70 mm or greater, was collected from a packing-house for 30 cultivars. Years 2004, 2005 and 2008 were above average years for larger fruit size and 2002, 2009 and 2010 were below average fruit size years. Packout for large fruit sizes in years with GDD of 468 or less 30 days post-bloom were not affected. However, years that GDD were near 700 at 30 days post-bloom had significantly less large (>70 mm) fruit packed. GDD greater than 494 the first 30 days post-bloom resulted in less than 60% packout of large fruit in the years studied suggesting that potential fruit size may be lost unless early flower/fruitlet thinning is initiated under warm post-bloom temperatures.