Optimizing Preharvest Calcium Application Frequency, Timing, Rate, and Sources to Increase Tissue Calcium Content and Shipping Quality of Sweet Cherry

Northwestern U.S. sweet cherries that are shipped to distant markets are subject to various arrival issues including fruit softening, flavor loss, skin darkening, pitting, splitting, pedicel browning, and decay. The objectives of this study were to develop preharvest Ca spray protocols to improve cherry shipping quality and determine the response of fruit growth, fruit size, yield, and return bloom to Ca. Fruit firmness and pitting susceptibility of ‘Lapins’ and ‘Sweetheart’ were related to fruit tissue Ca concentration, but not N concentration, in fruit sampled from different orchards. Six weekly applications of Ca were required between pit-hardening and 1-week before harvest (WBH) to increase fruit tissue Ca concentration of ‘Lapins’. Nine weekly applications applied from full bloom to 1 WBH did not improve tissue Ca concentration compared to the 6-application regime. The optimum Ca application rate was 0.1% to 0.15%.  Leaf burning was observed at rates exceeding 0.15% for several different Ca sources.  Two applications (one at pit-hardening + one at 1 WBH, or one at 2 WBH + one at 1 WBH) did not increase tissue Ca concentration compared to control. Frequent applications are more important than exact spray timing. The efficacy of Ca was not dependent on fruit development. When applied six times on a weekly basis, “Cal-8” at 0.2% Ca and “Chelate Ca” at 0.05% Ca were more efficient than CaCl₂ at 0.07-0.15% Ca, Ca(NO₃)₂ at 0.1% to 0.15% Ca, and Ca citrate at 0.07% Ca for improving fruit firmness. Soluble solid (SSC), titratable acidity (TA), and skin color at harvest were not affected by the Ca applications. Beginning 2 to 3 WBH, fruit growth was negatively affected by CaCl₂ and Ca citrate, but not by other Ca treatments. Enhanced fruit tissue Ca concentration improved shipping quality by reducing pitting, decay, pedicel browning, skin darkening, and loss of firmness and TA after three weeks of cold storage.