Will Postharvest Foliar Calcium Increase Apple Fruit Calcium the Following Spring?

Calcium deficiency in fruit of bitter pit-susceptible apple cultivars may be due to early breakdown of xylem function. If calcium is primarily moving into the fruit early in the season while the xylem is active, increasing the amount of calcium in stored reserves may increase fruit calcium. A study was initiated in 2009 to determine if postharvest foliar calcium applications can increase calcium in apple fruit in the following year and reduce the incidence of bitter pit in fruit at harvest or during storage compared with a standard summer foliar calcium program.  The study was conducted on cv. ‘Honeycrisp’/M.7 and included four treatments arranged in a RCBD with five replications.  The treatments included an unsprayed control, four postharvest applications of foliar calcium at 10-14 d intervals in 2009, a standard program of seven foliar calcium sprays at 10-14 d intervals beginning at second cover in 2010, and a combined treatment of four postharvest foliar calcium sprays in 2009 and a standard program of seven foliar calcium sprays in summer 2010.  Fruit and leaf tissues were sampled for mineral analyses at three times during 2010.  Fruit were harvested upon commercial maturity and yield determined.  Fruit exhibiting bitter pit were counted at harvest and discarded.  The number of fruit that developed bitter pit in post-harvest storage (32°C) was counted at 14 d intervals. Neither the postharvest program nor the standard program of summer foliar calcium were effective in increasing fruit or leaf calcium concentration.   The overall incidence of fruit with bitter pit was high, with almost 50% of the fruit on control trees exhibiting of bitter pit at harvest.  All three foliar calcium treatments reduced the incidence bitter pit at harvest to around 30%.  The development of bitter pit increased during storage by approximately 1% each week for all treatments. While calcium in fruit and leaf tissues at harvest were not affected by treatments, all of the foliar programs reduced the incidence bitter pit at harvest compared with the control.  The program of four postharvest foliar calcium sprays in 2009, followed by no foliar calcium sprays during summer in 2010, reduced the incidence of bitter pit as effectively as a summer program of seven applications of foliar calcium.    We have extended this study for a second year and have initiated a second site on mature cv. ‘Golden Delicious’/M.7 trees.