Foliar Nutrient Sprays Affect Early Harvest Season Fruit Cracking of ‘Wonderful' Pomegranate

Fruit split of pomegranate (Punica granatum) is a physiological disorder resulting in rind cracking late in fruit development. This exposes arils, reducing the number of marketable fresh market fruit and the mass of processing fruit. To assess the efficacy of foliar nutrients in the mitigation of pomegranate fruit split in California’s San Joaquin Valley, research was conducted at two commercial 9-year old ‘Wonderful’ orchards (Site 1 and Site 2, respectively). Zinc sulfate (ZnSO₄ at 3000, 4000, or 5000 mg·L^-1), magnesium sulfate (MgSO₄ at 1%, 2%, or 3%), potassium nitrate (KNO₃ at 1%, 2%, or 3%), or a control treatment of deionized water was applied by backpack sprayer at early fruit set (July; fruit green to breaker with caliper ~ 50 mm). MgSO₄ and KNO₃ treatments were applied again at late fruit set (August; fruit red with caliper of ~ 70 mm). A randomized complete block design with whole-tree experimental units and 25 blocks was utilized, for a total of 250 trees at each site. Trees at Site 1 were harvested at the beginning of their commercial harvest period (October) and trees at Site 2 were harvested at the end of their season (November). Nutrient analyses conducted prior to and after treatment applications demonstrated that nutrients were taken up by the leaves and increased fruit nutrient content. Zinc treatments resulted in phytotoxicity at Site 2. Data collected included yield (fruit number and mass) of split and unsplit fruit at harvest, mass and caliper of individual fruit and postharvest evaluation of arils, including number and mass per fruit. At Site 1, percent fruit split decreased in response to 5000 mg·L^-1 ZnSO₄, 1% KNO₃ and all MgSO₄ without negative effects on other fruit quality parameters. At Site 2, treatments did not result in a significant reduction in any harvest parameter measured. This indicates that foliar applications of MgSO₄ or KNO₃ have potential commercial use to decrease pomegranate fruit split, though treatments may not be effective during late-season harvests. Further studies are needed to determine how long harvest windows can be extended by foliar nutrient applications.