EFFECT OF VERY HIGH CALCIUM SPRAYS JUST BEFORE HARVEST ON APPLE FRUIT FIRMNESS AND CALCIUM CONCENTRATION

A Polish report in Fall 2001 indicated that very high rates of Ca chloride applied one week before harvest consistently enhanced firmness retention by about 20% in Jonagold apples stored in refrigerated air for 120 d. The authors conducted a study in 2002 examining the effect of CaCl₂ and MiraCal (an organically complexed Ca oxide-based proprietary spray product) application at 24 kg Ca per 1,000 L, 7 to 9 d before harvest on firmness and Ca content of Gala, Golden Delicious, Braeburn and Fuji apples. The field experiments were conducted in semi-arid central Washington State, USA. The high Ca sprays failed to improve fruit firmness at harvest or after 30 d storage in refrigerated air. The apples treated with CaCl₂ had a readily observable film of moisture on their surfaces, consistent with the high hygroscopicity of the compound and its ability to absorb and retain moisture from the air. The CaCl₂ caused no injury on the fruit, but did cause leaf burning. The MiraCal dried as a powdery white coating. It caused no injury on the Braeburn, Golden Delicious and Gala apples but caused lenticel burning on all of the MiraCal-sprayed Fuji apples. It caused no leaf injury. Considerably more Ca was taken up by the fruit treated with CaCl2 than with MiraCal. It appears that the published benefit of the high Ca treatment on apple firmness may depend on cultivar or initial fruit Ca status, or may be expressed only when harvested fruit is unusually stressed, such as by long-term refrigerated air storage.