Calcium-spray treatments for ameliorating albedo breakdown in navel oranges

Albedo breakdown is a rind disorder of orange fruit that results in significant economic cost to the Australian citrus industry. The effectiveness of calcium sprays as a control measure for albedo breakdown of navel orange fruit was investigated in a series of experiments in which single fruit, 1 m2 quadrats of tree canopy or whole trees were treated with different calcium sprays. Repeated spraying (24×) of single Bellamy navel orange fruit throughout the season with 1% calcium solutions increased the proportion of unaffected fruit from 30 to 65-80%. Repeated applications (15×) of 1% calcium as CaCl2 and Ca(NO3)2 to quadrats increased the proportion of unaffected fruit from 43 to 70-95%. The effect of anion was in the order Ca(NO3)2 CaCl2 >CaCO3. Repeated spraying (15×) of a commercial CaCl2 formulation (Stopit) at a calcium concentration of 0.33% increased the proportion of unaffected fruit in quadrats from 17 to 47%. Generally, early and mid-season calcium sprays were equally effective and tended to be marginally better than late-season sprays. Spray-applications of calcium at a concentration of 0.33% were more effective than calcium sprays at 0.11% applied early in the season. Calcium sprays were less effective than gibberellic-acid sprays based on the number of sprays but calcium treatments did not affect rind quality characteristics such as rind colour; in contrast, gibberellic acid induced regreening of fruit. Calcium sprays (as CaCl2) applied early in the season at 0.33% caused unacceptable fruit drop. CaCl2 (0.33%�calcium) treatment (5×) resulted in leaf chloride concentrations as high as 100 mmol/L in leaf-tissue water (0.7% on dry weight basis) - a level sufficiently high to substantially reduce photosynthesis. Ca(NO3)2 is the preferred calcium source in citrus because it is (i) as or more effective than CaCl2, (ii) does not result in leaf damage and fruit drop and (iii) may provide a small controlled nitrogen supplement to leaves and fruits.