Calcium-spray treatments for ameliorating albedo breakdown in navel oranges

Abstract

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.