The Effect of Preharvest Coatings and Foliar Nutritional Sprays on Postharvest Peel Breakdown of Citrus Fruit

The fresh citrus industry in Florida generates more than one billion dollars in economic activity for the state. Marketability of fresh citrus and customer satisfaction are greatly affected by fruit peel condition. Peel breakdown is a physiologic disorder that usually develops during postharvest handling and may result in economic losses of a million dollars or more in some years. Previous studies found that peel breakdown can be reduced by applying a foliar preharvest application of mono-potassium phosphate (MKP), magnesium (Mg), or an antitranspirant (Vapor Gard).

Further studies were conducted during the 2011-12 Florida citrus season to evaluate additional preharvest materials and their possible mechanism(s) in reducing peel breakdown. All fruits used in this project were from commercial citrus groves in Vero Beach and Ft. Pierce, FL. The treatments consisted of MKP at 23.5 lb MKP per acre (0-52-34; 8 lb K2O per acre) with 4 lb per acre low-biuret urea (46-0-0); 1%, 1.5%, or 2% Vapor Gard (antitranspirant); 2% calcium chloride (CaCl₂); 1% WashGard (a carnauba coating formulated for field applications); or 1% Polymer Delivery System (PDS). The materials were applied at 5 gallons per tree using a hand-gun sprayer. Fruit samples were harvested 3, 7, or 14 days after application, washed and waxed using a commercial carnauba coating, stored under ambient conditions, and evaluated weekly for the development of decay and peel disorders.

Preharvest foliar treatments with CaCl₂ never significantly reduced peel breakdown compared to the water-sprayed control, whereas differences with MKP, Vapor Gard, PDS, or WashGard were almost always significant. Of the latter group, 2% Vapor Gard, PDS, and WashGard were most effective, usually resulting in 20% to 50% less peel breakdown. Leaf stomatal conductance was significantly reduced compared to the control 6 and 21 days after application with WashGard, and tended to be lower after treatment with Vapor Gard or MKP. There were no consistent treatment effects on fruit peel integrity, measured as peel puncture resistance. There was also no relationship in peel thickness between fruit that remained healthy and those that ultimately developed peel breakdown. As expected, fruit that developed peel breakdown lost more water during storage than healthy fruit.