Sunburn damage is a skin discoloration of the exposed side of the fruit. Three different types of sunburn can be distinguished based the conditions of their formation;
- Sunburn browning is the most common type caused by the concomitant action of radiant heating and UV-B exposure. The symptoms are yellowish, brownish patches on the sun-exposed side of the fruit. Symptoms occur on the surface of the fruit (epidermis and hypodermis) but do not penetrate into the core tissues.
- Sunburn necrosis is a pure thermal death of the skin and underlying cortex tissues of the fruit caused by excessive radiant heat. No light is necessary for the formation of this sunburn type.
- Photooxidative sunburn is the third sunburn type which develops on previously shade-grown fruit upon sudden exposure to the direct sun. This happens for instance after summer pruning when the covering foliage is removed. Photobleaching of the suddenly exposed fruit surface causes white discoloration.
As sunburn symptoms do not disappear once formed, their protection methods involve prevention only. Sunburn formation can be prevented three different ways.
- Climate ameliorating techniques reduce fruit surface temperature therefore the possibility of the formation of heat-dependent types of sunburn (sunburn browning, sunburn necrosis). These techniques involve evaporative cooling, the use of shading nets and fruit bagging
- Sunburn suppressants mechanically cover the fruit surface with particle films (e.g., Surround) or carnauba wax emulsion (RAYNOX) protecting the fruit from the direct sun.
- Other chemicals and ingredients (vitamin C, vitamin E, antitranspirants, etc.) have also been tried against sunburn with more or less success. Recently, abscisic acid showed significant effect in reducing sunburn damage to apple fruit.
Jozsef Racsko, Ohio State University