HEAT AND LIGHT-INDUCED APPLE SKIN DISORDERS: CAUSES AND PREVENTION

In Washington State (USA) and many other major fruit-growing regions of the world, several fruit skin disorders are induced by heat and/or light stress in apples (Malus × domestica Borkh.). Three types of apple sunburn have been characterized by the authors. The first type (sunburn necrosis) is caused by heat alone [high fruit surface temperature (FST) of 52±1°C] and results in thermal death followed by necrosis. The second type (sunburn browning) is caused by both high FST and UV-B radiation. The third type (photooxidative sunburn) appears to be caused by light alone and occurs on green peel (non-acclimated to light) that is suddenly exposed to full sunlight so that photobleaching occurs first, followed by necrosis. This third type is thought to be photooxidative damage and can occur at much lower FST and without UV-B radiation. Another disorder that results from stress is ‘Fuji’ stain. This skin disorder appears only after a period of cold storage. This stain disorder appears primarily in sunburned fruit, and its incidence rises sharply as the severity of sunburn increases. This indicates that stress conditions that cause sunburn, also predispose certain fruit prior to harvest to develop this skin disorder during cold storage. We have evidence that UV-B radiation is involved. Another disorder that results from heat and light stress is lenticel marking. Its incidence also increases in fruit that have more severe sunburn. All of these disorders can be significantly decreased by use of RAYNOX®, a sunburn protectant. Evaporative cooling or a combination of RAYNOX® and evaporative cooling are also effective. Evaporative cooling reduces the FST, and RAYNOX® decreases both FST and UV-B absorption by fruit.