Sunburn of Apples: Causes and Suppression of Sunburn Damage

Sunburn of apples causes large economic losses to growers in several regions of the world.  Sunburn is caused by stress from high temperatures and excessive solar radiation.   Fruit surface temperature (FST) on sun-exposed apples is frequently 11 to 17 ºC higher than air temperature.  The three types of sunburn we have identified and characterized are each caused by different factors.  Sunburn necrosis occurs when FST reaches 52 ºC for only 10 minutes.  Thermal death of epidermal cells occurs at that temperature and necrosis appears subsequently on the peel.  Sunburn browning, which is usually the most important type commercially, occurs in full sunlight when FST reaches 46 to 49 ºC for an hour; damaging ultraviolet (UV) radiation is also involved. The minimal FST required for sunburn browning varies with cultivar.  Photooxidative sunburn, the third type, is caused by sudden exposure of apples that have not previously been exposed to full sunlight (i.e. non-acclimated to high light).  Photooxidative damage can occur at much lower FST and without UV-B radiation.  Radiation in the visible range causes photooxidative sunburn and can occur during summer pruning, thinning, or shifting of branches with heavy crop loads.  Photobleaching occurs on the sun-exposed side, and necrosis may then appear subsequently.  Sunburn can be suppressed by several means.  Overhead evaporative cooling (EC) is the most effective means for reducing FST, but does not reduce stress from UV.  RAYNOX^® is a wax-based formulation that reduces transmission of UV, visible and infrared rays to fruit; blocking damaging UV rays is most important in suppressing sunburn.  RAYNOX^® should be applied three or four times during the season to augment natural waxes of the fruit.  Several particle film products (based on kaolin clay, talc, or calcium chloride) are also available to growers, but RAYNOX^® is the only rainfast sunburn suppressant that can be combined with EC to obtain excellent protection from both high FST and damaging UV rays.  ‘Fuji’ stain and “sunburn scald” of 'Granny Smith' are other skin disorders that can markedly increase crop losses.  Both disorders can appear during cold storage on apples with sunburn browning, suggesting that induction of sunburn also predisposes those sunburned apples to develop the other disorders during storage.  Incidence of both disorders can be reduced by using RAYNOX^® for sunburn suppression.