Karl Foord, UMN Extension Educator
Sunburn to apples should be distinguished from sunscald. Sunscald is damage to the bark of the tree when strong winter sun warms up tissues on the south facing side of the tree. When the sun sets, the temperature plummets and this softened tissue is damaged by freezing.
Sunburn is damage to the peel of the fruit. The temperature of the apple peel can be significantly greater than the ambient temperature ranging from 18°F to 29°F above ambient temperature on a clear day when other conditions are favorable (Schrader et al., 20011). Factors influencing peel temperature include solar irradiation, air temperature, relative humidity, wind speed and direction, and tree vigor.
David Bedford, a research scientist at the University of Minnesota Horticultural Research Center, noted seeing a lot of this type of damage due to the high temperatures and high dew points experienced this summer.
There are two types of sunburn damage, necrosis and browning. Sunburn necrosis involves the thermal death of the tissue and occurs at 126 ± 2°F (Photos 1 & 2). Sunburn browning describes the presence of a yellow, bronze, or brown spot on the sun-exposed side of an apple and occurs at peel temperatures between 115 to 120°F.
Proper pruning use of shade cloth and application of particle films are methods that growers can use to protect fruit and avoid sunburn.
Thanks to Janelle Daberkow, horticulture educator in Stearns and Benton counties, and the Benton County Master Gardeners for providing photos.
¹Schrader, L. E., Zhang, J., and Duplaga, W. K. 2001. Two types of sunburn in apple caused by high fruit surface (peel) temperature. Online. Plant Health Progress doi:10.1094/PHP-2001-1004-01-RS.