Initial symptoms are white, tan or yellowed patches found on the sun exposed side of the fruit. With severe skin damage, injured areas can turn dark brown on the tree. During unusually hot weather following cool or moderate conditions, and especially when accompanied by water stress, injury to the skin and flesh can occur. Injured cortex tissue is brown and firm and may become spongy and sunken. Fruit exposed to the sun after removal from the tree, either on the orchard floor or in field bins, can develop severe sunburn.
Sunburn results from heat stress to the fruit leading to injury of the affected cells.
The transpiration or evaporation of water from the apple fruit helps to cool the fruit while it is attached to the tree. Upon harvest from the tree or during periods of water stress on the tree, fruit surfaces and flesh can increase to much higher temperatures resulting in greater sunburn. The greatest problem occurs when fruit are suddenly exposed to high temperatures and intense sunlight such as when the weather rapidly shifts from being cloudy and cool for many days to sunny and warm. The shifting of branches during the season due to heavy fruit loads can suddenly expose fruit to the sun as can harvesting fruit into field bins which are left in the sun. A gradual increase in temperature and solar radiation could allow the fruit to adapt whereas a sudden change does not allow for adaptation and often results in injury.