Found more frequently on fruit on the southwest quadrant of the tree. Heavy crops that cause branches to bend over mid-season can increase sunburn incidence as a sudden exposure to heat and sun promotes sunburn development. Water stress can increase the incidence of sunburn. Granny Smith and other light skinned apple varieties are more sensitive to sunburn. Also, sensitivity may be associated with low calcium concentration in the fruit.
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.