"Sunburn" is the name applied to a golden or bronze skin discoloration of apples (top photo) caused by exposure of one side of the fruits to intense sunlight on the tree. It detracts from the appearance of the fruits, but normally the skin is not killed, and the tissues show no sign of breakdown. It fact, the flesh in the sunburned area may be firmer than that of the rest of the fruit at harvest, but it tends to soften rapidly in storage.
True sunscald occurs when an apple that has been shaded is suddenly exposed to the sun. The injury appears as white or tan spots, but in severe cases the skin and flesh present the appearance of having been held in a flame (second photo). The injury is most common on apples on the south side of a tree, or on fruits that have fallen on the ground. The disorder is seldom found on the market.
Occasionally in storage, apples are found with brown, somewhat shriveled and sunken areas (third photo) that were not in evidence when the fruits were stored. This condition, known as delayed sunscald, affords entrance points for decay organisms such as the fungus causing alternaria rot.
Sunburn with secondary infection, Ozark Gold
Sunburn, Golden Delicious
Sunburn with secondary infection, Elstar