Two Types of Sunburn in Apple Caused by High Fruit Surface (Peel) Temperature

Sunburn of apple costs growers millions of dollars in economic losses annually. We have identified two types of sunburn in apple and their causes. The first (sunburn necrosis) is caused by thermal death of epidermal and subepidermal cells (peel), and causes a necrotic spot on the sun-exposed side of the fruit. Thermal death occurs at 126 ± 2°F (52 ± 1°C). Electrolyte leakage increases significantly with necrosis, indicating that membrane integrity is lost during thermal death. The second type (sunburn browning) is sublethal and results in a yellow, bronze, or brown spot on the sun-exposed side of the fruit. Sunburn browning occurs at a fruit surface temperature of 115 to 120°F (46 to 49°C) and has little effect on membrane integrity, but sunlight is required for sunburn browning. Accepted for publication 24 September 2001. Published 4 October 2001.