The Fruit Finish section of the card set includes sun and heat-induced disorders found in Washington apples. This includes sunburn browning, sunburn necrosis and photo-oxidative sunburn (previously referred to as Type III sunburn). The upcoming revision of the Defects & Disorders Guide will include any emerging or reclassified disorders. To navigate directly to the pest damage of interest use the links below this text block. The card images shown on this site are slightly modified from their original form to accommodate web formatting. Any reproduction of the card images or content without permission is in violation of WSU Copyright policies.
Comparison of sunburn symptoms on Gala
Sunburn Browning is the most common form of sunburn. The symptoms appear as a yellow, brown or dark tan spot on the sun exposed side of the apple. This disorder is induced by ultraviolet-B radiation from sun light along with high fruit surface temperature. The sunburned area may contain desiccated lenticels depending on severity.
Sunburn Necrosis is heat-induced while the fruit is on the tree. Like Sunburn Browning it occurs on the sun exposed part of the fruit, but UVB is not a major factor. When the fruit surface temperature exceeds 126 degrees F for as little as 10 minutes, skin cell death occurs resulting in a dark brown or black (necrotic) spot that appears later. Cracking of the affected area is common.
Apples that are suddenly exposed to sunlight after growing in shade, as would occur with hand thinning or late season pruning, may develop Photo-oxidative Sunburn. Unlike the previously mentioned sunburn forms, this is neither surface temperature, nor UV-B dependant. In addition to happening on the tree, it can also occur after harvest if apple bins are left sitting out in the sun in the field, on a truck, or at the loading dock. Appearance of Photo-oxidative Sunburn varies by severity. In general, it appears first as whitened peel (i.e., photo-bleaching) with browning appearing later.