Symptoms and Signs
Bark, fruit, and leaves exposed to direct sunlight are injured by heating and drying of tissue. Damage typically is most severe on the south and southwest sides of trees. Sunburn initially causes a pale yellowish area on the exposed side of fruit. The center of discoloration may turn black, brown, or red, then necrotic or withered. Sunburned leaves develop chlorotic then necrotic blotches, which initially form between veins. Sunburned twigs become cracked, discolored, purplish, or roughened on their exposed (usually upper) side. When severe, sunburned trunk and limb bark and the cambium underneath can discolor and die, causing cankers that can girdle and possibly kill limbs.
Comments on the Disease
Sunburn, sometimes called sunscald, typically occurs when trees defoliate, exposing fruit or previously shaded bark. Newly planted trees that grew with bark shaded in the nursery, and trees that are unable to take up enough water because of unhealthy roots or inappropriate irrigation, are highly susceptible to sunburn.
Prevent sunburn by providing trees with good growing conditions and proper cultural care, especially appropriate amount and frequency of irrigation. Where feasible, prevent conditions that cause foliage to drop prematurely, including Phytophthora root rot and high persea mite numbers. If trees defoliate, do not irrigate until soil in the root zone approaches dryness. Defoliation reduces tree use of water, so soil will remain wet longer than with unaffected trees. Examine soil carefully and frequently and modify irrigation to prevent excess moisture in the root zone.
Whitewash young trees routinely at planting. Whitewash the trunk and major limbs of older trees if they develop sparse canopies or are severely pruned, such as when cut back to trunks and grafted with new scion (stumped). Special whitewash products are available, or white interior latex paint diluted 50% with water can be applied. An inexpensive whitewash formula is 50 lbs hydrated lime and 4 lbs zinc sulfate to each 100 gallons of water. Certain white film kaolin clay particle products can be sprayed onto foliage to reduce sunburn and tree heat stress, apparently without interfering with leaf photosynthesis.