Sunscald

Sunscald is a physiological disorder, not a pathogen-caused disease. It occurs when fruit or bark is exposed to intense direct solar radiation without sufficient foliage cover or prior acclimation. The mechanism differs by plant part: on fruit (tomatoes, peppers, apples), rapid surface heating denatures the outer cell layers, producing pale tan to white papery patches that soon collapse. On young tree trunks, south- and southwest-facing bark absorbs heat during clear winter days, then refreezes overnight when temperatures drop sharply, rupturing the cambium in a pattern distinct from frost crack.

Conditions that favor sunscald are predictable: sudden defoliation from disease, heavy pruning, or pest damage; heat waves pushing ambient temperatures above 95°F; and low humidity that accelerates tissue desiccation. Container-grown transplants moved abruptly from shade into full sun are especially vulnerable in the first week. In apple orchards, trunk sunscald is most common on trees less than four years old, whose bark has not yet developed the thickness to buffer large diurnal temperature swings.

Management centers on preserving the canopy that already functions as a shade buffer. On vegetables, keeping foliage healthy through late summer is more cost-effective than any reactive treatment once damage appears. Where heat waves are forecast, 30-50% shade cloth deployed before temperatures peak prevents most fruit damage. For young fruit trees, trunk whitewashing with diluted white latex paint (roughly 1:1 paint to water) applied in late fall is a well-documented preventive. Affected fruit should be removed promptly: the collapsed tissue is a reliable entry point for secondary bacterial and fungal rots that spread to adjacent healthy fruit.