Diagnosing Sun & Heat Related Injuries in Plants
by Michelle Le Strange, Master Gardener Advisor

Sunburn, Sunscald, High Temperature Injury and High Light Injury: What's the difference?

Sunburn
Sun exposure can generate critically high temperatures in plant tissues, leading to dehydration and death. Sunburn is injury to aboveground plant parts (leaves, bark, flowers, and fruit) caused by excessive exposure to solar radiation. High temperatures are closely linked to sunburn injury. Plants deficient in water have an increased potential for injury, but sunburn can also occur on sensitive species when soil moisture levels are adequate. Unlike sunscald, sunburn is not preceded or followed by freezing temperatures.

Sunburn on leaves looks like reddish or red-brown areas that eventually die. You've probably seen sunburn on camellias and azaleas if their leaves are exposed to afternoon sun, but you can also see sunburn on Indian hawthorn (rhaphiolepis), photinia, euonymous, and other heat tolerant shrubs. New growth is particularly sensitive to sunburn. Sunburned bark initially appears discolored (often reddish brown) and then becomes dry. Cracking and peeling is typical, and damage is usually most severe on the south or southwest sides of branches and trunks. Wood-boring insects and wood decay often follow. Sunburned flowers and fruit start as water-soaked areas on the most exposed surfaces. Eventually flower tissue turns brown and shrivels, while fruit tissue appears rotten.

Sunburn is most common in summer, but can occur at any time of year, even winter. It typically happens on plant parts receiving greatest exposure to the sun, usually south and southwest sides.

Sunscald
Sunscald is winter injury of the bark of limbs and trunks of woody plants. Rapid changes in temperature are believed to cause death of cells just under the bark leading to a separation of bark and wood. Tissues are injured when freezing temperatures precede or follow daytime warming. Factors thought to contribute to sunscald include thin bark, trunk injuries (wounds), root injury, borers, and insufficient plant moisture. Of these, water deficiency has been determined to be a crucial factor contributing to injury in newly planted trees. Sunscald injury is most common on the south and southwest sides of trunks and branches.

Young trees with thin bark and newly planted trees are particularly prone to sunscald. Mild sunscald symptoms include red discoloration of smooth bark. When thick bark trees are exposed to sunscald the bark shrinks, appears sunken, splits, and then peels back in chunky patches, exposing sapwood underneath. Once sunscald injury has occurred, the tree responds by developing callus tissue. The trunk can become quite lumpy and deformed on small diameter tree trunks. To avoid or protect against sunscald orchardists paint tree trunks and limbs white. Sunburn and sunscald symptoms are similar on trees.

Species more sensitive to sunscald include maples, pears, stone fruit, tulip, walnut, and photinia and laurels when grown as standards.

High Temperature Injury
Critically high air temperatures can lead to thermal injury in leaves, flowers, fruit, bark, wood, and roots. When a critical temperature is reached or exceeded for a species, the chemicals inside plant cells are altered, their membranes melt and disrupt, and then they dehydrate and die. High air temperatures can be generated from several sources including fires, heat releases from vents, pipes, or other sources like asphalt paving equipment.

Fire is the most common cause of thermal injury. Typically injury from fire is distinctive and not easily confused with other disorders. Aboveground heat releases in localized areas cause scorching of leaves and dieback of stems closest to the thermal source. Releases of greater magnitude can injure the entire canopy. Usually the onset of symptoms is rapid. Unlike fire injury, stems are not charred.

High Light Injury
High light intensity can injure plant foliage, which is expressed as a foliar chlorosis (bleaching). In sensitive species the chlorophyll is oxidized by the light. Typically high light injury occurs when shade-requiring plants are placed in the sun. Unlike sunburn damage, high light damage can take place when air temperatures are relatively low. The critical level for injury varies with species, acclimatization, and maturity of foliage.

Some common plants that are sensitive to high light conditions include aralia (Fatsia aralia) caladium, cast-iron plant, Japanese aucuba and Kaffir lily (Clivia miniata).

Look for pictures of these disorders and more in "Abiotic Disorders of Landscape Plants: A Diagnostic Guide" recently published by the University of California and available for sale from the Master Gardener Offices.