Leaf scorch and sunburn on Plants

Symptoms

Sunburn looks different on leaves, needles, fruit and bark. Sunburned broadleaf foliage may look glazed (abnormally shiny), silvery or reddish brown. Some leaves show blotches between the veins that turn yellow first and then die. Sun-exposed leaf, stem or pod surfaces may show water-soaked spots.

Conifer needles turn black or brown, and they may drop early. Fruit can show a pale yellowish discoloration on the side that faces the sun.

Bark on trunks and limbs may discolor, and sometimes it oozes sap. A reddish-brown discoloration can appear on the south and west sides of the trunk and limbs. Bark may also crack and peel in patches, exposing the wood.

Branches can become girdled and start to die. Trunks and limbs can end up cankered or girdled and dead. Damage on the sun-facing side of fruit, or on the south and west sides of a trunk, points toward sunburn. Leaf, needle, fruit and bark symptoms are all listed for sunburn, so the damage is not limited to one plant part.

Causes

Sunburn is usually associated with warm weather, and often with insufficient soil moisture. Exposure to intense sunlight, high temperatures, or both, is another listed cause.

Unhealthy or restricted root systems are a listed cause. Insufficient water uptake from unhealthy roots or inappropriate irrigation is another. Restricted soil volumes, inappropriate soil moisture, or anything else that makes roots unhealthy can all contribute.

Bark can burn when it is suddenly exposed to the sun after pruning or premature leaf drop. Removing shade, or adding heat-reflecting surfaces nearby, can do the same. On rose canes, reflected heat from masonry, vinyl siding or rock mulch causes sunburn.

Winter brings its own cause. Rapid temperature fluctuations in winter can damage bark. Bark may warm in sunlight and then cool rapidly after sunset.

Premature foliage drop can also lead to sunburn when it is caused by Phytophthora root rot or persea mite infestation. Defoliation exposes bark and fruit to sunburn. Pruning, leaf drop and the removal of shade all work by exposing bark that was previously shaded.

Diagnosis

The sources behind this page name no look-alike problems, so the best clue is where the damage sits. Fruit with a pale yellowish discoloration on the side facing the sun fits sunburn. Reddish-brown bark on the south and west sides of the trunk and limbs fits as well. Water-soaked spots on sun-exposed leaf, stem or pod surfaces fit too.

Look at leaves and needles as well. Glazed, silvery or reddish-brown broadleaf foliage, or conifer needles that turn black or brown, fit the symptom list.

Next, ask whether the weather has been warm and the soil moisture low, since sunburn is usually associated with both. Ask whether you pruned heavily, whether leaves dropped early, or whether shade was removed. A new heat-reflecting surface nearby counts as well.

Check the roots and watering too. Unhealthy roots, restricted soil volume or inappropriate irrigation can all leave a plant short of water. In winter, ask whether bark warmed in sunlight and then cooled rapidly after sunset.

Treatments

Most of the listed treatments are cultural, and one is organic. Start with the cultural steps, which change the conditions around the plant.

Plant where roots will have adequate soil volume and sufficient growing space as they mature. Provide good overall cultural care, especially adequate irrigation. Apply and maintain appropriate mulch to conserve soil moisture and reduce soil temperatures during summer.

Avoid pruning off more than about 20% of the plant canopy in any one year, to prevent sudden exposure of previously shaded bark. Retain temporary, lower branches for several years after planting to shade the trunk. Whitewash young or thin-barked trunks with white interior latex paint, not exterior paint.

Choose plants well adapted to the local climate to reduce susceptibility to winter sunscald. During the warm season, remove any trunk wrap or insulation used to prevent winter sunscald, since leaving it on can cause other damage. Control conditions that cause premature foliage drop, such as Phytophthora root rot and persea mite infestations, since defoliation exposes bark and fruit to sunburn.

The one organic option listed is kaolin clay. Spray white film kaolin clay particle products onto foliage to reduce sunburn and tree heat stress. The product label is the legal instruction, and you must follow it for the crop and site being treated. No chemical treatment is listed for this problem.

Prevention

Prevention comes down to the conditions around the plant. Give roots enough soil volume and growing space as they mature. Keep up adequate irrigation, and use mulch to conserve soil moisture and reduce soil temperatures in summer.

Keep bark shaded where you can. Prune off no more than about 20% of the canopy in a year, and retain temporary lower branches for several years after planting to shade the trunk. Young or thin-barked trunks can be whitewashed with white interior latex paint.

Watch for changes that expose bark, such as removing shade or adding heat-reflecting surfaces nearby. Choose plants well adapted to the local climate, and remove trunk wrap in the warm season. Deal with Phytophthora root rot and persea mite infestations, which cause premature foliage drop.

Plants most often affected

Toyon, avocado and rose are the plants named for this problem. On rose, excessive heat from the sun causes sunburn on canes, and so does reflected heat from masonry, vinyl siding or rock mulch. The symptoms section also describes conifer needles.

No plant page in this build records this problem yet.