Sunscald
Plant disorder
Description
Sunscald is noninfectious injury caused when direct sunlight heats plant tissue beyond what it can tolerate. It often follows a sudden increase in exposure, such as moving a container plant outdoors, removing shading leaves, or pruning heavily. Heat and intense light can bleach or kill patches of leaves and the sun-facing sides of fruit. Tomatoes and peppers are especially prone when their fruit loses its leaf cover. Young or thin-barked trees can also develop sunscald on trunks: winter sunlight warms bark during the day, then a rapid temperature drop damages the tissue. The injury remains confined to exposed areas, although damaged fruit may later develop a secondary rot. Susceptibility depends on the plant, its stage of growth, and how gradually it became accustomed to sun. Sunscald does not spread from plant to plant.
Symptoms
- Bleached or scorched patches
- Brown or crispy leaf edges
How to Identify
Look for pale, bleached, tan, or brown patches on surfaces that face the sun. Leaf tissue may become dry and papery; exposed fruit may develop a firm, light-colored patch that later sinks or rots. On trees, injured bark can crack or die, often on the south or southwest side in the Northern Hemisphere. Check for a recent change in exposure or a spell of intense sun or temperature swings. Unlike many fungal diseases, sunscald typically begins on the most exposed surfaces and has no spreading pattern of spots across shaded tissue. Insect damage usually leaves feeding marks or visible pests.
Prevention
Introduce indoor-grown plants and seedlings to outdoor sun gradually over about one to two weeks, increasing their exposure a little each day. Keep watering consistent during hot weather, and mulch soil to reduce moisture loss, but do not leave roots waterlogged. Avoid removing large amounts of healthy foliage that shades fruit; use light, well-timed pruning instead. During extreme heat, provide temporary, ventilated shade, especially for newly planted crops and exposed fruit. Position containers where they receive afternoon shade if the plant tolerates it. For susceptible young trees, protect trunks from winter sun with a suitable tree wrap applied in autumn and removed in spring. Keep wrap loose enough to avoid trapping moisture or constricting the trunk.
Treatment
First, stop further exposure where practical: move a container plant to a less intense position, or provide temporary shade without enclosing it or blocking airflow. Water the root zone when the soil needs it, and add mulch if appropriate; extra water cannot repair burned tissue. Leave healthy foliage in place to shade fruit. Scorched leaf tissue will not turn green again, but the plant may produce healthy new growth. Remove a leaf only if it is mostly dead, and avoid heavy pruning while the plant is stressed. Harvest or remove fruit that has begun to rot so it does not attract pests; sound fruit with a small superficial sunscald patch may still be usable after the damaged portion is cut away. For trees, protect exposed trunks against future injury, but do not peel away attached bark or apply wound sealant. Seek professional advice if trunk damage is extensive. Fungicides and insecticides do not treat sunscald.
Plants Affected
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