Scorch, Sunburn, and Heat Stress
Click for larger image Leaf scorch on Japanese maple leaves (Acer palmatum)

Leaf scorch may occur on any species of tree or shrub as well as herbaceous plants. It is a widespread noninfectious disease or disorder. Scorch most often occurs following prolonged periods of dry, windy weather or bright sunshine when the roots are unable to supply water to the foliage as rapidly as it is lost by transpiration from the leaves. Unfavorable locations, such as sandy or gravelly soil, near obstructions or pavement that restrict root growth, or exposed windy slopes usually promote scorch. Anything that affects the plant’s ability to take up water, including insect and disease problems, can result in leaf scorch. Herbicides and pesticides may also contribute to scorch. Do not spray on windy days to eliminate drift problems and do not allow mist to settle onto trees.

In mild cases of leaf scorch, the leaves remain attached, and little damage results. In more severe cases, plants may drop many of their leaves prematurely, although such plants do not die. Where leaf scorch occurs each year, such annual stress will gradually weaken the plant, making it more susceptible to insects and diseases.

Symptoms and Diagnosis

Browning of leaf margins and/or yellowing or darkening of the areas between the main leaf veins are symptoms of leaf scorch. Due to environmental causes, leaves may dry, turn brown, and become brittle. Look for damage to trees and shrubs on the upper portion on the sunny, southern side and on the windy side. Premature dropping of leaves and twig dieback may occur during the late summer. Symptoms usually appear after drying winds in conjunction with periods of hot, dry weather.

Leaf scorch on narrow leaf evergreens appears as brown or purple brown discoloration of the needle tips. If unfavorable conditions become more severe, browning of needles increases. This should not be confused with the browning and shedding of older interior needles. Scorch may result from hot, dry weather in summer or from strong, dry winter winds when the ground is frozen. Symptoms may not become apparent for a month or more after the initial injury.

Winter leaf scorch in evergreen plants usually appears as two long, brown areas paralleling the main leaf vein.

Life Cycle

Although plants can experience scorch with no insects or disease pathogens involved, insect and disease damage can also affect the plant’s uptake of water. In some cases, insect damage such as leafhopper (hopper burn) or specific scorch diseases caused by fungi or bacteria can produce similar symptoms. Scorch can also reduce the health of a plant making it more susceptible to attack by insects and diseases.

Integrated Pest Management Strategies

1.Water when needed and maintain plant vigor. Plant in a fertile, well-drained soil at the same depth the plants grew in at the nursery and with an adequate supply of organic matter. Soil should be moist to a depth of 10 to 12 inches after a thorough watering. This should be done about once weekly. It is very important for the trees, especially broadleaf and needled evergreens, to be well watered going into the winter period. Water during this period if seasonal rains are not adequate or during the winter when the soil is not frozen. Annuals and perennials require more frequent watering than most trees and shrubs.

2. Fertilize. Fertilize plants in early spring based on a soil test and the directions printed on the fertilizer container. Nitrogen should be applied annually based on the area to be fertilized, the type of plant, and the diameter of the trunk. Do not fertilize with nitrogen in the late fall as it may cause new soft growth easily damaged by cold weather.

3. Mulch. Organic mulches (pine needles, wood chips, composted leaves, pine bark, cypress mulch) can help retain moisture during the summer and fall droughts. In the winter this mulch prevents the alternate freezing and thawing of the soil. Mulch will also cool the soil in summer.

4. Screens. Screens may be used to protect trees and plants in areas exposed to wind and sun, but it is best not to plant tender plants in these exposures.

5. Exposure. Other factors which can result in scorch are excessive fertilizer, deicing salt, herbicide, dog urine, trash fires, leaking sewer or gas mains, girdling roots or strangling wires, vehicle exhaust, and heat reflected from buildings. Elimination of the exposure to these elements is the only correction.

6. Physical hazards. Avoid root injury when digging near trees and shrubs. Care should be taken to not injure the bark and roots when using lawnmowers, weed whips, and edging tools.

Organic Strategies

Strategies 1, 3, 4, 5 and 6 are strictly organic approaches. Using an appropriate organic fertilizer would be a viable organic approach to Strategy 2.

More images:

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Sunscald or sunburn on watermelon (Citrullus)
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This Japanese maple (Acer palmatum) may look variegated from a distance, but it's actually suffering from leaf scorch probably due to its position in the landscape: a southern exposure and surrounded by a brick wall and pathway
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Leaf scorch just beginning on Japanese maple leaves (Acer palmatum)
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Leaf scorch on Japanese maple leaves (Acer palmatum)
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Sunburn on winged euonymus (Euonymus alatus); note distinct line between sunburned tissue and normal tissue protected by shadow of upper leaves
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Sunburned patches on jade plant (Crassula)
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Close-up of sunburned patch on jade plant leaf (Crassula)
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Heat stress on rhododendron (Rhododendron); note that spots are only on upper leaf surface and distinct edges between affected and healthy tissue
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Heat stress on rhododendron (Rhododendron); note that spots are only on upper leaf surface and distinct edges between affected and healthy tissue
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Scorch on Bradford pear leaves (Pyrus calleryana 'Bradford')
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Scorch on maple (Acer) leaves
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Scorch on dogwood (Cornus) leaves
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Sunburn on African violet leaf (Saintpaulia); distortion was caused by cyclamen mites (Acari)
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Close-up of scorched leaf of fragrant snowbell (Styrax obassia) that died of stem cankers
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Ragged hosta leaves caused by sunburn
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Many varieties of hostas get burned if planted in too much sun and suffer from heat stress when planted too close to hardscaping, such as, sidewalks.
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Sunburn on variegated hosta
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Leaf scorch on sugar maple (Acer saccharum) caused by egg-laying of 13-year cicada (Hemiptera)
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Dead areas in a zoysia grass lawn probably due to a combination of the extreme heat and drought in the summer of 2012; areas next to hardscaping were particularly vulnerable due to reflected heat.
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Sunburn on an orchid leaf
Pests and Problems

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