Leaf scorch is a non-infectious, physiological condition caused by unfavorable environmental situations. It is not caused by fungus, bacteria, or virus. The problem may appear on almost any plant if weather conditions are favorable, such as high temperatures, dry winds, and low soil moisture. When large amounts of water evaporate from leaf surfaces, the plant roots are unable to furnish enough water to compensate for the transpirational loss. Leaf tissue dies as a result. Young trees or those that are already in stress due to insect infestations, diseases, or other factors are more susceptible than those growing vigorously and in good condition. Plants that are prone to leaf scorch include Japanese maple, Norway maple, sugar maple, beech, ash, oak, linden, birch, alpine currant, horse chestnut, white pine, rhododendron, viburnum, and flowering dogwood.
Scorch symptoms may differ between plant species, but it typically appears in July and August as a yellowing between leaf veins and along leaf margins, and a browning on the tips of leaves. Since these leaf parts are the last to be supplied with water from the roots, they are usually the first to be affected. Browning of dead tissue often appears without any previous yellowing, extending into the leaf between the veins. Entire leaves may curl and wither when leaf scorch is severe. Scorched leaves are usually abundant on the side of the plant most exposed to prevailing winds and strong sunlight. Leaves on the same branch often show similar symptoms but an entire plant may not be uniformly affected.
On narrowed-leaved evergreens, such as arborvitae, hemlock, fir, pine, spruce, and yew, scorch injury begins from the needle tip progressing inward. When severe, half or all of the needle may turn brown. Scorch injury on evergreens may occur in winter from drying winds when soil is still frozen, as well as during warm, dry summers.
Scorch is a condition and not a cause. Symptoms occur when one or more adverse factors are affecting the plant. In some cases, it is simply a sign that a particular plant is not suited to its exposure or the site it is growing in. Prolonged high temperatures, hot, drying winds, and low rainfall are the most common reasons for leaf scorch. Less obvious causes for scorch include damaged roots, such as from construction or recent transplanting, soil compaction, restricted root space, poor drainage, girdling roots, nutrient deficiency, and high concentrations of de-icing salt, fertilizer, or chemicals. Disease or insect damage to a plant’s root system may cause an imbalance of water between leaves and roots. Wilt diseases, such as verticillium wilt, affects the water conducting vessels in the plant, which sometimes creates conditions for scorch.
Scorch damage alone is insufficient to kill an otherwise healthy plant. Proper treatment depends upon the reason for scorch symptoms; however, good cultural practices that improve general plant health and promote good root growth will reduce the chances of leaf scorch.
- When dry weather conditions occur over an extended period of time, plants should receive deep supplemental watering every 10 to 14 days. Newly transplanted trees and shrubs should be watered every 7 to 10 days. A slow soaking of the soil is most effective.
- Conserve soil moisture by mulching plants with a 3-4" depth of organic mulch, such as woodchips, leaf mold, or bark. Because mulches absorb water from the surface, be sure to water thoroughly so water penetrates into the soil.
- Apply fetilizers in early spring or late fall, after leaf drop, to minimize the potential of root injury. Always water in well. Avoid applying fertilizer during the summer when soil is drier.
- Keep lawn fetilizers outside of the dripline of trees and shrubs.
- Prune any dead, diseased, or crossing branches to reduce the amount of foliage the root system must support.
- If the cause of leaf scorch is chemical injury, recovery in some cases may be minimal. If de-icing salt or fertilizer burn are suspected, leaching the soil with a slow trickle of water for 24 hours may help in recovery.