Leaf scorch is a mid-summer physiological disorder common in our state. Most often it represents the reaction of a plant to an unfavorable environment. Though there might be multiple reasons for this condition to develop, it frequently is the result of an inadequate water supply. Cool temperatures and abundant rainfall in spring and early summer followed by hot, dry weather sets the stage for this disorder to show itself, especially on newly planted trees and shrubs.
After being taken up by the roots, water movement within a plant occurs in vascular tissue called 'xylem.' Anything that disrupts water uptake and/or movement can result in leaf scorch. In short, the disorder develops when water is lost from the leaves of a plant faster than it can be replaced. As a result, leaf cells dehydrate and die in areas of the leaf where water becomes depleted. This usually occurs first on the outer edges and spreads inward toward the midrib. The leaves may then either partially or completely dry up and turn brown.
Plants with rapidly growing shoots may express the same problem which results in the death of an entire shoot or branch. This type of "branch dieback," however, should not be confused with dieback caused by diseases or insects. When dieback is caused by either of the latter, additional and more specific evidence usually is present that helps to identify the offending pathogen or insect pest. For example, dieback caused by the feeding of beetle larvae such as the emerald ash borer or Asian longhorned beetle usually is accompanied by "exit holes" in tree trunk bark.
The first apparent symptom of scorch is a slight yellowing of the leaves, especially along their edges or between their veins. As the problem becomes more severe, the yellow areas die and turn brown. Gradually, the leaf may develop more and more dead, brown tissue until the leaves totally dies while still attached to the tree. The area of brown tissue may increase more rapidly when the plant in question is exposed to prolonged hot and dry conditions.
When plants suddenly wilt and leaves turn brown totally and abruptly without dropping from the tree, most often the root system of the tree has died. Newly transplanted trees and shrubs are most likely to experience this problem, since their root system is often reduced during transplanting. The likelihood for leaf scorch is increased with improper transplanting procedures. These include digging a hole too small for the root ball, leaving the wire cage or other constricting material around the root ball, planting the root ball too deep, and improper watering following transplanting. Such practices retard the development of new roots soon after transplanting, resulting in scorch with the arrival of hot or dry weather.
Other conditions also can lead to leaf scorch. Very wet weather followed by drought can be particularly damaging. Excess water leads to a lack of oxygen in the root zone which results in root suffocation. This occurs deep in the soil, while very dry weather leads to the death of roots near the soil surface. When root loss of this type is compounded with damage done to the plant by diseases, insects or people, scorch and dieback often appears very suddenly and dramatically.
Prevention is the best cure for leaf scorch and branch dieback. Although no one can control the weather, there are several things that can be done to help overcome the stress of weather extremes. Start with proper fertilization, since it will improve the overall vigor and health of plants. It is important to choose the right fertilizer analysis, the proper timing for application and the amount applied. Fertilizers are salts, excessive fertilization is detrimental since excessive salinity in the root zone will pull moisture away from the tissues, leaving roots unable to function properly. Additionally, protecting plants from insect and disease pests will help reduce stress.
Proper watering is key in preventing leaf scorch. Ironically, too little or too much water both can cause scorch symptoms to occur. In the case of too much water (as mentioned above), excessive moisture will force oxygen from the soil pores that normally hold it, creating anaerobic conditions. The latter is not conducive to new root development and can lead to a toxic root environment. As a general rule, less frequent but thorough watering will help induce the development of a deep root system better able handle occasional drought events.
Tree trunks also are subject to injury mechanical in origin or from diseases that can interfere with water and nutrient movement throughout the plant. Check for loose bark or slightly depressed, darkened areas. Borers or other insects in tree trunks or limbs may also lead to abnormal leaf drop, branch dieback or leaf scorch. Look for small holes with sawdust-like material coming from them. If damage is not too advanced, appropriate pesticides may help to stop the work of insects or diseases and reduce further injury.
Normally, increasing the vigor of a tree or shrub can help to stimulate good growth and repair damage that has been done. Watering during dry periods and fertilizing in fall or early spring may help trees and shrubs survive and speed the healing process. When a tree or shrub loses all of its leaves in midsummer, there is little hope for survival. This especially is true when leaves dry without dropping from the plant and continue to hang on until branches become dead and brittle.
If plants need to be replaced, give careful consideration to the location. Poor locations are often responsible for poor root development which, in turn, can increase the likelihood of leaf scorch. Shallow, rocky soils are a perpetual problem and careful watering each year may be the only means for maintaining large plants through drought periods. Tight soils, which usually are accompanied by poor drainage, also limit the development of roots that may lead to future problems. Some plants are better able to endure stress as young plants but have increasing difficulty absorbing sufficient amounts of water as they become larger. Therefore, scorch and dieback may appear on old established plants as well as those that have been newly planted.
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REVISED: August 5, 2021