Leaf scorch on aspen

During the hot days of summer, many trees, shrubs, flowers and vegetables develop dry brown leaf margins. The tissue between the main veins may also be affected; in severe cases, entire leaves will turn brown or black and die. Some trees, such as aspens, are especially susceptible, but the condition will occur in many species. Evergreens can also suffer; the needle ends turn a light tan or pinkish color, and if the condition persists, needles will turn brown or develop brown bands, and may drop off.

This condition is called leaf scorch. Its basic cause is that the tree or shrub is unable to take up sufficient water to meet its needs under harsh summer weather conditions. Water is taken up by a plant through its fine feeder roots, and is transported through the vascular system to the leaves. When it is unable to take up enough water, the leaf tissue that is farthest from the major veins will dehydrate first.

Most frequently, the problem is a root system inadequate to meet the plant's needs. This may be due to root loss from tilling or construction in the vicinity of the tree, from drought stress, or from a loss of roots due to dehydration during a dry winter. An injury or infection on the trunk or branches severe enough to interfere with the movement of water and nutrients can also cause a scorch problem. Often, scorch is due to over watering, for a plant cannot grow new feeder roots without oxygen. Soggy soils therefore prohibit root development, and scorch is the result. The combination of no water in the winter and too frequent watering in the summer will practically guarantee that leaf scorch will develop.

Related Stress Symptoms

Heat stress on euonymus

Heat Stress may cause results varying from wilt to discolored leaves to severe desiccation and death, depending on the severity of the stress. Plants suffering from heat stress shut down basic growth functions, and cannot draw sufficient water up into their leaves. Thus, this type of stress is considered to be related to drought stress. Heat injury can cause damage to young plants at the soil line, resulting in collapse. Plants on the south or west sides of buildings or fences may show the effects of reflected, concentrated heat, as may plants surrounded by rock mulch. Black plastic mulch can cause the soil to heat up to levels beyond a plant's tolerance, shutting down growth processes and severely damaging roots.

Cladoptosis (or Kladoptosis) is the dropping of leaves or twigs under conditions of stress. Ethylene gas, which is stimulated by drought or other stress conditions, causes a separation layer to form at the base of the leaves or twigs. This is a defense mechanism of the plant, which sheds some leaf tissue in order to preserve the life of the entire plant. The leaves may turn yellow before dropping, mimicking fall conditions. Many deciduous species will drop leaves under stress, but the condition is most commonly seen in aspens, cottonwoods, and willows. Over watering, under watering, excessive heat, or root damage may all cause leaves to drop.

Prevention

Once scorch has occurred, there is no cure. The dehydrated leaves will not turn green again, but with proper water management, the plant should recover.

Prevention of scorch needs to begin with winter watering. A deep soaking once a month, when there is no snow cover, will help prevent root die-back due to dehydration. The roots of mature trees extend outward several times the height of the tree, and this entire area needs water. To water in winter, choose a day when the air temperature is above freezing, and water early enough in the day for the water to soak in before the nighttime freeze. Water that freezes on the surface will cause root suffocation, and make the problem worse.

During the growing season, water deeply and as infrequently as possible. This can be difficult, when trees are located in or near lawns, but a deep and infrequent watering schedule will benefit your lawn as well, encouraging deep root development. Trees typically need less frequent irrigation than lawns do, but the two can coexist. Remember that roots need oxygen, and this means the soil must be allowed to dry out somewhat between watering.

Scorch on blue spruce

Finally, if scorch occurs, resist the urge to continually apply more water. Just keep up with the deep and infrequent schedule. Some years, especially if conditions are especially hot and windy, some scorch is likely. Don't panic and kill off your trees with an excess of kindness by over watering.

References:

Fosket, D. E. 1994. Plant Growth and Development. Academic Press, San Diego, CA.
Gresshoff, P. M., ed. 1993. Plant Responses to the Environment. CRC Press, Boca Raton, FL.
Maloy, O. C. and Murray, T. D., eds. 2001. Encyclopedia of Plant Pathology. John Wiley and Sons, Inc., New York, NY.
Pirone, P. P. 1970. Diseases and Pests of Ornamental Plants. Ronald Press Co., New York, NY.
Sinclair, W. A., Lyon, H. H., and Johnson, W. T. 1987. Diseases of Trees and Shrubs. Cornell University Press, Ithaca, NY.

Reviewed by Curtis E. Swift, Ph.D..
Area Extension Agent (Horticulture)