Leaf Scorch, a noninfectious, environmental condition, occurs when young trees experience prolonged periods of dry, windy weather, low rainfall and high temperatures with bright sunshine when the trees roots are unable to supply water to the foliage as rapidly as it is lost by transpiration from the leaves. Plants draw in water from the soil, use it for plant growth, and then release it in the form of a vapour, from stems and leaves through a process called transpiration. Trees begin to wilt and suffer drought stress when the transpiration rate exceeds water take-up.

Trees planted within the past two years are particularly susceptible to leaf scorch as their roots have not regenerated sufficiently to supply the moisture demands of the foliage. They are still getting established in their new environment.

Symptoms

Leaf scorch typically  appears in July and August as a yellowing between leaf veins and along margins and a browning of the leaf tips.  Since these 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 area between the veins. Entire leaves may curl and wither when leaf scorch is severe. Scorched leaves are more evident on the sides of the trees most exposed to prevailing winds and strong sunlight.  Leaves on the same branch often show similar symptoms but an entire tree may not be uniformly affected. Premature dropping of leaves and twig dieback may occur during late summer.

Leaf Scorch can reduce the health of the tree making it more susceptible to disease and insect attack.

Example Photos – All trees were watered every 5 – 7 days with 45 litres or more per tree per watering since planting. Trees were exposed to 32 days without any measurable rainfall, 2 hot spells, each with several days with temperatures above 32 degrees Celsius and warm, drying winds. (Click photos to enlarge.)

What to Do

While you cannot control the weather and there is no “cure” once the damage has been done, there are steps that can minimize the effects of leaf scorch. Good cultural practices that improve general tree health and promote good root growth will reduce the risk and severity of leaf scorch. Frequent watering during periods of drought help to maintain growth and tree vigor. Growth produces new leaves that offsets the loss of the damaged leaves.

• During periods of dry weather, newly planted trees should be watered every 7 – 10 days. Water each tree slowly and watch to see that the water is soaking into the soil, not just running off the surface. Let the water soak down to where the roots are. Water as needed, frequency will increase during dry conditions. The amount of water needed will vary depending on the type of soil that exists, where the tree is planted and the weather conditions.
• When dry weather conditions persist over an extended period of time, trees should receive supplemental, deep watering.
• Conserve soil moisture with an 8 – 10 cm depth of organic mulch around the base of the tree. (woodchips, shredded bark or leaf mold). Keep the mulch back 15 – 20 cm from the trunk. A mulch also helps to regulate soil surface temperatures. Mulches absorb water from the surface, so be sure to water thoroughly to get the water down to where the roots are. Mulches help to retain moisture during the summer and fall droughts. Mulches should be installed at the time of tree planting.
• Do not fertilize drought stressed trees. Chemically, fertilizers are salts and will pull water from the roots, further dehydrating them. Nitrogen fertilizer can encourage canopy (top) growth that the root system cannot support during periods of dry, hot weather.

References: 

• The Morris Arboretum and Plant Clinic 3/7/02
• Common Diseases of Maples, Dr. Sharon M. Douglas
  Dept. of Plant Pathology and Ecology
  Connecticut Agricultural and Experimental Station
  New Haven, CT

Photos by Maple Leaves Forever
Written by Carl Mansfield, Arboreal Consultant, Maple Leaves Forever
Setup and Editing by Mary Bella, Webmaster & Communications, Maple Leaves Forever