By: Bonnie L. Grant, Certified Urban Agriculturist
Your shade tree may be in peril. Landscape trees of many types, but most frequently pin oaks, are getting bacterial leaf scorch disease by the droves. It was first noticed in the 1980s and has become a rampant enemy of deciduous trees across the nation. What is bacterial leaf scorch? The disease is caused by a bacterium that interrupts the flow of water in the vascular system of the tree with often dire consequences. Read on to learn more.
Shade trees are prized for their regal dimensions and comely leaf displays. Bacterial leaf scorch disease seriously threatens not only these trees’ beauty but also their health. The symptoms may be slow to notice at first, but once the disease takes fire, the tree is often close to death. There is no treatment or bacterial leaf scorch control for this disease, but there are some cultural steps that can be made to ensure a beautiful tree for the last few years of its life.
Bacterial leaf scorch is caused by Xylella fastidiosa, a bacterium that is spreading across the eastern and southern United States. The first signs are necrotic leaves with browning and finally leaf drop.
Leaf scorch starts at the edges or margins of the leaf and produce browned edges while the center remains green. There is often a yellow band of tissue between the brown edges and green center. The visual symptoms differ from species to species. Pin oaks exhibit no discoloring, but leaf drop does occur. On some oak species, the leaves will brown but will not drop.
The only true test is a laboratory test to rule out other diseases and cultural causes of marginal browning.
There are no chemicals or cultural methods for treating bacterial leaf scorch. Expert recommendations on how to treat bacterial leaf scorch are just panaceas at best. Basically, if you baby your tree, you can get a few good years out of it before it succumbs.
Death occurs in 5 to 10 years in most plants. Applying supplemental water, fertilizing in spring and preventing weeds and competitive plants from growing in the root zone will help but cannot cure the plant. Stressed plants seem to die more quickly, so it is advisable to watch for other disease or pest issues and combat them immediately.
If you wish to try to keep the tree longer or removal is impossible, use good cultural methods to improve the tree’s health. Prune off dead branches and twigs.
You may also wish to enlist the aid of an arborist. These professionals can provide an injection containing oxytetracyclen, an antibiotic used in treating leaf scorch. The antibiotic is injected into the root flare at the base of the tree and must be repeated annually to add a few years to the tree. The injection is not a cure but simply a method of treating bacterial leaf scorch and enhancing the tree’s health for a period of time.
Sadly, the only real way to effectively combat the disease is to choose resistant tree species and remove infected plants.
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Steve Nix is a member of the Society of American Foresters and a former forest resources analyst for the state of Alabama.
Leaf scorch is a noninfectious condition caused by an unfavorable environment - there is no virus, no fungus, no bacterium to blame. It can not be helped by chemical control so you will have to discover the underlying causal factor which can be drying winds, drought, root damage and other environmental problems.
Still, infectious diseases can attack the tree and make the condition even worse. Major target trees are Japanese maple (plus several other maple species), dogwood, beech, horse chestnut, ash, oak and linden.
Brown Rot (Monilinia spp.) affects the fruit, flowers and twigs of the plum tree. Flowers will turn brown and dry up, but they do not fall from the tree. Brown spots develop and spread over fruit, followed by lighter colored spores. Where the flowers attach to the twigs, sunken areas and brown sticky goo appear and the leaves at the ends of the twigs shrivel. Prune out diseased branches, and avoid getting the foliage wet when watering to help limit the disease. A copper-containing fungicide can be applied after buds appear, but before the tree blooms.
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.
If BLS sounds like bad news all around, it is. Along with other pests and diseases that have no remedies, Bacterial Leaf Scorch threatens our familiar shade trees and we can’t yet cure it.
We’re here to give you the best information to keep your trees and your garden healthy, as well as provide realistic options for treating pests and diseases.
If you have trees you think might be infected or at risk of developing BLS, contact us for an assessment. We can diagnose the issue and see whether treatment will be helpful. If it isn’t treatable, we can safely remove trees as needed before they become hazards.