Q: Numerous oak trees in our area have begun displaying scorch symptoms. How do we determine if the symptoms are due to bacterial leaf scorch versus other diseases or abiotic problems? If it is bacterial leaf scorch what can be done at this time of year for our clients ’trees?
— Todd Mastrobuoni, Integrated Arbor Solutions LLC, Fair Lawn, N.J.
October 2006 (left) and October 2007 (right). Before and after pictures of an oak tree treated with oxytetracycline hydrochloride. Treatments were applied in spring of 2007.
Photo by Dr. John Hartman, University of Kentucky.A: Leaf “scorch” is a general term for symptoms that can be caused by a variety of different problems that interfere with or limit the flow of water to the leaves of a tree. Symptoms of leaf scorch consist of browning or discoloration beginning at the leaf margin and continuing to the inner parts of the leaf. Scorch may be caused by a non-living (abiotic) problem such as drought, nutritional deficiency, girdling roots, damaged or limited roots, salt injury, and mechanical injuries to the trunk. Scorch can also be caused by living (biotic) causal agents, primarily vascular wilt pathogens such as Xylella fastidiosa, which causes bacterial leaf scorch (BLS). Given the general nature of scorch symptoms and the numerous potential causal agents, correct diagnosis of the problem can be challenging. In addition, symptoms may be a result of multiple problems within an individual tree. To determine if the scorch symptoms are caused by BLS, it is important to distinguish symptoms caused by abiotic problems versus the symptoms of BLS.
The first step in diagnosing BLS is to determine if the tree showing symptoms is commonly infected by BLS and if the disease is common in your area. To date, BLS has been reported as far north on the Eastern Seaboard as New York and is prevalent in the Southeast, Texas, extends northward to Illinois and has been detected in California. Frequently infected trees include numerous oak species, elms, sycamores and maples. In California, the disease can be found commonly on Liquidambar species and ornamental olive.
Symptoms of BLS may vary slightly between tree species, but there are some common traits to look for regardless of the tree. Trees infected with BLS will typically develop normal leaves each spring. Symptoms begin to develop in mid-summer and intensify throughout the growing season, peaking in August and September each year. Leaves of BLS-infected trees will brown along the leaf margin and spread toward the veins and petiole in an irregular pattern. Often, green healthy leaf tissue is separated from brown discolored tissue by a yellowish-brown or reddish band. The onset of symptoms in a given year and the severity of symptoms can increase on water-stressed trees or during times of drought. Premature leaf drop will occur in late summer in a non-uniform pattern in localized areas of the crown, and will reappear in the same area of the crown and spread to new areas in subsequent seasons, causing dieback and decline over multiple years. In the landscape, multiple BLS-infected trees can be noted in close proximity; however, each tree will display varying levels of symptom severity.
In contrast, scorch symptoms caused by abiotic problems are typically more uniformly distributed throughout the canopy of the tree. Symptoms can be more severe in areas of the canopy that are exposed to high temperatures and more wind. In some cases, symptoms may only be found on the side of the tree that received mechanical damage or root damage. Similar patterns of uniform symptoms may be noted on multiple trees growing under the same conditions within a site. Abiotic problems generally arise suddenly and in some cases do not progress in severity as compared to symptoms caused by pathogens or insects.
Bacterial leaf scorch is a frustrating problem to manage because there is no known cure. Infected trees die prematurely, and their appearance deteriorates over their lifespan. A variety of management practices are aimed at extending the longevity of infected trees. These include treatment with antibiotics and water stress reduction through mulching, irrigation and growth regulation.
Tree injection treatments with the antibiotic oxytetracycline hydrochloride work directly on the pathogen, resulting in suppressed symptom development and reduced premature leaf drop. Treating trees in the earliest stages of infection has shown more promise than treating trees in more advanced stages of decline. Treatments should be applied after full leaf development in the spring and can continue until early July. Treating past early July will reduce the effectiveness of the treatment. Although the antibiotic tree injection treatments are effective in reducing annual symptoms, they do not cure the tree of BLS and have to be repeated annually. Additional practices should be combined with the tree injection treatments to increase the likelihood for success.
As mentioned earlier, drought plays a role in the severity of BLS, so practices aimed at improving tree health such as supplemental irrigation, mulching and correcting site conditions should be considered as part of the management strategy for BLS. The use of growth regulators to aid in the tree’s ability to withstand drought may also provide a benefit to the tree. Managing opportunistic secondary pests, such as wood borers, is also important to alleviate additional stress on the tree. Although BLS on shade trees is vectored by a variety of different leafhoppers and treehoppers, it is not known whether or not controlling these insects with soil-applied or trunk-injected insecticides will provide additional protection against BLS on an individual tree.
It is important to set reasonable expectations and make sure that the client understands that it will take a multi-year approach to managing BLS. Late summer/early fall is an excellent time of the year to diagnose symptoms and discuss management options with your clients.
Shawn Bernick is director of research and technical support at Rainbow Treecare Scientific Advancements, Minnetonka, Minn. He will be answering one tree health care question in each issue of Arbor Age throughout 2010. To submit your question for consideration, please e-mail Arbor Age editor John Kmitta at firstname.lastname@example.org Be sure to indicate that the question is for the tree health care Q&A, and include your name and contact information.