Horticulture News: Leaf scorch a plague on Kentucky trees

By Kim Cowherd Published:

Bacterial leaf scorch has devastated many landscape and shade trees in Kentucky’s urban forests in recent years. Especially hard hit have been the mature pin oaks lining many urban streets. First diagnosed in the U.S. in the early 1980s, this epidemic shows no signs of abating. Dr. John Hartman, University of Kentucky, Extension Plant Pathologist, discusses this tree disease.

SYMPTOMS

Bacterial leaf scorch is a chronic, eventually fatal disease that will become most noticeable soon, in the early fall. Symptoms include premature leaf browning, leaf death along the margins, and defoliation. Infected trees leaf-out normally the following year, however leaves on a few more branches turn prematurely brown in late summer. These events repeat themselves over a period of several years until the entire tree turns prematurely brown.

Trees gradually decline over the years as twigs, branches, and limbs die from continual defoliation. Because symptoms of this disease can sometimes be confused with other environmental, stress-related problems, it is advisable to have the diagnosis confirmed with a special laboratory test.

Leaf tissue can be tested for the presence of the bacterium at the University of Kentucky Plant Disease Diagnostic Laboratory. Bring samples of suspect leaves, along with a 6” portion of the limb attached, in a plastic bag to the Franklin County Extension Office. This service is free of charge.

CAUSE AND SPREAD

Bacterial leaf scorch is caused by the bacterium Xylella fastidiosa. This bacterium is spread by leafhoppers and treehopper insects, although it does not appear to be spread from tree to tree very rapidly. Nevertheless, in some neighborhoods where the disease has been present for many years, a high proportion of mature oaks may show symptoms of bacterial leaf scorch.

There is some evidence that X. fastidiosa is present in symptomless shrubs, grasses and weeds in the landscape. Thus, leafhoppers may not necessarily pass the disease from tree to tree, but may be acquiring the bacterium from other hosts.

The pathogen infects the xylem (interior vascular system) of the tree, where it partially blocks the flow of water to the leaves, resulting in leaf scorch symptoms. Symptoms are often associated with various moisture and heat stresses occurring that season.

HOST RANGE AND LOCATION

Bacterial leaf scorch is found throughout much of the Eastern and Southern U.S. In Kentucky, it is present in landscape trees in many urban areas, including Frankfort. This disease has not been detected in Kentucky’s forest trees.

Bacterial leaf scorch has been commonly observed in oaks, especially pin oak and red oak, and in sycamore in Kentucky. It is also occasionally found here in red maple, sugar maple, silver maple, London plane, hackberry, mulberry, elm, and sweetgum.

DISEASE MANAGEMENT

There is no cure for bacterial leaf scorch, so one should expect diseased trees to be gradually lost over the years. Because infected trees decline gradually, it may be 5 to 10 years before there are many dead limbs and branches present. In the meantime, tree owners can provide good growing conditions for the trees to prolong their survival and to enhance their aesthetic value.

Here are some ways that homeowners can keep trees healthy.

Pruning: Newly infected trees can be made to look somewhat presentable for a few more years if the dead wood is pruned out.

Watering: Kentucky summer weather can sometimes be hot and accentuated by periods of drought lasting anywhere from a week to several months. Apply deep, slow, supplemental water during the hot, dry parts of the growing season.

Tree–injections: Injections, done by licensed, certified arborists, may provide temporary remission of the symptoms. These treatments do not cure infected trees, but they may prolong the life of the tree. This should be done by licensed, certified arborists.

Tree replacement: The best remedy for bacterial leaf scorch is tree replacement. Begin planting non-susceptible replacement trees early so that they will attain a reasonable size before the diseased ones are removed.

To maintain species diversity and to prevent catastrophic tree losses because of diseases or insects, avoid planting all the same tree species in a neighborhood, or a community. Select trees that do well in Kentucky.

For more information on bacterial leaf scorch and tree health, contact the Franklin County Cooperative Extension Service at 695-9035 or email DL_CES_Franklin@email.UKy.edu.

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