Landscape - More on Bacterial Leaf Scorch on Oaks

The following is more information on bacterial leaf scorch of oaks which can be seen in Delaware landscapes at this time.

Bacterial leaf scorch on oaks in the red oak group can be found now. Infected trees have leaves with brown leaf margins (like drought damage). Since we have not been in a drought, lack of water is an unlikely cause of the symptoms, but since the bacterial infection restricts water flow, drought symptoms appear. This is a debilitating disease that can take several years to a decade to kill the tree and there is no cure. Symptoms can be reduced or masked with oxytetracycline injections, but trees cannot be cured. Plant hoppers vector the bacteria from symptomless weed hosts and transmit it to oaks during feeding. If you have suspect trees, tests will confirm the presence of the bacteria in leaf petioles. Contact the Plant Diagnostic Clinic at 320-831-1390 or on the web for more information on testing. The web has many articles on BLS-- here is a good one http://www.apsnet.org/online/feature/bls/

Leaf scorch symptoms on red oak. Photo from the Minnesota Department of Natural Resources Archive, Minnesota Department of Natural Resources, Bugwood.org

Information from Bob Mulrooney, Extension Plant Pathologist, UD.

Landscape - Bacterial Leaf Scorch

Hot weather in mid-summer is the time that you often see bacterial leaf scorch on infected trees. The following is more information.

Bacterial leaf scorch (BLS) is a disease with a name that definitely understates the impact on infected trees. It causes far more than leaf scorching. The bacterial pathogen can slowly kill mature trees in 8-10 years.

Here's what you may see. About mid-summer, infected trees appear to have environmental leaf scorch. The same trees appear healthy again the next spring, but scorching returns each summer, becoming progressively worse over 5 or 6 years, thinning the canopy and eventually killing the tree. I have seen the problem on Illinois oak trees, but other trees may also host this disease.

Bacterial leaf scorch is an infectious plant disease caused by a bacterium called Xylella fastidiosa. The pathogen is systemic, living only in the xylem. The most frequent hosts of this disease in the U.S. include elm, oak, sycamore, mulberry, sweetgum, sugar maple, and red maple. At the University of Illinois Plant Clinic we have confirmed BLS on pin, red, shingle, bur, and white oaks. Kentucky pathologists report BLS on pin, red, scarlet, bur, white, willow, and shingle oaks; silver, sugar, and red maples; sweetgum, sycamore, planetree, hackberry, American elm, and red mulberry. Look for scorch symptoms that occur in early summer to midsummer and then intensify in late summer. The scorched leaf edges or tissue between veins may be bordered by a yellow or reddish-brown color, but not in all cases. The symptoms may occur first on one branch or section of branches and slowly spread in the tree from year to year. The pin oaks we have seen infected have general thinning. It is one of those situations that you hope will be better next year but only gets worse.

There is no cure for this disease. Some have tried injections with oxytetracycline, but none have shown more than disease suppression with this antibiotic. Since the pathogen is in the xylem, cleaning pruning tools before moving to another tree is important to reducing spread of the disease. Xylem-feeding leafhoppers and spittlebugs are thought to spread the bacterium in landscape trees. Transmission between trees through root grafts has been reported.

Unlike most other bacteria, Xylella fastidiosa cannot be isolated in the lab. However, it may be confirmed using serological techniques. ELISA (enzyme linked serological assay) testing can be done and is used to help identify the Xylella pathogen. The most reliable test results occur in August and September, possibly because the bacterial population in trees is higher late in the season.

Bacterial leaf scorch on oak. Photo by Randy Cyr, Greentree, Bugwood.org

Information reprinted from an article by Nancy Pataky in the current edition of the Home, Yard, and Garden newsletter from the University of Illinois http://hyg.ipm.illinois.edu/article.php?id=100

Landscape - Bacterial Leaf Scorch in Oaks

The following is a good article on bacterial leaf scorch in oaks and its management using antibiotic injections.

Bacterial leaf scorch, caused by Xylella fastidiosa, affects many landscape trees including oaks (pin, red, scarlet, shingle, and white) maples (Norway, red, silver, and sugar), planes (American sycamore and London plane) sweetgum, hackberry, elm and mulberry. Oaks are widely planted in Delaware and the disease is most common in them. Leaves of infected trees typically show marginal necrosis (scorch) late in the summer followed by premature defoliation. Infected trees re-foliate normally in spring and the process of late summer scorch and premature defoliation is repeated. The disease begins on one or a few branches and over several years gradually spreads throughout the tree. After many years, dead twigs, then dead branches and limbs begin to appear in the tree and the condition continues to worsen over the years until the tree needs to be removed. Bacterial leaf scorch is a very problematic plant disease in Delaware.

In Kentucky research in 2006, they were able to show in replicated pin oak plots that root flare injection of oxytetracycline antibiotic in late spring delayed the appearance of scorch symptoms by about 2-3 weeks. In 2007, they determined that the best results were obtained when injections were made about 3 weeks after the first emerging leaves were fully expanded. Arborists with clients who want their trees injected with antibiotic may wish to concentrate their efforts for sometime in the next 2 weeks.

In the Kentucky work, they used Bacastat®, a commercial injectable form of oxytetracycline. This antibiotic is used for management of bacterial diseases in agricultural crops and is active against gram negative bacteria such as Xylella. Mycoject® is also an injectable formulation of oxytetracycline, but our results indicated that the more viscous formulation of this product was not as effective.

Although the time of appearance of scorch symptoms was delayed as a result of these treatments, none of the treatments prevented disease or cured trees with bacterial leaf scorch. When bacterial leaf scorch symptoms are delayed with antibiotic treatments, it is not known if this will prolong the life or vitality of the treated trees, but it might. Nevertheless, if clients are demanding a treatment, certified arborists trained in micro-injection techniques will want to make the injections in late May to provide the greatest effect. It appears that the effect of antibiotic injection lasts only one year, so annual injections may be needed. More research is needed to determine these long-term needs and effects.

Adapted from "It Is Near Time for Injecting Oaks for Bacterial Leaf Scorch" By John Hartman in the current edition of the Kentucky Pest News http://www.uky.edu/Ag/kpn/kpn_09/pn_090519.html#STO

Ornamentals Hotline Disease Pictures for Issue 17, July 20, 2007

Photo from Iowa State University Forestry Extension Website.

Bacterial Leaf Scorch on Pin Oak

Photo by Jim Peter, The Plant & Pest Diagnostic Laboratory at Purdue University

Bacterial Scorch on Pin Oak
Photo by Jim Peter, The Plant & Pest Diagnostic Laboratory at Purdue University

Bacterial Leaf Scorch on Oak

Photographer Edward L. Barnard, Florida Department of Agriculture and Consumer Services

Peony blotch on peony (Paeonia)

Image from Missouri Botanical Garden, Kemper Center for Home Horticulture Web Site