Issue 14, August 14, 2009

Time to Test for BLS

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. The images show two examples of Illinois oaks with confirmed BLS.

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 in one day 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. The University of Illinois Plant Clinic staff will be testing for BLS the week of August 24th this year. If you have a sample you want tested for BLS, and you live in the state of Illinois, give us a call at 217-333-0519 and we will arrange to run an ELISA with the next batch of samples. It is not economical to run this test for only one sample, but the standard Plant Clinic fee of $15.00 covers expenses when a group of samples are processed together. We use the ELISA reagents available from AGDIA, Inc in Indiana.

Here is what you need to send: Since testing for Xylella fastidiosa is done on new growth, send 3" of live, symptomatic twig tip and all leaves attached to that twig tip. Place four or five such tip cuttings in a zip lock plastic bag, label with tree species, include a check ($15.00) payable to the University of Illinois, a completed specimen data form (Adobe PDF) and mail to the Plant Clinic. We can only accept Illinois samples at this time.--Nancy Pataky

Nancy Pataky

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