Environmental stress, root injury, drought, and many other factors can cause leaf margin necrosis, or scorch. This condition is usually widespread in a tree and fairly uniform. It is not necessarily repeated in following years and is noninfectious (see issue no. 5). Bacterial leaf scorch (BLS) is an infectious disease that spreads systemically and causes slow decline and death of a tree. BLS is not new but is appearing more frequently in the Midwest. This may simply be because more people recognize the symptoms.
Infectious leaf scorch is caused by the bacterium Xylella fastidiosa. This disease has become famous for its presence in oaks and elms on the mall in Washington, DC. Although mainly found in eastern and southern states, BLS has now frequently been found in western Kentucky and Indiana and reported in Illinois. This past summer, the U of I Plant Clinic confirmed BLS on six pin oaks in central Illinois.
Frequent hosts include elm, oak, sycamore, mulberry, sweetgum, sugar maple, and red maple. Scorch symptoms occur in early to midsummer and intensify in late summer. The scorched leaf edges or tissue between veins may be bordered by a yellow or reddish brown. Symptoms occur on one branch or section of branches and slowly spread in the tree from year to year. BLS often appears on the oldest leaves first, unlike environmental scorch, which first appears on the newest leaves. Of course, diagnosis is never that simple, and oaks are an exception. We did not observe this pattern on pin oaks in Illinois. Most references say oaks show symptoms on an entire branch at once. In trees with bacterial scorch, infected leaves often remain on the tree until the fall. Oaks are again the exception; they drop leaves early. If your oak has been in slow decline with leaf scorch symptoms each July and August, and fall leaf drop occurs about a month ahead of healthy oaks, BLS may be present.
The bacterial pathogen is found only in xylem tissue. Xylem-feeding leafhoppers and spittlebugs are thought to spread the bacterium in landscape trees, but it can also be transmitted through root grafts. Transmission methods must not be very effective, though, as disease does not spread rapidly from tree to tree.
We cannot test for this bacterium at the Plant Clinic, so we send our samples to AGDIA, Inc., a private lab in Indiana. AGDIA has a serological (polyclonal antibody) test that can be done on young twigs and leaves. As of this writing, the fee for the test is $48.25 for one sample and $6.25 for each additional sample. Call ahead to be certain you have prepared the sample correctly; this will avoid resampling at your expense. Leaf petiole tissue is preferred. If you have questions, consult AGDIA at www.agdia.com or call them at (219)264-2014 or (800)62-AGDIA.
What can you do if bacterial scorch is present? There is probably nothing you can do to keep the tree from dying. Prune out dead wood as it appears. Start thinking of tree replacement, and plant a variety that is not known to host this disease. Be sure to pick a species that does well in your site. Investigate drainage pattern, soil type, amount of sunlight, and any oddities about the location. No fungicides, insecticides, or bactericides can be sprayed on a tree to prevent or cure this disease. The antibiotic oxytetracycline is present in some commercially available injectable products. There has been little research in this area, but some work shows that oxytetracycline suppresses Xylella in some cases and may provide temporary symptom suppression when injected into trees. Researchers in Kentucky who have tried such injections do not see any benefit, and National Park Service researchers have seen only short-term benefits. You may need to repeat costly injections as frequently as every year; and there are no guarantees. We will keep you posted as new information on managing this disease is available.