BLS is short for bacterial leaf scorch, a disease that has been in the news in Illinois since 2001. Symptoms include scorching of foliage in mid- to late summer. Each spring the tree leafs out normally, but by midsummer the symptoms spread further in the tree. Although this scorching seems harmless at first, the disease becomes worse each year and can kill a mature tree in 4 to 6 years. Look for scorch symptoms that appear on older leaves first and late in the growing season to help distinguish this infectious disease from environmental scorch, which occurs in spring or early summer and affects newest growth first. BLS has been reported and confirmed on elm, hackberry, maple, mulberry, oak, sweetgum, sycamore, and planetree. To date in Illinois we have confirmed BLS only on oaks, but this includes positives on pin, shingle, bur, red, and white oaks. In 2004 we found seven more positive cases of BLS on oaks, but no new species were involved.|
The bacterial pathogen causing bacterial leaf scorch is Xylella fastidiosa. It is found only in xylem tissue. Xylem-feeding leafhoppers and spittlebugs are thought to spread the bacterium in landscape trees. It can also be transmitted between trees through root grafts. The transmission methods must not be very effective, though, because we do not see rapid spread of the disease from tree to tree.
The bacterial pathogen cannot be seen in standard lab sections or isolated in the lab like most other bacteria. It can be confirmed using serological techniques. We cannot test for this bacterium at the Plant Clinic, so we send our samples to a private lab such as AGDIA with a serological (polyclonal antibody) test for the bacterium that can be done on young twigs and leaves. Consult AGDIA at http://www.agdia.com, 219-264-2014, or 800-62-AGDIA. If you prefer to go through the Plant Clinic with your sample, we can test for other problems but would have to bill for AGDIA testing as well. Call if you have questions.
What can you do if bacterial scorch is present? There is probably no way to keep the tree from dying, but you can help by pruning out dead wood as it appears. Start thinking of replacement options and plant a tree that is not known to host this disease. Pick a species that does well in the site you have in mind. Investigate drainage pattern, soil type, amount of sunlight, and any oddities about the location. I do not have any new information on treatment of this disease. There are no fungicides, insecticides, or bactericides that can be sprayed on a tree to positively, effectively prevent or cure it. An antibiotic called oxytetracycline is present in some commercially available injectable products intended to combat Xylella. There is not a great deal of research in this area, but work shows that in some cases oxytetracycline will suppress Xylella and may provide temporary symptom suppression when injected into trees. Researchers in Kentucky have tried such injections and do not see any benefit. National Park Service researchers have seen only short-term benefits. Injections may need to be repeated as frequently as every year, can be costly, and afford no guarantees.
The best advice I can give concerning BLS is to become familiar with the symptoms. The National Arboretum has an excellent Web site with images and information at http://www.usna.usda.gov/Research/BacterialLeafScorch.html.