Bacterial leaf scorch (BLS) is a disease caused by the bacterium Xylella fastidiosa that has been found to affect shade trees of many species in various parts of the U.S. It has been confirmed for some time in coastal states from New York to Texas, and more recently, in Indiana, Kentucky and Ohio. Although large scale, systematic surveys of Midwestern and Plains states have not been conducted, samples have tested positive for BLS in Kansas, Nebraska, Oklahoma and Missouri. According to Simeon Wright,with the University of Missouri Plant Diagnostic Laboratory, BLS has been confirmed in Missouri in samples from St. Louis, St. Charles, Boone and McDonald Counties. Dr. Gerald Adams, at Michigan State University is coordinating statewide surveys in 11 states in the North Central and Plains regions. Unfortunately, pin oak samples from Columbia, MO, sent in 2009 by Wright to MSU were confirmed to have the disease.
One reason why more trees in the Midwest have not been confirmed to be infected with BLS is that a definitive diagnosis of the disease is not easy to make. Bacteria infecting the host plant are confined to the xylem, where they and their byproducts clog the vessels, leading to drought stress. Affected trees exhibit various symptoms, some of which could be confused with those caused by stress from drought, soil compaction or salt damage. The most obvious symptom of BLS is scorching starting at the margin and extending inward, toward the main veins. In some species, there is a “halo” of light tissue at the inner margin of the scorch. Unlike with oak wilt, scorched leaves tend to stay on the tree and trees may live many years with the disease. BLS symptoms recur annually and may be intensified by drought and other stresses, often leading to dieback and facilitating attack by other organisms, such as borers, anthracnose and Armillaria. The only practical methods for definitively diagnosing BLS are ELISA (Enzyme linked immune-sorbent assay) and PCR (Polymerase chain reaction). ELISA testing can be rapidly conducted by trained diagnosticians using kits that require little laboratory equipment. They are not, however, inexpensive. PCR, while more technically difficult, expensive and labor intensive, is about 100 times more sensitive than ELISA.
BLS is a difficult disease to manage. It affects a wide species range, including sycamore, mulberry, sugar maple, red maple, sweetgum, American elm and a number of oaks. In surveys coordinated by Dr. Adams at Michigan State, oaks in the red oak group have been among the species most commonly testing positive by PCR. Researchers in Dr. Adams’ lab have also confirmed BLS in swamp white oak. Another factor making BLS management a challenge is that it is spread efficiently by insect vectors, primarily leafhoppers, that are mobile and unpredictable. Leafhoppers commonly arrive, en masse, carried on wind currents. Since they are xylem feeders, they are highly effective at transmitting the bacterium from infected to healthy trees. Therefore, controlling vectors with insecticides is unlikely to be effective in preventing infection. Trunk injections with antibiotics may reduce symptoms, but cannot eradicate the bacterium with current technology. Mulching and irrigation during drought may also reduce symptoms and slow the rate BLS-related decline.
If you see scorch symptoms on a tree and would like to determine whether they are caused by BLS, the best approach would be to contact the diagnostic lab at your State University and see if they have an arrangement for doing the testing themselves or sending the samples to another lab. Do not send individual samples directly to Dr. Adams at Michigan State. In some cases, if BLS is confirmed, it may be best to remove the infected tree to reduce the potential for hazard development and spread of the bacterium to other trees in the area. For more information on BLS and to see images showing symptoms, see the following web links.
REVISED: August 1, 2012