Shade trees getting ’scorched’ by plant disease

Bacterial leaf scorch is severely affecting urban shade trees grown not only to provide shade, but to help clear the air, reduce noise, and improve the aesthetics in many U.S. communities, say plant pathologists with The American Phytopathological Society (APS).

According to Ann Brooks Gould, associate extension specialist at Rutgers University, New Brunswick, NJ, bacterial leaf scorch (BLS) affects many shade tree species such as American elm, red maple, sweet gum, sycamore and London plane, and a number of oak species. The disease has been found in landscapes, street plantings, and small woodlots throughout the eastern U.S. and as far west as Texas.

According to Gould, BLS affects as many as 35 percent of susceptible street and landscape oaks in some central New Jersey communities. Current loss of value plus replacement costs for older trees affected by this disease is estimated at $8,000 per tree. Landowners and tree care professionals in these locations must plan for the loss of property values and high costs of replacement as shade trees in landscapes, wood lots, and golf courses affected by BLS decline and must be removed.

BLS is caused by a bacterial pathogen, Xylella fastidiosa, which has a wide host range that includes common landscape ornamentals and weeds. X. fastidiosa is spread by insects, mainly sharpshooters.

Symptoms of BLS are very similar to those caused by environmental stresses. Because of this, the disease is often overlooked or misdiagnosed. On oak trees, BLS symptoms include scorching in late summer or early fall on leaves of all ages at about the same time. On sycamores and elms, symptoms progress from older to younger leaves. Affected leaves may curl and drop prematurely, and as the disease progresses, branches die and the tree declines. Elms may be killed outright by the disease. Other affected species eventually decline to the point where the dead branches pose a safety risk and the tree must be removed.

According to Gould, current management options of BLS in urban trees include:

• Maintain plant vigor. The development of BLS is enhanced by other diseases, insects, and environmental stresses such as drought. BLS may also predispose infected plants to other disease and insect problems.
• Practice sanitation. Branches that have died due to BLS should be routinely removed. Infected trees that are in a severe state of decline should also be removed.
• Use tolerant plants. In areas where BLS occurs, avoid planting highly susceptible trees, and design new tree plantings with a diverse complement of tree species.

More on the symptoms of BLS and disease management options is available in this month’s APSnet feature article at http://www.apsnet.org/online/feature/bls/.