Plant Diagnostic Clinic
573-882-3019
Cause: Xylem-limited bacterium, Xylella fastidiosa. This bacterium is transmitted by leafhoppers. The bacterium grows in the xylem of the plant and physically clogs the vessels. This in turn creates water stress.
Primary hosts in Missouri are not known, but as of October 2000, bacterial leaf scorch was detected in pin oaks from the eastern and central parts of the state. X. fastidiosa has a very wide host range that encompasses over 30 plant families. This disease is a reported problem of fruit and landscape trees. Landscape tree hosts include elm, sycamore, bur, pin, red and shingle oak, sugar and red maple, mulberry and sweetgum. Bacterial leaf scorch has been reported as the major cause of leaf scorch in the coastal states from New York to Texas and in Kentucky and Tennessee.
Figure
1
Red oak leaves infected with bacterial leaf scorch showing the marginal scorch.
The scorch associated with oak wilt, in contrast often starts at the tip of the
leaf and extends downward to the petiole.
Although symptoms of bacterial leaf scorch vary slightly with species, they typically include late-season (August, September) marginal leaf scorch that begins on the older leaves and moves toward leaves at the branch tip (figure 1). In most, but not all tree species, browned, dead areas of the leaf are separated from green tissue by a narrow yellow border. In some tree species scorched leaves will abscise early. Leaves on severely affected branches will appear normal in the spring but later show symptoms. Initially a tree may only have one infected branch, but in succeeding years the scorch will spread to all parts of the tree. Over time the tree will experience a gradual decline, during which it may succumb to secondary infections. Typically infected trees will decline over a period of five to ten years from the time bacterial leaf scorch first infects the tree. These symptoms at a glance may be mistaken for those produced by vascular wilt diseases, oak wilt and Dutch elm disease. The difference is that the scorch and decline occurs over several seasons rather than occurring over a period of two or three months.
Since leaf scorch symptoms may be caused by a number of factors such as drought, salt damage, root or trunk damage and poor general root health, it is difficult to diagnosis bacterial leaf scorch on symptoms alone. Therefore positive diagnosis requires testing of symptomatic tissue with an enzyme-linked immunosorbent assay (ELISA) laboratory test. Fresh, symptomatic leaf tissue from around the mid-vein and petioles are used to run the test.
Management of bacterial leaf scorch involves early recognition so affected branches can be pruned to stop further spread in the tree. Fertilizing and irrigating may prolong the life of diseased trees. Although the spread of the disease occurs slowly, severely infected trees may be removed to reduce spread to neighboring trees. Control of the insect vectors with insecticides is ineffective in slowing disease.
Unfortunately there are no cures for this disease, although recent research has shown that micro-injections of the antibiotic oxytetracycline will suppress the symptoms. However, the effect is temporary and additional injections are needed to maintain tree health. The continual wounding made by the injections raises concerns the wounds will provide an entrance for secondary disease organisms. More research needs to be done in this area.
Updated 4/1/05
On this page
On this site
Contact
