Bacterial Leaf Scorch in Landscape
Trees
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
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.
Symptoms
and diagnosis
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.
Integrated
management strategies
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 6/8/09