Common Sense Protects Trees from Leaf ScorchBy
Nurserymen and landscapers can help rein in a disease that is slowly but
surely dooming hundreds of century-old oak and elm trees near the historic Mall
in Washington, D.C. Currently, 20 percent of the oaks and 30 percent of the
elms there are infected with the disease known as bacteria leaf scorch.
Nothing can be done to save those trees--or others infected with the
disease. But future generations of oaks, elms, sycamores and maples would
benefit by effective screening, according to researchers in the
Nursery Plants Research Unit, Beltsville, Md. The research group is part of
the U.S. National
Arboretum operated by USDA's Agricultural
Leaf scorch is caused by the bacterium Xylella fastidiosa. It clogs
tree xylem, the tissue that carries water from roots. Scientists found that
xylem-feeding leaf hoppers and spittlebugs transmit other crop diseases caused
by this bacterium. Yet insect transmission is just now being studied in
Leaf scorch has been found not only in Washington, but also in New York, New
Jersey, Maryland, Kentucky, Texas, Nebraska and California. It's also found in
Brazil, where it attacks coffee trees.
Arboretum researchers say nursery operators should carefully check young
trees for symptoms and destroy infected ones. A warning sign: leaves that begin
browning on their outer edges, spreading inward. Symptoms recur each year,
spreading over the tree's crown with stunted growth and branches that don't
revive in spring. A laboratory test using tree sap is conclusive.
Landscapers--particularly for large projects such as housing subdivisions--
should be vigilant. Planting a number of different tree species seems to raise
the odds of some surviving an outbreak.
ARS scientists, in cooperation with the U.S. Department of the Interior,
have identified several control strategies. ARS scientists have identified
several new xylem-feeding insects that harbor--and spread--the bacteria.
Synthetic chemical insecticide is not feasible in public places like the Mall.
But natural enemies of these insects may be found. ARS scientists are also
searching for trees with natural resistance, along with treatments for stricken
Scientific contact: Jo-Ann Bentz, ARS
U.S. National Arboretum,
Nursery Plants Research Unit, Beltsville, MD 20705, phone (301) 504-8260,
fax (301) 504-5096, firstname.lastname@example.org.
U.S. Department of Interior contact: James Sherald, phone (202)
342-1443. Ext: 208