Common Sense Protects Trees from Leaf Scorch

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 Floral and Nursery Plants Research Unit, Beltsville, Md. The research group is part of the U.S. National Arboretum operated by USDA's Agricultural Research Service.

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 landscape trees.

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 ones.

Scientific contact: Jo-Ann Bentz, ARS U.S. National Arboretum, Floral and Nursery Plants Research Unit, Beltsville, MD 20705, phone (301) 504-8260, fax (301) 504-5096, [email protected].

U.S. Department of Interior contact: James Sherald, phone (202) 342-1443. Ext: 208