Fighting the Blight

The battle against Dutch elm disease continues, but the front grows broader every year. A fungus borne by tiny bark beetles that kills the stately Ulmus americana by causing its circulatory system to clog up, the disease first arrived in the U.S. from Europe in the early 1930s. In the past few years, it has crossed the Rocky Mountains and reached California and the eastern portions of Oregon, where it threatens to spread as rapidly as it has in the rest of the nation. At present, the fungus is killing trees at a rate of 400,000 or more every year, at a total cost to communities of as much as $100 million annually in infected-tree removal and replacement.

For years the most effective chemical treatment was DDT, but a near-total ban was placed on that insecticide in 1972. Since then scientists have considered other ways of combatting the disease, including breeding armies of tiny parasitic wasps that would attack the fungus-bearing beetles, and defending elms with sticky traps coated with beetle-attracting odors. The newest weapon is a fungicide called Lignasan BLP, manufactured by Du Pont and put in commercial use for the first time this summer in about 1,000 locations in the U.S., including New York's Central Park, the White House lawns and in Minneapolis-St. Paul, where as many as 18,000 diseased trees may be cut down this year.

Elm Research. Lignasan BLP is dissolved in water and pumped into a tree's circulatory system through holes drilled into the tree at or just below the root "flare" at ground level. In experiments conducted on 7,500 trees last year and an anticipated 100,000 elms this summer by the Elm Research Institute of Harrisville, N.H., early test results indicate that Lignasan is 99% effective when used preventively--before the disease strikes--on healthy trees. But many plant scientists refuse to endorse the product until they know more about proper dosages, duration of protection and the chemical's effectiveness when used without other preventive measures--such as insect control and prompt, careful pruning or removal of blighted elms. Just as cautiously, Du Pont emphasizes that Lignasan is only an aid for controlling the disease, not a cure.

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