<P> Multistriatin is a pheromone produced by female elm bark beetles, which can be produced synthetically . It has potential in being used to trap male beetles, which carry the fungus . </P> <P> Because of the ban on the use of chemicals on street and park trees in the Netherlands, the University of Amsterdam developed a biological vaccine by the late 1980s . Dutch Trig is nonchemical and nontoxic, consisting of a suspension in distilled water of spores of a strain of the fungus Verticillium albo - atrum that has lost much of its pathogenic capabilities, injected in the elm in spring . The strain is believed to have enough pathogenicity left to induce an immune response in the elm, protecting it against DED during one growing season . This is called induced resistance . Trials with the American elm have been very successful; in a six - year experiment with the American elm in Denver, CO, annual Dutch elm disease losses declined significantly after the first year from 7 percent to between 0.4 and 0.6 percent; a greater and more rapid reduction in disease incidence than the accompanying tree sanitation and plant health care programs . </P> <P> Preventive treatment is usually only justified when a tree has unusual symbolic value or occupies a particularly important place in the landscape . </P> <P> Research to select resistant cultivars and varieties began in the Netherlands in 1928, followed by the USA in 1937 . Initial efforts in the Netherlands involved crossing varieties of U. minor and U. glabra, but later included the Himalayan or Kashmir elm U. wallichiana as a source of antifungal genes . Early efforts in the USA involved the hybridization of the Siberian elm U. pumila with American red elm U. rubra to produce resistant trees . Resulting cultivars lacked the traditional shape and landscape value of the American elm; few were planted . </P>

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