as that of 'Jefferson'. Thus far, plantings of these four varieties generally appear to be successful. In 2005, approximately 90 'Princeton' elms were planted along Pennsylvania Avenue in front of the White House in Washington, D.C. The trees, whose maintenance the National Park Service (NPS) manages, remain healthy and are thriving. However, it has been noted that U. americana cultivars are not recommended for more than singular plantings as they have unresolved DED and elm yellows concerns. It has also been noted that monoculture plantings of U. americana cultivars, such as those along Pennsylvania Avenue, have disproportionate vulnerabilities to disease. Further, long-term studies of 'Princeton' in Europe and the United States have suggested that the cultivar's resistance to DED may be limited (see Pests and diseases of 'Princeton'). The National Elm Trial evaluated 19 elm cultivars commercially available in the United States in scientific plantings throughout the nation to assess and compare the strengths and weaknesses of each. The trial, which started in 2005, lasted for ten years. Based on the trial's final ratings, the preferred cultivars of U. americana are 'New Harmony' and 'Princeton'. 'Jefferson' was released to wholesale nurseries in 2004 and is becoming increasingly available for planting. However, 'Jefferson' has not been widely tested beyond Washington, D.C. The National Elm Trial provided no data on ‘Jefferson’ because an error in tree identification had occurred earlier in the nursery trade. The error may still be causing nurseries to sell 'Princeton' elms that are mislabeled as 'Jefferson', although one can distinguish between the two cultivars as the trees mature. In 2007, the 'Elm Recovery Project' from the University of Guelph in Ontario, Canada, reported that cuttings from healthy surviving old elms surveyed across Ontario had been grown to produce a bank of resistant trees, isolated for selective breeding of highly resistant cultivars. In 1993, Mariam B. Sticklen and James L. Sherald reported the results of NPS-funded experiments conducted at Michigan State University in East Lansing that were designed to apply genetic engineering techniques to the development of DED-resistant strains of American elm trees. In 2007, AE Newhouse and F Schrodt of the State University of New York College of Environmental Science and Forestry in Syracuse reported that young transgenic American elm trees had shown reduced DED symptoms and normal mycorrhizal colonization. Hybrids and hybrid cultivars Ulmus 'Rebella' (U. americana × U. parvifolia) Thousands of attempts to cross the American