Ulmus is a genus of mostly large, deciduous trees found throughout much of the Northern Hemisphere. Until the introduction of the Dutch elm disease pathogen in 1930, the American elm (U. americana) was one of the most commonly planted ornamental trees in the United States, known for their graceful, arching form. The devastating effects of Dutch elm disease on native elms in North America prompted efforts to develop tolerant or resistant cultivars and hybrids, including introductions by the National Arboretum of the Agricultural Research Service (USDA) and The Morton Arboretum. The National Elm Trial, coordinated by Colorado State University, was established in 2005 to evaluate commercially available elms at multiple locations across the United States. Dutch elm disease is the most serious disease of elms, but there are many potential pests. Other diseases include phloem necrosis (elm yellows), bacterial leaf scorch, cankers, and Verticillium wilt. Insect pests include elm bark and leaf beetles, Japanese beetle, gypsy moth, leafminers, and aphids.