Elms are loved for their graceful, stately shape, with branches like spreading fountains, and their green leaves that turn gold in fall. Sadly, the American elm (Ulmus americana) can no longer be recommended because it is vulnerable to a devastating pathogen called Dutch elm disease. However, due in part to research at The Morton Arboretum, other species and hybrids that are more resistant to the disease are available for planting. The biggest lesson learned from the devastation of Dutch elm disease is the importance of having a variety of trees along streets, in parks, and in home landscapes so that no disease or pest that may arrive can kill a large proportion of the trees. The American elm was the most popular tree to plant in the booming cities of the 19th century, so that by the 20th century many streets were lined with only elms and were shaded in summer by a cathedral-like ceiling of their branches. When Dutch elm disease (which actually originated in Asia) spread to the US in the 1950s, it was able to mow down elm after elm through their grafted root systems or with the help of a beetle. Today, arborists and foresters are careful to plant a diverse range of trees that will not all be vulnerable to any particular pest, disease or weather conditions.