Dutch elm disease (DED) is caused by a fungus that invades and plugs the water-conducting tissues of elm trees. It enters the tree through wounds made by the elm bark beetle. The beetles carry the spores of the fungus from tree to tree. The disease can also spread from tree to tree through root grafts. When trees are planted close together, as elms lining streets were, the root systems become intertwined, and plant sap flows from one tree to another through naturally formed grafts. As water flow to the crown of the tree is cut off, branches die. Foliage wilts, turns yellow or brown, and drops. Eventually the entire tree becomes infected and dies. Quick removal and destruction of infected limbs may prolong the life of the tree. Controlling the bark beetles that spread the disease is not practical. Plant breeders are working on resistant elm hybrids, so that one day you may again see the graceful forms of elms arching over streets. A few resistant cultivars include Accolade, 'Dynasty', 'Frontier', 'Homestead', 'Patriot', 'Pioneer', 'Sapporo', and 'Urban'. However, it's unlikely that city foresters will again return to the practice of planting entire sections of the city with the same species. Diversity in plant types helps prevent epidemics such as DED.