Dutch Elm Disease defoliating half a treeThis disease is from a fungus which was introduced to North America from Europe.  It has resulted
in the death of many mature elm trees.  The fungus is carried to host plants by a bark beetle.  Efforts have been made in various areas to save valuable mature elms with tree injections. This can work if the tree is healthy and has not been infected by DED. There are efforts being made to breed elm trees that will be resistant to this damaging disease.