There are two main pests of Elm and they go hand in hand. There are Elm Bark Beetles that feed just under the bark and doing so, disrupt the flow of food and water within the tree and with enough damage, cause limbs to die and eventually the whole tree. The beetle carries with it a fungal disease called Dutch Elm disease. This pathogen is spread by the beetle. Once inside the tree, the disease grows and blocks the movement of water and food as well. While you can treat for the beetle, treating the disease is much more difficult. As the disease moves within the tree, branches will "flag" and die, moving from branch to branch until the tree is dead. DED can kill the tree in a single season. Consider getting an Arborist to assist in determining the exact cause and the cost of treatments vs removal and replacement.