Dutch elm disease is caused by a fungus that is transmitted through beetles. This fungus kills the tree or creates a chronic stage of stress that will eventually lead to death of the tree. It is considered the most costly shade tree disease ever and will remain active within a community as long as there are susceptible trees. The fungus invades the water-transporting vessels and produces toxins to which the tree reacts. In defense to the toxins, the tree produces gums and internal growths designed to block the advance of the fungus. The combination of the toxins and the defense mechanisms of the tree inhibit water flow to the crown, which causes wilting and death. Because the water-conducting vessels are clogged, the first symptoms seen are “flagging” or wilting throughout the canopy. Usually starting at the branch tip, leaves turn a dull green to yellow, curl, and become dry and brittle. As the infection spreads, the wood beneath the bark displays a brown discoloration. The primary vector for the disease is the elm bark beetle. As the disease progresses, Dutch elm disease can pass from infected trees through grafted roots and spread through the root system without the beetle acting as a vector.
Can I treat or prevent Dutch Elm Disease?
Most infected American elms cannot be saved. In some rare cases, the tree can undergo surgery to remove all infected limbs and continued fungicide treatment. Here in Utah, most of our elms are Siberian elm, Ulmus pumila, or sometimes miss called Chinese elm. This variety of elm is still infected by the fungus. It doesn’t respond the same as American elms or other varieties. Most of the time, the fungus places the tree under constant chronic stress and makes it susceptible to all other types of problems that will eventually kill the tree.
You can prevent Dutch elm disease in your trees by a few treatment options. The first is a fungicide that is treated every two years and insecticides to prevent the beetle. This prevention does not protect from the spread of the fungus in grafted roots. Trenching at least 36” down and severing all roots around unaffected trees is the only available option to prevent the transmission of the fungus underground.