Dutch elm disease is caused by two strains of an aggressive fungus (Ophiostoma-ulmi Ophiostoma-novo-ulmi) that provoke elm trees to die regardless of their health. It is considered the most costly shade tree disease and will remain active in 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 tyloses designed to block the spread 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 tree death.

Dutch elm disease symptoms begin to develop 4–6 week after infection. The first noticeable symptom that results from the fungal occupation of the water conducting vessels is wilting or “flagging” of one or more branches, usually starting at the branch tip. Leaves on infected branches turn dull green to yellow, curl, and become dry and brittle. As the infection spreads, the wood beneath the bark displays a brown discolouration.

Female elm bark beetles lay their eggs beneath the bark of dead and dying elm trees. If the elm is infected with Dutch Elm Disease, the newly hatched beetles will emerge from the tree carrying the deadly fungus on their bodies. The beetles fly to healthy trees to feed on 2–4-year-old branches, and thereby spreading the disease.

Dutch elm disease can also be spread by root contact/root grafting from one diseased elm to a healthy elm.

Most infected elms cannot be saved. In rare cases, if the fungus has not moved into the root system, physically cutting out the infected portions of the tree, with a process called tracing, can save the elm.

Sanitation is the most important tool for controlling Dutch elm disease on a community-wide basis. It involves identification and rapid removal of diseased elms. Such practices eliminate beetle breeding sites and reduce the number of disease carrying beetles.

The goal when protecting elms from Dutch elm disease is to evenly and completely distribute a fungicide throughout the entire canopy of the tree.

• To protect a tree from beetle-transmitted fungal infection, Arbotect 20-S, must be evenly and completely distributed throughout the 2–4-year-old branches.
• The only way to get even distribution is by macro-infusion of a large volume of solution into the root flares of the tree.
• Arbotect is a fungicide that protects elms from beetle-transmitted infection, and is the only fungicide that provides three growing seasons of protection.
• Arbotect does not protect elms from root graft infection.

• Macro-infusion can be done after the leaves are fully formed on the tree up until the tree begins to show fall coloration.
• Treatment must be done into the root flares to get even coverage. Do not infuse into the trunk tissue: doing so results in poor distribution of the treatment and incomplete coverage of the protection.

The purpose of macro-infusion is the complete and even coverage of two to four-year-old branches so that if a beetle comes to feed, the disease cannot enter the tree. Infusing into the root flares is critical to the success for even and complete distribution.

The tissue in the root flares allows for lateral movement of Arbotect as it is infused. Trunk tissue, on the other hand, is hard and rigid. It is highly compartmentalized. Infusion into trunk tissue provides very limited lateral movement of the Arbotect solution and will result in small portions of the crown receiving a large dose of Arbotect with other areas receiving none.

Macro-infusion rapidly introduces a large volume of solution directly into a tree’s vascular system. Macro-infusion delivers complete and even distribution of solution throughout the tree’s canopy, which provides for predictable results.