Dutch Elm Disease (DED)

Description

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.

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.

Sign and Symptoms

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.

DED-Fungus-in-Branch-cross-section-300x201

Most elm tree species are susceptible including American Elm

Close up

Foliage

Canopy

Bark

Diagnosing Dutch Elm Disease

A few key distinctions will help in accurately making sure that you are in fact dealing with this dreaded disease.

Leaves on infected trees are almost always curled or wilted looking. They often drop off.
Symptoms progress as the fungus grows in the tree. Thus, the pattern of leaf death visible outside the tree reflects the fungus growth inside. Remember that the initial infection grows downward in a narrow band until it has reached the roots. It then spreads sideways and grows back up the rest of the tree. See the Technical Description of Dutch elm disease for more information.
Check under the bark close to where there are external leaf symptoms. Dutch elm disease always causes the tree’s water-conducting vessels to turn a dark brown. Finding this discoloration along with wilting leaves is a very strong indicator that Dutch elm disease is present. Use a chisel and a hammer to open a hole in the bark to check for the discoloration.
Trees in the spring can die rapidly – causing the appearance that the disease is moving very quickly. What is happening in reality is the fungus was in the tree from last summer, the tree grew new tissue over the top of the infected wood, and then the fungus colonized in the newly formed wood causing rapid dieback. Infections that happen in the current year can be seen reflected in leaf dieback as the fungus grows.

Managing Dutch Elm Disease

Dutch elm disease is difficult to control and, without proper management, it will wipe out a large population of elms in just a few years. However, with a properly implemented management program, the devastating effects of the disease can be greatly reduced.

An effective Dutch elm disease management program includes four parts for control:

Rapid detection and rapid removal of diseased elms
- A tree inventory and monitoring program are the foundation. Early detection and rapid removal can keep mortality just under 2%/year, and without treatment, there is no certainty of which elms will become infected.
Disruption of root graft transmission
Saving diseased elms (Tracing)
Protecting high-value elms with Arbotect 20-S

1. Prompt detection and removal of diseased elms

Scouting for Dutch elm disease and identifying diseased elms is the first step of any DED management program. Scouting also involves checking peoples’ yards and garages for elm wood. Scouts will usually move through an area every 2-4 weeks during the growing season to make sure dying trees are identified and properly dealt with.

Promptly removing and disposing of elms dying from Dutch elm disease is the key to effectively managing Dutch elm disease on a community-wide basis. It involves identifying diseased elms that cannot be saved by tracing and immediately removing those trees. This will reduce the number of disease carrying beetles. A single dead elm can produce tens of thousands of contaminated beetles. Without such a program, a substantial majority of a community’s elm population will succumb to Dutch elm disease and die within a few years. Removed elms need to either have their bark removed, be chipped, burned, or buried.

2. Root Graft Prevention

Severing grafted root systems between diseased and healthy elms can save many trees. Accordingly, prompt disease detection as well as installing root graft barrier trenches is very important to the overall success of an integrated control program.

The only way to stop the spread of Dutch elm disease through root grafts is by physically breaking the root connections between the infected tree and the healthy tree. This is most often accomplished with trenchers or vibratory plows. In situations where there are buried utilities, an Air Spade can be used to remove the soil and expose the root grafts without damaging the utility lines. Trenches should be at least 3 feet (1m) deep in clay soils and 5 feet (1.5m) in sandy soils, although it is always best to go as deep as your equipment allows.

Trenches are typically installed midway between the infected tree and the healthy tree, although this can be adjusted in either direction depending on how far the infection has spread. It is important to know where the fungus is located in the diseased tree before the trench is installed. If the disease stain is already at ground level in the infected tree it is impossible to determine how far it has traveled through the root system toward the healthy tree, so a second trench may be necessary. This trench would isolate the healthy appearing tree from any others that might be root grafted to it.

It is important to sever the root grafts before removing the diseased tree. Because each tree is transpiring, removal of the diseased tree must allow the healthy tree to pull all of the moisture and fungal inoculum out of the other tree’s root system quickly. An Air-Spade can be used to remove soil and expose root grafts in situations where utilities are present.

3. Saving Infected Elms

Tracing is a method of saving elms recently infected with Dutch elm disease. It is far more cost effective than removing and replacing an elm. By utilizing this procedure, a city can save many thousands of dollars. Since the fungus that causes Dutch elm disease grows very predictably in a narrow band downward and only in the current year’s water-conducting vessels, tracing is easy to learn and incorporate for a qualified arborist.

4. Protecting High-Value Elms with Arbotect

There have been hundreds of proposed “cures” or protection products for Dutch elm disease over the years, but only one treatment has been proven to work through replicated University trials. When applied properly, Arbotect provides a 99% success rate against Dutch elm infection for up to 3 years.

Arbotect does not work if the tree already is infected with Dutch elm disease or if the disease fungus enters the tree via root grafts.

Treatment should be used in conjunction with an insect control and sanitation program (early detection and rapid removal of infected trees) in order to obtain best results. Use of Arbotect 20-S does not prevent the transmission of Dutch elm disease via root grafts.

Applications can be made after leaves have fully flushed in the spring. Treatments can continue throughout the growing season, or until leaf senescence in the fall.

One application of Arbotect 20-S protects the entire tree for up to three years yielding significant cost savings.

Multi-year control prevents transmission by elm bark beetle at peak flight times early in the growing season. To protect a tree from a beetle-transmitted fungal infection, Arbotect 20-S must be evenly and completely distributed throughout the two to four year old branches.

To ensure even distribution, Arbotect 20-S is injected using the Macro-Infusion injection method. Macro-Infusion injects a large volume of solution into the root flares of the tree. This solution is then transported throughout the canopy providing a protective fungicide barrier.

Only Arbotect 20-S has the ability to move into the newly formed sapwood, while resisting degradation resulting from cold, heat, and other adverse conditions. Arbotect 20-S was first registered over 30 years ago and has a proven 99% success rate when applied as per manufacturer’s directions.

Protecting Elm Trees Using Arbotect 20-S

Arbotect 20-S is a preventative treatment for Dutch Elm Disease (DED). It is the only scientifically proven systemic fungicide that provides multi-year protection (up to 3 years) from DED, with a proven success rate of 99% when included in an DED IPM program.