Dutch Elm Disease: Symptoms and Management

At Hendricksen Tree Care, we have helped you get to know the trees in the Chicago area, and possibly the trees on your own property, through our blog series on the native trees of the Chicago area.  While we certainly believe it is important to know about and appreciate the trees in our area for their natural beauty, it is also important to understand the threats to our trees.  There are many tree diseases and pests that can cause serious damage to the tree, especially if the problem is not noticed or treated.  That is why we have started a new series on the common trees and insect pests that can affect your trees in the Chicago area.

This week, we start this series with a case study on Dutch elm disease.  Dutch elm disease (DED) is a fungal disease that mainly attacks elm trees, including the American Elm.  This disease originated in Europe and is called Dutch elm disease because it was first discovered and identified by Dutch phytopathologist Bea Schwarz in the Netherlands in 1921.  Dutch elm disease is believed to have arrived in the U.S. in 1928 when a shipment of logs from the Netherlands arrived containing beetles that carried the disease.  By 1960, Dutch elm disease had spread to the Chicago area and by 1989, more than 75 percent of elm trees in the U.S. had been lost to the disease.

The fungus that causes Dutch elm disease is transmitted by insects such as bark beetles and it causes the vascular system of the tree to clog, restricting the flow of water and nutrients.  The main symptoms of the disease are browning and wilting of the leaves which will lead to defoliation and branch dieback.  If the disease is not noticed and treated early, it will likely lead to the death of the tree.  In this guide, we will explain the life cycle and main symptoms of Dutch elm disease as well as discuss the best ways to treat and manage the disease.

The best defense against Dutch elm disease is to prevent your trees from getting infected in the first place.  At Hendricksen Tree Care, our professional arborists provide tree care and maintenance services to help protect your elm trees against diseases such as Dutch elm disease.  We will make sure your trees are effectively maintained with fertilizer applications, pruning, and preventative treatments to help your trees better resist the disease.  If your trees do become affected, we can diagnose the problem and immediately come up with effective solutions to treat the disease.

Dutch Elm Disease Life Cycle

Dutch elm disease is caused by the fungus Ophiostoma ulmi and as the name suggests, American elm trees are highly susceptible to this disease.  There are two main ways that an elm tree can contract Dutch elm disease; from the elm bark beetle or through root grafts.  Once the fungus enters the tree, it attacks the xylem, or water-conducting cells and the tree will respond by producing a chemical that will further clog the xylem.  The restriction of water and nutrients will cause the tree to wilt and eventually die.

When are Trees Susceptible?

Elm trees are the most susceptible to Dutch elm disease from the spring through mid-summer.  This is because the water producing elements within the tree are produced during this time of year.  Elm trees are less susceptible to this disease in drought conditions and when experiencing vigorous growth.

Dutch Elm Disease Transmission

• Elm Bark Beetles: The most common way Dutch elm disease is transmitted is through elm bark beetles. There are two types of elm bark beetles that can affect trees in the U.S.; the native elm bark beetle and European elm bark beetle.  These elm bark beetles look similar as they are brown or black in color with oval shaped bodies.  They lay their eggs in stressed or dying elm trees and when they emerge as adults, they feed on healthy trees and infect them with the fungus that causes Dutch elm disease.
• Grafted Roots: Grafted roots are the second way trees can contract Dutch elm disease. This occurs when the roots from two different trees that are close together fuse to each other in the soil.  The fungus that causes Dutch elm disease can travel through the grafted roots from an infected tree to a healthy tree and move upward through the tree.  This can be prevented by planting elm trees far enough apart to avoid root grafting.

Life Cycle of Elm Bark Beetles
Because elm bark beetles are the main cause of the spread of Dutch elm disease, the life cycle of the disease is closely related to the life cycle of these beetles.  It is important to note that the life cycle of the native elm bark beetle differs from that of the European bark beetle.  Both beetles tend to breed in stressed or dying elm trees and lay their eggs in tunnels they create in the bark called galleries.  If the tree they choose to lay their eggs is affected by the fungus, the fungus will multiply within the gallery and the beetles will have the fungus both in and on their bodies when they emerge from the wood.

European elm bark beetles overwinter within the tree that they hatched as both adults and larvae.  During the winter, they will feed on the host tree and emerge as adults in the spring.  They will then find a healthy elm tree and feed on the inner bark and small branches before repeating their breeding cycle two or more times during the growing season.  These beetles transmit the Dutch elm disease fungus when they leave the stressed or dying tree that they hatched in and feed on crown of a healthy tree.

The native elm bark beetle differs from the European bark beetle in that it overwinters within the bark of lower stems of a healthy elm tree.  They emerge in the spring to feed on the inner bark and branches of the healthy elm before leaving to lay their eggs in a stressed or dying elm.  These beetles often transmit the Dutch elm disease fungus when feeding in the spring or when making its overwintering site in the fall.

Life Cycle of the Dutch Elm Disease Fungus

Once the fungus is transmitted to the tree, it will find its way to the xylem and restrict the flow of water and nutrients through the tree.  European elm bark beetles tend to transmit the fungus in the upper branches of the tree.  In this case, the fungus will cause browning and wilting of the leaves and branches while traveling down the crown.  If the fungus enters the tree in the lower bark from the native elm bark beetle or through root grafting, it will move its way up the trunk to the canopy.

The speed in which the disease progresses depends on several factors including the size of the tree, the location of the infection, time of year, climactic conditions, and the response of the tree.  Dutch elm disease can progress quickly and kill a tree within one growing season or develop more slowly over a couple of years or longer.  Some trees can even survive and recover from Dutch elm disease.

Signs of Dutch Elm Disease

As explained above, the fungus that causes Dutch elm disease attacks water producing cells within the tree and restricts water and nutrients from moving freely.  While this can’t be seen, the effects of the disease can be seen on the outside of the tree.  The following are the main signs of Dutch elm disease.  These symptoms typically become visible in the late spring and beyond, but they can be visible in early spring if the tree was infected the year before without showing symptoms.

• Defoliation: The leaves on affected branches will begin to wilt and become discolored from the lack of water and nutrients. They will turn yellow then brown and they will become wilted and curled.  Eventually, the leaves will fall off the tree.  Areas of brown, wilted leaves in the canopy could be a sign of Dutch elm disease.
• Branch Dieback: When the leaves start to discolor and wilt, the branch itself is also affected. As the leaves die off, the branch will eventually die too.
• Dark Streaks in the Branches: While the defoliation and dead branches caused by Dutch elm disease are similar to the symptoms of other diseases, dark streaks within the branches and stems is usually a sure sign of the disease. The dark streaks will appear in the sapwood underneath the bark.  To check your branches for the streaks, you need to remove the bark.  A professional must test the brown streaks to positively identify the problem as Dutch elm disease.
• Galleries: Both the larvae and adults create galleries in the sapwood as they feed. These galleries are found under the bark and appear as lines that come out in every direction from a deep center line.

Dutch Elm Disease Treatment

There is currently no cure for Dutch elm disease, but it can be treated effectively to save the tree.  The most effective way to treat trees infected with Dutch elm disease is to interrupt its cycle.  The following are the most effective Dutch elm disease treatments:

• Remove affected branches: If you have elm trees on your property, you should be on the lookout for weakened, dead, or dying branches the entire growing season. Removing dying and dead branches as soon as possible will eliminate breeding sites for elm bark beetles and remove the beetles themselves if the pruning is done before they emerge.  If you find affected branches while the tree is dormant, they must be removed before the spring.  Affected branches found during the growing season should be removed within 2 to 3 weeks of finding them to prevent the beetles from emerging.  Branches that are removed must be destroyed by chipping, burning, or burying them.
• Insecticide applications: There are insecticides that can be applied to elm trees that will help kill off the elm bark beetle before it can spread the disease. The time and location of the application will depend on which type of elm bark beetle is in your area.  If the native elm bark beetle is more common in your area, then insecticides should be applied to the lower stems in the late summer to kill the beetle when it prepares to overwinter.  If the European elm bark beetle is more common in your area, then the insecticide should be applied to the crown of the trees in the spring.  It is best to have a professional arborist apply the insecticide.
• Disrupt root grafts: Disrupting root grafts between trees will help prevent the disease from transferring. If an affected elm tree is close to a healthy elm tree, the root grafts need to be severed immediately.  This should be done before the affected tree is removed because removing the tree while the roots are still grafted will cause the healthy tree to quickly pull the contents of the infected tree.  If you have two healthy elm trees close together, you should sever their root grafts as a preventative measure.
• Fungicide injections: There are fungicides that can protect trees from Dutch elm disease when applied properly, but this is an expensive treatment and it can pose other health risks to your tree. This treatment is most effective when applied through macroinjection into the roots so it can be distributed to the crown.  This treatment should be done every 1 to 3 seasons and should happen after the earliest leaves have fully grown.  The negative effects of this treatment include leaf scorch or loss and discoloration and decay from drilling injection holes.  Flushing the injection holes with water after applying the fungicide can help prevent damage.  It is best for the fungicides to be applied by a professionalarborist.

Tree Care and Maintenance from Hendricksen Tree Care

If you have elm trees in your yard, it is important to always be on the lookout for signs of Dutch elm disease.  Make sure you call a professional arborist right away if you believe your tree might be infected so they can properly diagnose and treat your tree in time to save it.

At Hendricksen Tree Care, our professionals can accurately diagnose Dutch elm disease, as well as other tree diseases, and put together a plan for treatment.  We will discuss your treatment options with you and help you choose the right option based on the situation and your budget.  We take great pride in caring for your trees and we will try treating your tree before resorting to removal.  Our arborists at Hendricksen Tree Care are available to treat and diagnose trees in Arlington Heights, Northbrook, Mount Prospect, Lake Bluff, Lake Forest, Wilmette, Park Ridge, Libertyville, and the surrounding north and northwest Chicago suburbs.

Watch for the next installment of our series on the common tree diseases and insect pests in the Chicago area.