Ontario Tree Diseases: Dutch Elm Disease

Ontario Tree Diseases - Dutch Elm Disease

There are a number of common Ontario tree diseases that pose significant threats to our province’s tree populations. This article looks at one of the most prevalent and dangerous Ontario tree diseases – Dutch Elm Disease.

Ontario Tree Diseases: Dutch Elm Disease

Affected Trees: all species of Elm in Ontario, but particularly American or White Elm and Rock Elm

Is Dutch Elm Disease the most dangerous of Ontario tree diseases?

Ontario Tree Diseases - Dutch Elm Disease

Out of all the Ontario tree diseases, Dutch Elm disease has proved to be one of the most devastating. Since reaching the province, it has spread rapidly through southern Ontario, killing thousands of native elms. This, combined with an aggressive tree felling project to save remaining elms, has nearly eliminated our entire population of native American elms.

What is Dutch Elm Disease?

Dutch Elm Disease (DED) is a fungal infestation caused by two species of fungi that have decimated the elm population in Canada.

Dutch Elm Disease affects all species within the elm genus (Ulmus). However, Dutch Elm disease has particularly impacted two of the three native elm species in Ontario. These are the White or American Elm (Ulmus americana), known for its towering stature and distinctive vase-shaped canopy and the Rock Elm (Ulmus fulva).

What does Dutch Elm Disease do to trees?

Dutch Elm disease will eventually kill the tree. The disease rapidly spreads and so Dutch Elm Disease has had a devastating effect on both individual trees and entire populations of trees. 

While efforts have been made to manage and slow the spread of the disease, vigilance and proactive measures are crucial to protecting the remaining elm populations.

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What causes Dutch Elm Disease in Ontario?

Dutch Elm Disease (DED) is caused by two closely related species of pathogenic fungi – Ophiostoma ulmi and Ophiostoma novo-ulmi. The latter is the most aggressive of the two and is the main cause of Dutch Elm Disease in Ontario today.

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How does Dutch Elm Disease kill trees?

The fungus damages the water transporting cells within the tree. This blocks the system of transporting water around the tree starving it of the water and nutrients it needs to live. Once infected with Dutch Elm Disease, the tree will rapidly decline and eventually die. 

Is there a cure for Dutch Elm Disease?

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There is no cure for Dutch Elm Disease, although treatment may stop the spread of damage within an individual tree. Damaged tissue cannot be repaired, so whether the tree remains viable will depend on how far the disease has spread. However, due to the risk of the disease spreading to other elms, the best decision is often to remove any infected trees.

Where did Dutch Elm Disease come from?

Dutch Elm Disease originated in Europe at the beginning of the 20th Century. It is thought that it reached North America around 1930 through the importation of affected lumber, making its way to Eastern Canada by 1940. It is now established in every province of Canada except British Columbia and Alberta. It is found in some areas of eastern and northern Ontario, but it has been most widespread in southern Ontario, where the disease has killed the majority of native elms.

Ontario Tree Diseases Dutch Elm disease

Can Dutch Elm Disease spread to other trees?

Dutch Elm Disease primarily affects all species of elms, native and non-native. It has been known to spread to other tree species, but this is not common.

How does Dutch Elm Disease spread?

The fungus that causes Dutch Elm Disease is spread by three routes – root grafts, spread of infected elm material and being transported from infected trees to healthy trees by beetles. 

Root grafts naturally happen when elm trees grow close together and their roots become intertwined. Healthy trees growing within about 7 meters of infected ones are very likely to get infected. Once the fungus gets into a tree, it spreads within the stems and roots of living elms.

Fungal spores or beetles carrying fungal spores can be transported by infected elm material. 

The third way DED is spread is by insects. The beetles that will carry the fungus that causes the disease are attracted to weakened trees infected by DED. They burrow into the bark, creating egg galleries under the bark to lay their eggs. The larvae feed on the tree’s vascular tissue where the pathogenic fungus is present. Once the beetle’s life cycle has been completed and the pupae have become adults, they will fly onto healthy elms to feed and breed, taking the spores of the fungus that cause the disease with them. 

What does the Dutch elm disease beetle look like?

If beetles are present you may see tiny exit holes on the bark or sawdust indicating that a beetle has burrowed into the tree. You may also find sawdust on the bark, indicating burrowing beetles. 

There are three different types of beetle that spread Dutch Elm DIsease. These are the smaller European Elm Bark Beetle (Scolytus multistriatus), the banded elm bark beetle (Scolytus schevyrewi) and C (Hylurgopinus rufipes).

European Elm Bark Beetle

The European Elm Bark Beetle is a small insect around 2 to 4 millimeters long. It has an elongated, cylindrical shape with a dark brown to black coloration. The distinguishing feature of SEEBB is its body, which is covered in rows of small pits or striations along its length, giving it a striped appearance. Its head is equipped with powerful mandibles for burrowing into the bark of elm trees. These features distinguish it from other beetles and contribute to its role as a vector for Dutch Elm Disease.

Banded Elm Bark Beetle

The Banded Elm Bark Beetle typically measures around 2 to 3 millimeters in length. It has an elongated body with a dark brown to black coloration. The distinguishing feature of BEBB is the presence of transverse bands or stripes across its elytra (wing covers), giving it a banded appearance. These bands are often lighter in color than the rest of the body.

Native Elm Bark Beetle

The Native Elm Bark Beetle is 3mm long and very dark brown to black in color. Its body has a rough surface covered with short, stiff yellow hairs. 

How to identify Dutch Elm Disease – what does Dutch Elm Disease look like?

Ontario Tree Diseases trees affected by Dutch Elm Disease

One of the reasons why Dutch Elm Disease has been one of the most destructive of Ontario tree diseases is that it is hard to detect in the early stages of an infestation. 

The beetles that spread the disease are very small and not easily visible when they take up residence.

In the early stages of the disease the damage the fungus causes is inside the tree, so there may not be any external signs of disease. As the disease progresses, leaves will wilt and yellow as they are starved of water and nutrients. It is only in these later stages that there are visible signs and by the time whole sections of the tree turn brown and branches die it is too late.

Early Dutch Elm Disease Symptoms

Symptoms first appear in June or early July. The earliest symptom is often the wilting of leaves at the ends of individual branches. Leaves then begin to turn yellow or brown and curl but stay attached to the tree so that you start to see bare, brown areas in the canopy. This is called “flagging”.

You can also see signs of Dutch Elm DIsease under the bark of infected branches. Healthy wood under the bark is cream in color.  A diseased tree will display dark brown or red streaks, signs of where the beetle has burrowed into the tree.

Ontario Tree Diseases trees affected by Dutch Elm Disease

Later Dutch Elm Disease Symptoms

The following year, leaves will not develop to a normal size. Leaves on infected branches will droop, turn yellow and then brown and drop early. Entire branches will die back and eventually, entire sections of the tree will begin to die.

How to Prevent Dutch Elm Disease

Ontario Tree Diseases Dutch Elm Disease Prevention

Beetles are attracted to weakened trees, so a good pruning regime promoting the health and vigor of your trees is essential. Immediately remove any damaged or dead branches. Remove any suspect branches to eliminate breeding grounds for the beetles.

Avoid pruning elms during the growing season (April to October) to minimize the risk of attracting the bark beetle vector that spreads the DED fungus.

Promptly remove and destroy any elm wood showing signs of DED to prevent the spread of the fungus. 

Don’t use elm as firewood or transport elm wood between locations.

Fungicides can be injected into the trunk to halt the infection and protect healthy elms from becoming infected. Insecticides can be used to control bark beetles but their success depends on accurately identifying the particular species of beetle.

Severing root grafts between elms can help slow the spread of DED within a stand of trees.

How to Treat Dutch Elm Disease

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Regularly inspect leaves for wilting, yellowing, or browning, especially in the upper branches. Early detection in individual branches can aid in timely intervention. Promptly remove and destroy branches showing symptoms to prevent the spread of the disease. Remove all dead branches from the site. 

There are fungicidal treatments available to treat Dutch Elm Disease, but these are expensive and have no guarantee of success. Damage that has already taken place cannot be reversed, so to save a tree, the disease should not have advanced too far.

The disease is usually fairly widespread within the tree by the time the first signs of infection show, so it may not even be possible to save the tree even if treated.

Infection is usually fatal within 1 to 3 years, so removing the trees at the first sign of infection may be the most responsible strategy to prevent further spread to healthy trees.

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