Tree Diseases

The forests in Eastern North America do not resemble what they were over 100 years ago. Invasive species from Europe and Asia have decimated the forests that once dominated this region, and it's not over. First the Chestnut, then Elm, now Ash. There are also threats to Hemlock, Beech and Maple.

Chestnut Blight
Chestnut trees once accounted for 25% of forest trees in North America. The trees were deeply rooted in the culture of the local people. The nuts were sold, creating income, and fallen nuts provided food for domestic animals such as pigs. The wood is resistant to decay, and was once prized for fencing, furniture and flooring. Here is an excellent video describing the Chestnut Tree and the culture of Appalachia.

Greatest Forest Loss in History

Chestnut blight was accidently brought from Asia in 1904. In thirty years, 99% of the Chestnut trees were killed, greatly altering the ecosystems in which they grew. Today, only a very few large trees survive. Many healthy Chestnuts were cut down in hopes of preventing the spread of the disease. This eliminated any trees that may have had natural immunity and contributed to the demise of the Chestnut.
Above: A beautiful flowering Chestnut planted at the former St. Williams Forestry Station.  Below left: The same tree two years later showing die-back. The tree was removed the following year. Below right: A large canker that formed near the base of the trunk.
For more information about Chestnut Blight, go to:
Canadian Chestnut Council

Recommended Book
Freinkel, S. (2007). American chestnut: The life, death, and rebirth of a perfect tree. Berkeley: University of California Press.


Dutch Elm Disease
Dutch Elm Disease was accidentally brought to Canada in 1944, via wooden crates. White Elm are the most affected by the disease. Once a tree is infected, nothing can be done. Preventative pesticides can be used but are costly. Development of a disease resistant hybrid has been unsuccessful.

White Elm trees once lined many streets in Southern Ontario. Today, only a few stately Elms can still be spotted, indicating some immunity to the disease. But occasionally, some of the surviving large Elm still succumb to Dutch Elm Disease.

Elm trees produce abundant seeds every year, and the seeds easily germinate. This has helped Elm to survive.

For more information about Dutch Elm Disease:

Dutch Elm Disease History  


Emerald Ash Borer
Emerald Ash Borer is an invasive beetle accidently brought to North America from Asia in shipping boxes in 2002. It has quickly spread killing millions of Ash trees. Blue Ash has shown some immunity, while Black Ash is the most susceptible.

The tell-tale marks of
Emerald Ash Borer

There is a pesticide that can be used to control Emerald Ash Borer, but it is very expensive, and must be administered every two years. Unfortunately, many non-infected Ash trees have been cut down as a preventative measure. This eliminates trees that may have had a natural immunity.

Above: A row of White Ash at Rock Point Provincial Park in 2016.  Below: These trees were removed in 2017, victims of the Emerald Ash Borer.

When the Chestnut and Elm trees were killed out by their respective diseases, a common replacement tree was White Ash. For example, Chicago has streets of Ash, now endangered by Emerald Ash Borer. But they have not given up on the Ash tree! They have a very good website with  encouraging news about saving Ash trees and information about the Emerald Ash Borer.

Save the Ash Tree Coalition 


Asian Long-horned Beetle
(Affecting Maples, Birches, Poplars and Willows)

The first infestation in Ontario was found in 2003. Unlike other diseases of trees, the Long-horned beetle affects multiple types of trees. It is the larvae that cause the most damage, by chewing tunnels which weaken the tree and cutting off transport of nutrients and water.

So far, the infestations in Ontario have been kept under control.

For more information:
Asian Long-horned Beetle in Ontario 

Butternut Canker
This devastating fungus spreads quickly and has infected most Butternut Trees in North America. Its origins are not known, but suspected to be from Asia. Once a tree is infected, not much can be done, and the tree eventually dies. This can take up to 40 years, with younger trees dying more quickly. About one third of Butternut in Ontario have already died due to the canker, and many of the remaining Butternut are infected.

Visit this site for more information:
Forest Invasives Canada


Beech Bark Disease
American Beech is susceptible to two invasive species; the insect Beech Scale which damages the bark, and a canker fungus which invades the wounds caused by the Scale. Both species originated in Europe and were brought to Canada on European Beech plants. Many Beech die from damage within a few years, while others are weakened and subject to damage by winds. The long term effects on populations is not yet known, as Beech Bark Disease has only been in Ontario since 1999.

For more information about Beech Bark disease:
Beech Bark disease in Ontario  


Hemlock and the Woolly Adelgid
Woolly Adelgid is an invasive insect from Japan brought to North America in the early 1950s. It is widespread in the United States, but only small populations have been discovered in Ontario beginning in 2012.

The Woolly Adelgid eats sap and injects toxic saliva that causes needle loss and inhibits new growth. In as little as four years, the tops of Hemlock trees can be dead. This also changes the ground below so that Hemlock saplings (requiring moist shade) may not survive.

The insect can be controlled with insecticides, but this is not practical for large-scale forest situations.

Here is a good documentary about the Woolly Adelgid. Cook Forest is a beautiful old growth Hemlock forest in Pennsylvania:

Cathedral: The Fight to Save the Hemlocks of Cook Forest

For more information about Woolly Adelgid in Ontario:
Canadian Forest Service


Dogwood Anthracnose
Anthracnose is an Asian fungus that was confirmed in Ontario in 1998. It starts on the leaves in moist condition, then causes dieback and eventually tree death. The disease can be somewhat controlled by cutting back damaged branches and using a fungicide.

With small numbers of Eastern Flowering Dogwood in Ontario, the Anthracnose fungus has contributed to the status of endangered species in Ontario.

For more information on the Anthracnose fungus in Ontario:
Dogwood Anthracnose