Oak wilt

What is Oak Wilt?

Oak Wilt is an aggressive vascular disease caused by the fungal organism Ceratocystis fagacearum (syn. Bretiziella fagacearum). The fungus restricts the flow of water and nutrients through oak trees and causes wilting foliage, quickly leading to tree death. Oak Wilt affects all species of oak trees, however oak trees belonging to the red oak family (red oak, black oak, pin oak) are the most susceptible species to the Oak Wilt fungus, killing a tree within a few weeks. Oak trees belonging to the white oak family (white oak, bur oak, English oak) can live with infection for decades.

The fungal organism grows on the outer sapwood and creates blockages in the tree’s vascular system, killing the tree as important water and nutrients cannot move throughout the tree.

The Oak Wilt fungal organism can be spread through the following ways:

  • contact with native sap feeding beetles (also known as Nitidulidae) as they move from tree to tree to feed.
  • by moving firewood from infected oak trees
  • or by grafted root systems below ground between nearby infected and uninfected trees.

Where is Oak Wilt?

Oak Wilt has not yet been found in Canada, but London is on the front line from this invading disease. Oak Wilt is widespread in Michigan and can be found less than 600 metres from Windsor, Ontario, in Belle Isle, Michigan. That’s only five football stadiums away! London is also strategically located along the 401 corridor where Oak Wilt could make entrance into Ontario. This was the case when the non-native insect Emerald Ash borer affected our region.

In 2020, researchers found Oak Wilt Environmental DNA or eDNA in Ontario.

What is eDNA?

eDNA is DNA, or genetic material that is sampled directly from the environment (water, soil, air, etc.). DNA originates from cellular material shed by organisms from skin, excrement, hair etc.

eDNA can be used as an early detection tool for invasive species as it can be found in the environment at very low levels. The presence of the oak wilt fungus eDNA does not mean that the fungus is currently affecting trees within Ontario, but it does mean that it is possible for the fungus to exist in these tested areas and potentially infect trees.

Signs and Symptoms

The most noticeable symptom will be a sudden wilting and premature dropping of foliage (leaves). Other signs and symptoms include discolouration / bronzing of foliage starting from the outside margins of the leaf, and the development of fungal mats appearing on the trunk or larger branches. These mats typically appear the spring following last years infection and can be associated with vertical cracks that are created by the outwards pressure of the mats. The fungal mats produce a sweet smell described as being identical to Juicy Fruit™ gum. Beetles are attracted to these fungal mats and may carry the fungal spores to the next host tree.

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Oak leaves with oak wilt causing green leaves to turn yellow around edges
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Oak tree with oak wilt causing leaves to turn yellow and fall prematurely

What Can Residents Do?

  1. Education

Learn about Oak Wilt. Having an educated and engaged community is especially important as we can all help with prevention and early detection of this devastating disease.

2. Don’t move firewood.

The Oak Wilt fungus can stay alive on cut firewood for up to year. If an infected tree was cut down for firewood, it should be burned or disposed of by a qualified professional as quickly as possible. Taking firewood to a camp or cottage can spread many different pests and diseases to other areas of the province.

3. Avoid routine pruning of oak trees between April and July... 

This is when sap beetles are active and are attracted to the sweet-smelling fungal mats located under the bark of infected Oaks, potentially carrying the fungal spores to Oak trees with open wounds from pruning cuts. If the work is an emergency than prune branches that are only needed to make the area safe. Immediately after the wound is created, paint the area with pruning paint or even latex paint to provide a barrier.

4. Be on the lookout for unusual changes.

Give your oak tree special attention and observe it during the seasons for changes. Does it look like it is suffering from drought when there is not one? Oak trees should have their leaves in the spring and summer months. If green or browning leaves are on the ground in July and dropping early that is a common sign of Oak wilt. 

5. Contact an Arborist.

It is good practice to have your trees inspected yearly. An Arborist may be able to see symptoms in oak trees that would perhaps not stand out to a homeowner.

What is the City doing?

Oaks comprise about 1% - 2% of our urban forest. According to The City of London’s tree inventory, we have an estimated 8,000 oak trees with about 50-50 makeup of red and white oak types.

The City’s Forestry Operations are following best management practices (BMPs) such as scheduling routine non-emergency pruning of oaks outside of the months of April-July during the active period of sap feeding beetles.

Although oaks make up a relatively small percentage of the urban forest they are very important “key” species, usually living for centuries and growing to a large size, providing food, shelter and a place to live for hundreds of other species of native wildlife. Many of London’s older mature trees are oaks which contribute to a diverse tree canopy cover.

How to Report Oak Wilt

By law, finding this disease, or suspecting it, must be reported to the Canadian Food Inspection Agency (CFIA). The City of London is supporting the CFIA with early detection of the disease in Canada and asks residents that have concerns with an oak tree that dies suddenly or appears to be showing signs or symptoms of oak wilt to contact the CFIA or the City's Urban Forestry division at treeprotection@london.ca

Report Oak Wilt CFIA

Last modified:Tuesday, September 28, 2021