Oak Wilt: An Advancing Threat

Tree leaves loosing colour

Invasive Species and Our Forests

We have a team dedicated to monitoring and managing priority invasive species in the Credit River Watershed. We’re closely monitoring oak wilt, a fungal disease caused by an invasive fungus, which isn’t in the watershed but is making its way north.

Oak wilt is causing damage to forests, natural areas and backyards throughout the northeastern United States. It’s now 500 metres away from the Canadian border. You can help keep this deadly disease out of the beautiful forests of the Credit River Watershed.  

Oak wilt is caused by a non-native fungus, Bretziella fagacearum, that grows in the sapwood under the bark of the tree. It cuts off the flow of water and nutrients. The disease progresses quickly, killing healthy trees within two weeks to a year from the time of infection. All oak trees are susceptible to the fungus but the red oak group (red, pin and black oaks) are especially vulnerable and decline more rapidly once infected.

The disease spreads one of two ways: close range through root to root contact with an infected tree or over longer distances by tiny picnic beetles (Nitdulidae) that feed on sap from tree wounds. After an infected tree dies, the fungus produces spores creating “pressure pads” under the bark that attract picnic beetles and other insects with their sweet smell. The beetles carry the fungal spores to new trees as they feed on sap from open wounds. The wounds provide the perfect entry point for the fungus, continuing the infection cycle.  

Signs and symptoms of oak wilt:

  • Browning or bronzing leaves beginning at the tip and moving back towards the stem, with a clear border between brown and green tissue
  • Leaves near the crown of the tree changing colour
  • Early leaf drop
  • Cracks in the bark
  • Grey or tan mats of fungus under the bark of infected or dying trees
  • Sweet smell coming from tree bark

There are currently no treatment options for oak wilt approved for use in Canada. Our best strategy is to prevent its spread. The Canadian Food Inspection Agency (CFIA) is working hard to prevent the fungus from spreading into Canada and we can help too.

How you can help

  • If you are pruning or removing trees on your property due to safety concerns, construction, to improve access or other reasons, take the following steps to protect trees from the spore-carrying picnic beetles and prevent the spread of oak wilt. Refrain from cutting, pruning, harvesting, or injuring any oak trees between April 1 and July 31. If cutting during this period is unavoidable or a tree is accidentally injured, coat the wound or cut stump with pruning paint or any latex-based paint.
  • Learn to identify oak trees on your property and become familiar with the signs and symptoms of oak wilt
  • Don’t move firewood. Oak wilt fungus may be present in the sapwood or under the bark and can easily spread to new areas
  • Report suspected oak wilt cases to the Canadian Food inspection Agency and to CVC’s invasive species team.

Managing invasive species can feel like a daunting task. However, together we can take steps to stop the spread of invasive species before they become established in the watershed. Learn more about invasive species.

Do you have questions about invasive species? Join the conversation online by following us on Twitter, Facebook, LinkedIn, Instagram and TikTok.

By CVC’s Bryana McLaughlin, Coordinator, Terrestrial Restoration and Management

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