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What You Need to Know About Oak Wilt

Disease Overview

Oak wilt is a fungal disease that is caused by the fungus Ceratocystis fagacearum. Trees that are infected with this disease develop tyloses and gums, which restrict the flow of water and nutrients throughout the tree. The infected trees will then wilt and die.

Distribution

Currently oak wilt is not present in Canada. However, it has been reported throughout the northwestern United States and southwest as far as Texas, as well as in the Great Lakes region. In 2016, oak wilt was found on Belle Isle Island, located 500 metres from Windsor, Ontario (Nienhuis & Wilson, 2018). As such, oak wilt is a real threat to ecosystems in Ontario and Canada. Preventative measures and proactive planning are required.

Tree Species at Risk

Oak wilt affects all species and varieties of oak trees. It has been found that red oak trees are the most vulnerable to oak wilt, and are usually killed within one year of being infected. White oak trees are more resistant to the fungus, meaning they can survive over a year with the fungus , and can sometimes even recover.

Red Oak Trees include: Red Oak, Pin Oak, Black Oak, Shumard Oak, and Hills Oak

White Oak Trees include: White Oak, Bur Oak, Swamp White Oak, Chinquapin Oak, and Chestnut Oak

How Oak Wilt Spreads

Figure 1.The disease cycle of oak wilt by Julie Martinez (2000).

Oak wilt is able to spread in two ways. It can pass from tree-to-tree above ground as well as below ground. Above ground, sap and bark feeding beetles carry the fungus from infected trees to wounds on healthy trees. This occurs when the beetles are breeding or feeding. Below ground, the oak wilt fungus travels from infected trees to healthy trees via interconnected roots. It is much more common for the oak wilt fungus to travel below ground.

Symptoms of Oak Wilt

Oak wilt is characterized by wilting and bronzing of a tree’s leaves. This is highly distinguishable on red oak trees. The bronzing and wilting starts at the top of the tree and spreads down through the rest of the crown. However, the symptoms of oak wilt can vary and are not always visible. White oak trees typically experience the symptoms of oak wilt one branch at a time.

Next Steps

Currently, there is no treatment for oak wilt. Best practice includes preventing the introduction of oak wilt into new areas.  Pruning oak trees between April-July should be avoided as this is when beetles are most active.

The Canadian Food Inspection Agency states the following measures to reduce the number of new infections in an area that is already exposed to oak wilt:

  • Surveillance and removal of diseased trees, removal of potential inoculum sources;
  • Preventing the formation of, or severing existing root grafts between diseased and healthy trees; and
  • Minimizing wounds on oak trees, including pruning wounds, especially during the flight period of insect vectors.

Key Takeaways

  • Oak wilt is a fungal disease that affects all types of oak trees, but especially trees in the red oak family.
  • The oak wilt fungus blocks the ability for oak trees to absorb water and nutrients – quickly killing the tree.
  • Some preventative measures include not pruning oak trees from April to July, since this is when beetles are very active, and monitoring oak trees for any symptoms of oak wilt.
  • Oak wilt is not currently present in Canada, but has been confirmed just across the border from Windsor.
  • It is important to be aware of oak wilt, and how it will affect our forests and urban trees.

Reporting Oak Wilt

If you have a tree that you believe might exhibiting symptoms of oak wilt, contact an arborist, who may be able to provide more information about what is causing decline in your oak tree.

By law, suspecting oak wilt must be reported to the Canadian Food Inspection Agency (CFIA). If you believe that your oak tree may have oak wilt, contact the City of London's Urban Forestry Division or the Canadian Food Inspection Agency for more assistance.

References

Ceratcystis fagacearum (Oak Wilt) – Fact Sheet. (2018). Canadian Food Inspection Agency. Retrieved from http://www.inspection.gc.ca/plants/plant-pests-invasive-species/diseases...

Martinez, J. (2000). How to identify, prevent, and control oak wilt. Retrieved from https://www.fs.usda.gov/naspf/sites/default/files/publications/identify_prevent_and_control_oak_wilt_print.pdf

Oak Wilt. (2018). City of London. Retrieved from https://www.london.ca/residents/Environment/Trees-Forests/Pages/Oak-Wilt...

OFAH/OMNRF Invading Species Awareness Program. (2012). Oak Wilt. Retrieved from: www.invadingspecies.com.

Nienhuis, S. & Wilson, R. (2018). Invasive species in Ontario: The threat, the strategy, and the law. The Forestry Chronicle, 94(2), 97-102.