City of Lake Forest, IL

Oak wilt is a serious fungal disease that affects all types of oak trees, especially oaks in the red oak group. 

Oak wilt can be difficult to diagnose and usually requires a laboratory test for a positive confirmation. Contact a certified arborist ASAP if you suspect any of your oak trees are showing signs of any of the symptoms listed below.

The Forestry Section has compiled some background information on oak wilt, how to identify it, and steps you can take to prevent it.

What is Oak Wilt?

• Oak wilt is a vascular disease caused by the fungus Caratocystis fagacearum.
• Oak wilt is found in all counties of Illinois and close to twenty other states.
• The Red Oak group is highly susceptible to the disease. Red oaks typically die within weeks to months after infection.
• The White Oak group show some resistance to oak wilt.
• Oak wilt spreads slowly via sap beetles, bark beetles, and root grafts.
  • Root-graft transmission is responsible for the majority of new infections.
  • Sap and bark beetles are only responsible for minimal disease transmission.
• Oak trees are most susceptible to the fungus in the spring and summer months.

 Identifying Oak Wilt

• Once infected, the tree will wilt from the top portion of the canopy causing the leaves to turn a dull green to bronze color, starting at the tips or outer margins. Defoliation will then occur. Some trees leaves will turn brown and stay attached to the tree through the winter months.
• Fungal mats/pads can develop on red oak trees
• Other diseases/insects that cause similar symptoms to oak wilt
  • Two-lined chestnut borer
  • Bacterial leaf scorch
  • Anthracnose
  • Drought

Preventing Oak Wilt

• Since oak wilt resembles other diseases, laboratory confirmation is important. Contact a certified arborist for testing.
• Follow the City’s Tree Preservation Ordinance by pruning oaks only during the winter months, November 1 through March 30.
• Remove infected oak trees. If you remove an infected tree, do not store infected wood on your property. If you choose to keep the wood from an infected tree, cover it with plastic for at least one summer to kill any fugal mats or bark beetles.
• Oak wilt control varies from site to site and can involve both underground and above ground methods. Trenching around infected trees may be necessary to disrupt root grafting in nearby oak trees
• Injections of propiconazole are recommended as a preventative measure for healthy oak trees.

City Management

The City will continue to monitor our trees for oak wilt during our normal visual surveying for Dutch elm disease and Emerald Ash borer. We have also begun drone canopy assessments in the area of the confirmed positive test. The drone assessments will continue through the spring and summer months.

If you have any questions, please feel free to contact the Forestry Section at 847-810-3564.