What is oak wilt and how do you treat it?



Oak wilt kills millions of oaks (Quercus spp and cvs. Zones 3-11) in the Midwest and beyond every year, but what is it? Oak wilt is caused by a fungus (Bretziella fagacearumFirst Ceratocystis fagacearum) is a deadly fungal disease that threatens oaks in residential environments and forests. The oak wilt fungus grows through the water-conducting system (xylem) of the infected oak and clogs the vascular system, causing significant symptoms and in many cases rapid decline and death. So far, oak wilt has been identified in more than 20 US states, and the first in Ontario in May 2023.

Northern red oak after oak wilt infection
Oak wilt mortality due to acorn wilt on northern red oak. Photo: Joseph O’Brien, USDA Forest Service, Bugwood.org

What types of oak trees cause oak wilt?

The disease is fatal to the red oak group, which includes the following common Midwest species with pointed leaf lobes:

  • Scarlet oak (Q. coccineazones 4-9).
  • Pine oak (Q. palustriszones 4–8).
  • Northern red oak (Q. Rubrazones 4–8).
  • Black oak (Q. velutinazones 3-9).

Oak wilt can kill a member of the red oak group within one to two months. Trees in the white oak group, which have rounded leaves, slow down the disease and rarely die in one season. They may eventually survive the infection. The white oak group includes these common Midwest species:

  • White oak (Q. Albazones 3-9).
  • Swamp white oak (Q. Two colorszones 3–8).
  • On the oak (Q. Macrocarpazones 3–8).
  • Post oak (Q. stellata(zones 5-9)

Research has shown that oak is considered intermediate in sensitivity.

What do symptoms look like?

Symptoms of the disease, usually seen between June and September, include wilting and early death of single branches. Leaves on affected branches often turn bronze, tan or dull green, with symptoms starting at the tips and outer margins of the leaves. Leaves can fall off the tree, bend or fall, even when green. These symptoms are more common in the months of July and August. Aggravation and spread of symptoms are much more pronounced and with a marked reduction.

How does oak wilt?

Grafting of a live oak root causes oak wilt.
Oak wilt can spread through the roots between trees similar to this live oak (Quercus Virginia) root transplantation. Photo: Paul A. Mistretta, USDA Forest Service, Bugwood.org

The spread of the disease from infected trees to healthy trees occurs in one of two ways. Naturally grafted root systems are not uncommon for adjacent oaks that typically share a communal system, allowing the disease to move quickly from tree to tree. This is the most common way the disease spreads between nearby oak trees. Additionally, some sap-feeding beetles can become vectors and carry spores from infected trees to new host trees. Beetles are attracted to “fruity” odorous fungal mats in infected trees and pick up spores that are then carried to other trees. Healthy oaks that have been cut or injured during the growing season become attractive targets for these beetles, which can also carry infected logs and woody seeds. Trees that have died of oak wilt can live above ground for up to one year and in the root system for up to two years.

What can I do to prevent the spread of the fungus?

Stump of a withered oak tree
The uprooted stump of a withered oak tree. Uprooting the stump and disconnecting the roots can help prevent the spread of oak wilt in situations where plowing or trenching is not possible. Photo: Joseph O’Brien, USDA Forest Service, Bugwood.org

In cases of oak dieback and mass decline, one method of controlling continued spread by root grafts is to mechanically collapse and cut the root system to a depth of 5 feet. This approach has been common in forests when groups of trees show evidence of oak wilt infection. Dead or dying trees should be removed, and infected wood should not be taken off site. Burying or burning wood is acceptable, and any equipment used to cut or handle infected wood must be disinfected and disinfected before leaving the area.

As a general rule of thumb, oak should only be cut during the dormant season because fresh cuts (or wounds) during the growing season in the Midwest (April to October) become a magnet for sap-feeding beetles that transmit the infection. can do Any necessary cuts or wounds made at this time should be immediately covered with a wound paint product to avoid attracting beetles. There is also some promising research examining the feasibility of injecting active fungicides into healthy trees in areas where oak wilt is a concern. Understanding aggressive mitigation and control efforts, as well as being aware of the disease’s symptoms, is critical to helping reduce the spread of oak wilt. There are many other sources for additional information about oak wilt. Check with your state. Cooperative Extension Service office for more information and assistance in confirming a possible oak wilt infection.

To learn more about tree diseases:

Learn how to control this common conifer tree disease.

What you need to know about beech leaf disease.

Controlling pine wilt disease

Mark Dwyer is the Garden Manager of the Edgerton Hospital Healing Garden in Edgerton, Wisconsin and runs Landscape Prescriptions through MD.



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