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Oak Wilt

French common name: Flétrissement du chêne

Scientific name: Ceratocystis fagacearum (Bretz) Hunt

Order: Microascales

Family: Ceratocystidaceae

 
Oak wilt is a vascular disease of oak trees, caused by the fungus Ceratocystis fagacearum. The fungus grows on the outer sapwood of oak trees restricting the flow of water and nutrients through the tree and causing the foliage to wilt. While some trees can recover from the infection, the fungus can eventually cause the tree to die.

Oak wilt is not yet present in Canada, but it occurs in 23 U.S. states with close proximity to southern Ontario. The origin of the oak wilt fungus is not known.

 
Photo: Paul A. Mistretta, USDA Forest Service, Bugwood.org 

Click on the following link to download a printable version of our oak wilt fact sheet:
Oak Wilt Fact Sheet



Learn about Oak Wilt

Science

Physical Description

The fungus, Ceratocystis fagacearum, develops sporulating mats between the sapwood and bark of dead red oak trees. These mats, called "pressure pads" by technicians, can vary in size, are generally produced in late fall or early spring, and remain visible for two or more weeks. They are usually observed on the trunk or large branches of the tree (CFIA, 2012).


Life Cycle

Oak wilt, a vascular disease, is caused by the fungus Ceratocystis fagacearum, which grows on the outer sapwood of oak trees. The main period of infection occurs during the spring. In the case of red oak, fungal spores can be transported through all parts of the tree as the tree nears death. On the other hand, in white oaks, the distribution of fungal spores is more restricted, occurring only in the xylem of the current year’s growth (CFIA, 2012).


Sporulating mats that develop on the bark of dead red oak trees attract sap beetles to feed. The sticky spores from these mats adhere to the insects’ bodies, after which they are carried to healthy trees and deposited in tree wounds.


 

 
A sap beetle (Nitidulidae) on an infected oak tree that was attracted to
sap seeping from the bark. 
 

The fungus can spread naturally in two ways: above-ground or below-ground.

Above-ground: when a diseased red oak dies, the fungus produces sporulating mats on the dead tree (these mats are not produced on living or white oaks). Nitidulid beetles, or bark beetles, then feed on these fungal mats and pick up spores on their bodies which they then carry from the infected tree to wounds on healthy trees.

Below-ground: the fungus can travel from infected trees to healthy trees through any interconnected roots. The Ceratocystis fagacearum fungus tends to survive on the above-ground parts of the infected tree for up to year after the tree has died. However, the fungus can survive considerably longer than this on the below-ground roots of the tree (CFIA, 2012). In addition, the fungus can be spread artificially, over longer distances, by humans through the transport of infected wood products or nursery stock.

                           

 
 Image: Julie Martinez, Scientific Illustrator 

                                       


 

Host Trees

All species of oak trees (Quercus sp.) have been found to be susceptible to oak wilt, with species of red oak being the most seriously affected (CFIA, 2012).

 

 
Images: Natural Resources Canada, Canadian Forest Service
 

Signs & Symptoms

Signs and symptoms of oak wilt vary depending on the species of oak that is infected.   Red oak species are most susceptible to oak wilt and can die quickly from infection.  White oak species can often survive infection for years with some symptoms and slow tree decline.

Symptoms to watch for:

  • wilting and bronzing of the foliage, starting from the top of the tree and moving down

  • dicolouration of the leaves beginning at the leaf margin and progressing to the midrib

  • premature leaf fall

  • white, grey or black fungal mats, also referred to as "pressure pads", just under the bark that sometimes emit a fruity smell

  • vertical bark cracks in the trunk and large branches as a result of the fungal spore mat exerting outward pressure on the bark.

 

 
Fungal growth on the sapwood of an
infected oak.

Cross-section of an infected oak with severe vascular
streaking. 

 

Distribution

Oak wilt and its causal fungus are currently found within 23 states in the U.S. and its origin is unknown. The fungus has not yet been introduced into Canada, but the proximity of oak wilt in forests of Central-North U.S.A. show that it could easily be spread to southern Ontario (City of Toronto, 2010).  In 2016, oak wilt was found on Belle Isle, Michigan in the middle of the Detroit River, less than 1 km away from the shores of Windsor, Ontario.

             
 
2016 map of oak wilt distribution in Michigan (Michigan DNR, 2016). Oak wilt is currently in close proximity to the Michigan/Ontario border. Ontario is at high risk for oak wilt introduction. 

           

Regulation

The following Canadian Food Inspection Agency (CFIA) plant protection policies relate to oak wilt caused by the fungus Ceratocystis fagacearum

  • D-08-04 - Plant Protection Import Requirements for Plants and Plant Parts for Planting: Preventing the Entry and Spread of Regulated Plant Pests Associated with the Plants for Planting Pathway

  • D-01-12 - Phytosanitary Requirements for the Importation and Domestic Movement of Firewood

  • D-99-03 - Phytosanitary Measures to Prevent the Entry of Oak Wilt Disease (Ceratocystis fagacearum (Bretz) Hunt) from the Continental United States

  • D-98-08 - Entry Requirements for Wood Packaging Materials Produced in All Areas Other Than the Continental United States.

Impacts

If it arrives in Canada, oak wilt could have large impacts on our oak population, so the Canadian Food Inspection Agency currently regulates the importation of oak materials in an attempt to prevent the introduction of oak wilt into Canada (OFAH/OMNR Invading Species Awareness Program, 2012). 

 

 

 
A stand of oak trees showing the devastating effects of oak wilt and oak decline



Economic Impacts

As red oak species are usually killed within one year of becoming infected, their populations could decline fairly quickly, reducing their availability for use by the forest industry. Because red oak is a valuable commercial species within parts of Ontario, the potential introduction of oak wilt into the province could have negative impacts on local forest-based economies.

Oak trees also commonly occur in urban areas on homeowners' properties. Those that are adjacent to homes can help to reduce energy costs by shading the house in the summer and protecting it from wind in the winter. Therefore, if these trees are killed off by oak wilt, these economic benefits to homeowners would be lost.


Ecological Impacts

The introduction of oak wilt could reduce the number of oak trees, especially red oak, which currently grow in urban and natural areas. The loss of these trees could lead to a decline in biodiversity, a reduction in habitat and food for other wildlife, and a loss of the environmental services previously supplied by these trees.

Oak trees also play an important ecological role in stabilizing slopes, limiting soil erosion, and reducing air pollution (OFAH/OMNR Invading Species Awareness Program, 2012). Therefore, elimination of oak trees by oak wilt would mean a reduction of these services.

Further, since oak trees produce acorns and are thus an important mast-producing species (species that produce fruit or nuts for other wildlife species), the loss of oak trees could impact the survival of other forest-dwelling animals by reducing their food supply.


 
 
The acorns produced by oak trees are an important food source for a wide variety of forest-dwelling animals. The loss of oak trees due to oak wilt could have resulting impacts on the survival of wildlife that depend on this resource.


Social Impacts

The impact of oak wilt on social values has already been large in affected regions of the U.S. In urban areas, susceptible oaks trees are abundant, so the loss of these trees to oak wilt has lowered property values and has reduced the contribution to ecological services (such as filtering of air and water) that these trees provide. The same can happen if oak wilt is introduced into urban centers in Ontario and Canada.

Manage

Prevent

Preventing oak wilt from entering and establishing itself in Canada is the best way to protect oak trees. To assist in preventing oak wilt establishment, follow these tips:

  • Do not move firewood, as there may be live fungal spores.

  • Do not prune or damage oak trees between April and July as this is the most vulnerable time for overland spore spread.  If pruning must occur, paint a thin layer of wound paint on the pruning wound immediately.

  • Learn how to identify oak wilt signs and symptoms.

  • Educate others on the potential threat of oak wilt.

  • Report summer leaf fall and sudden die off of oaks.


Read more about Prevention


Detect

Detection is an essential step to stop oak wilt from further spreading if it reaches Canada. Early detection allows for rapid response and control of the disease to protect Canadian oaks. Landowners should contact their local forestry agency to assess and test for oak wilt.

There are several signs and symptoms that may indicate an oak wilt infection, however laboratory confirmation using one of the following methods is required for a definitive diagnosis (Llewellyn & Kurzeja, 2017):

1. Culture Wood Tissue on Selective Agar Media
2. Polymerase Chain Reaction (PCR)

When sampling for oak wilt, wood samples are collected from main stems or branches, with a diameter greater than 6 cm, while the wood is still moist. Culturing takes 8-10 days of incubation while PCR takes 4 business days. PCR is the more accurate and rapid of the two laboratory methods but involves special tools and materials that not all laboratories will have. 

Join Phillip Kurzeja, "The Oak Wilt Guy", on a hunt for oak wilt in the following videos:





 

Read more about Detection 


Respond & Control

There are several methods currently being used to control the active spread of oak wilt in the U.S.A.:

Remove Infected Trees
Infected or dead oaks that have been diagnosed with oak wilt should be removed and disposed of to prevent spore mats from forming. This is done by:

  • debarking, chipping or splitting, and drying the wood;

  • wrapping cut trees in plastic and burying edges;

  • controlled burning or burying of cut trees (USDA, 2011).

Following removal, stumps should be pulled using a backhoe or bulldozer and then immediately flipped, burned or buried. This prevents regrowth and breaks possible root grafts (USDA, 2011).


Break Root Connections
Oak wilt infection can spread between trees through interconnected root systems. Root grafts create an opportunity for spores to spread from infected trees to healthy trees without the need of vectors as with overland spread. Unfortunately, we have no way of knowing which trees have grafted roots so many are sacrificed using this method of control. There are a few methods used to disrupt root grafts:

  • Vibratory plows are often used to break any grafted roots between an area of infection and area without;

  • Backhoes can be used in the same way and can be more effective in rocky soils (USDA, 2011).


Chemicals
Chemicals can often be used in the absence of large machinery. Holes are drilled into the soil of an affected area and pesticides are added resulting in root death of a localized area (USDA, 2011). Another option is to prevent oak wilt spreading to nearby healthy trees by administering a fungicide to unaffected oaks which is said to protect the tree from oak wilt establishment for two years after treatment (USDA, 2011). Under certain circumstances the use of chemical management and prevention methods can be dangerous and expensive options making them uncommon in practice. 

 
 Photo: Michigan DNR, State of Michigan

 

Read more about Respond & Control