Oak Wilt (Bretziella fagacearum)
French common name: Flétrissement du chêne
Oak wilt is a vascular disease of oak trees, caused by the fungus Bretziella 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 present in Canada, but it occurs in 24 U.S. states with close proximity to Ontario. The origin of the fungus is unknown.
*Previously classified as Ceratocystis fagacearum.
The Invasive Species Centre’s Oak Wilt Wire is a quarterly digital newsletter focused solely on oak wilt news. This newsletter will highlight oak wilt in the news, interesting new research, new developments in prevention and management, community oak wilt champions, and updates from oak wilt experts. Tips and trivia are also included to keep you sharp on oak wilt knowledge! If you consider yourself an oak wilt champion in your community and would like your work to be highlighted, please contact us and share your stories!
The fungus Bretziella 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).
Oak wilt is caused by the fungus Bretziella 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. In white oaks, however, the distribution of fungal spores is restricted to only 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 Bretziella fagacearum fungus tends to survive on the above-ground parts of the infected tree for up to one 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
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
- Wilting and bronzing of the foliage, starting from the top of the tree and moving down
- Discolouration 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.|
The following Canadian Food Inspection Agency (CFIA) plant protection policies relate to oak wilt caused by the fungus Bretziella 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.
If it arrived in Canada, oak wilt could have large impacts on our oak population. As a result, the Canadian Food Inspection Agency currently regulates the importation of oak materials in an attempt to prevent the introduction into Canada (OFAH/OMNR Invading Species Awareness Program, 2012).
A stand of oak trees showing the devastating effects of oak wilt and oak decline.
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 this species 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, these economic benefits to homeowners would be lost.
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, the elimination of oak trees would mean a reduction of these services.
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 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.
The impact of 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 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.
Preventing oak wilt from entering and establishing itself in Canada is the best way to protect oak trees. To assist in preventing establishment, follow these tips:
- Don’t move firewood, as it may contain live fungal spores.
- Don’t 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 signs and symptoms.
- Communicate with others about the potential threat.
- Report summer leaf fall and sudden die off of oaks.
Read more about prevention at www.forestinvasives.ca.
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 their area.
High risk areas of Ontario and Quebec are under surveillance. The map below indicates areas that were surveyed in Ontario and Quebec, green arrows indicating that oak wilt was not detected in these areas.
Map: CFIA areas of concern for oak wilt in Ontario, Canada, green points indicate CFIA survey sites that were negative; CFIA, 2018b. Click for larger image.
There are several signs and symptoms that may indicate an infection, however, laboratory confirmation using one of the following methods is required for a definitive diagnosis (Llewellyn & Kurzeja, 2017):
- Culture Wood Tissue on Selective Agar Media
- Polymerase Chain Reaction (PCR)
When sampling, 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.
If you have found a suspect oak wilt, download and complete the following sampling protocols:
- Survey Guidelines for Oak Wilt (Ceratocystis fagacearum ) – ENGLISH
- Lignes directrices pour les enquêtes sur le flétrissement du chêne(Ceratocystis fagacearum)- FRANÇAIS
Suspected sightings should be immediately reported to the CFIA email@example.com. CFIA will dispatch a technician to aid in your survey and take the sample directly.
Read more about detection at www.forestinvasives.ca.
There are several methods currently being used to control the active spread of oak wilt in the U.S.:
Removal of infected trees
Infected or dead oaks that have been diagnosed 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).
Breaking root connections
The 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 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 to prevent 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.
Read more about response and control at www.forestinvasives.ca.
The Invasive Species Centre aims to connect stakeholders. The following information below link to resources that have been created by external organizations.
Oak Wilt: People and Trees, A Community Approach to Management
(large download – entire contents of a CD created in 2003)
Wisconsin Oak Wilt Guide
Oak Wilt in Michigan’s Forest Resource
The season for oak wilt is about to begin. A bit of knowledge might prevent a lot of heartache. – Bill Cook
Protecting oaks from oak wilt disease starts with restricting times trees are pruned to the cold weather season. – Bob Bricault
Proper diagnosis of oak wilt disease may prevent loss of surrounding trees. – Bob Bricault