Oak wilt is an aggressive vascular disease caused by the fungal organism. The fungus grows on the outer sapwood and restricts the flow of water and nutrients through oak trees. This can quickly kill a tree within a few weeks.
Oak wilt affects all species of oak trees. Oak trees belonging to the red oak family (red oak, black oak, pin oak) are most susceptible to the oak wilt fungus and can die within a few weeks from the infection. Oak trees belonging to the white oak family (white oak, bur oak, English oak) can live with infection for decades.
How to report oak wilt
If you find this disease, or suspect it, it must be reported to the Canadian Food Inspection Agency (CFIA). The City of London is supporting the CFIA with early detection of the disease in Canada and asks residents that have concerns with an oak tree that dies suddenly or appears to be showing signs or symptoms of oak wilt to contact the CFIA or to the City at ForestHealth@london.ca.
Causes
Oak wilt can be spread in different ways:
- contact with native sap feeding beetles (also known as Nitidulidae) as they move from tree to tree to feed
- by moving firewood from infected oak trees
- by grafted root systems below ground between nearby infected and uninfected trees
Signs and symptoms
The most noticeable symptom of oak wilt will be a sudden wilting and premature dropping of leaves. Other signs and symptoms include discolouration and bronzing of foliage starting from the outside margins of the leaf, and the development of fungal mats appearing on the trunk or larger branches. These mats typically appear in the spring after a tree is infected and can be associated with vertical cracks that are created by the outwards pressure of the mats. The fungal mats produce a sweet smell described as being identical to Juicy Fruit™ gum. Beetles are attracted to these fungal mats and may carry the fungal spores to the next host tree.
Preventing oak wilt
We can all help with prevention and early detection of this devastating disease.
Don’t move firewood
The oak wilt fungus can stay alive on cut firewood for up to year. If an infected tree was cut down for firewood, it should be burned or disposed of by a qualified professional as quickly as possible. Taking firewood to a camp or cottage can spread many different pests and diseases to other areas of the province.
Do not prune oak trees between April and November
The sap beetles may be in flight anytime between the start of April to the end of November. These sap beetles are attracted to the sweet-smelling fungal mats located under the bark of infected oaks, potentially carrying the fungal spores to oak trees with open wounds from pruning cuts. If the work is an emergency, only prune the branches necessary to make the area safe. Immediately after the wound is created, paint the area with a non-toxic pruning paint or even latex paint to provide a barrier.
Be on the lookout for unusual changes
Give your oak tree special attention and observe it during the seasons for changes. Does it look like it is suffering from drought when there is not one? Oak trees should have their leaves in the spring and summer months. Green or browning leaves on the ground in July and dropping early is a common sign of oak wilt.
Contact an arborist
It is good practice to have your trees inspected yearly. An arborist may be able to see symptoms in oak trees that would not stand out to a homeowner.
Oak trees make up about 1% - 2% of London’s urban forest. There are an estimated 8,000 oak trees with about half being red oak types and the other half being white oak types.
The City follows best practices and schedules routine non-emergency pruning of oaks outside of the months between April and July during the active period of sap feeding beetles.
Although oak trees make up a relatively small percentage of the urban forest they are a very important “key” species, usually living for centuries and growing to a large size. They provide food, shelter and a place to live for hundreds of other species of native wildlife. Many of London’s older mature trees are oaks which contribute to a diverse tree canopy cover.