NEW PEST ALERT: First Canadian Detection of Oak Wilt In Niagara

FROM THE CFIA: Oak wilt has been found in a residential area in Niagara Falls, Ontario. This is the first confirmed detection of oak wilt in Canada. To help prevent the spread of this detection, movement restrictions are in place on affected properties. We are working with other federal, provincial and municipal governments to survey the area and determine next steps.

What is oak wilt?

Oak wilt is a serious disease pest of oak trees and is caused by the fungus Bretziella fagacearum. It is an invasive disease of unknown origin, and it grows on the outer sapwood of all oak trees.

Below is an aerial view of an expanding oak wilt patch. The disease likely entered from the field and yellowing trees indicate the advancing front.

Photo: City of Ramsey

Recently, Michigan State University began investigating if oak wilt was the cause of sudden death in chestnut orchards, as chestnuts are in the same family as oaks.

How oak wilt kills trees

Oak wilt works by restricting water and nutrient flow, eventually causing death in many oak species. Oak wilt is a vascular disease, meaning it spreads through the tree’s vessels and can thus spread from the tree’s roots all the way to the top of the tree.

Oak wilt affects different species with different degrees of severity. Red oaks, including red, black, and pin oaks are the most susceptible as the spores can reach all parts of the tree. Trees can die within the season. Red oaks are often identified by their pointed lobes. All photos from the Ontario Tree Atlas.

White oaks (except for English oak) and swamp oaks are less susceptible as fungal spore distribution is more restricted. It may take these trees years to succumb to the disease, if they do at all. Bur oaks are reported to have intermediate susceptibility. White and bur oaks are identified by their rounded lobes.

How to recognize oak wilt

  • Leaf bronzing starting from the top of the oak tree, or sudden wilt of a previously healthy oak tree. Oaks will generally hold onto their leaves for much longer into the fall than most deciduous trees. Premature shedding of oak leaves in the spring, summer or early fall is thus a telling symptom of oak wilt.
Mature oak tree with bronzing on the left half of its crown.
Photo: Michigan State University
  • Vertical cracks in the bark of the trunk, caused by fungal mats that grow into “pressure pads” underneath the bark. These pressure pads can be up to 2cm thick and can emit a fruity odor.
Oak tree with outer layer of bark removed. Under the bark is a dark, oval fungal stain with circular growths in the middle.
Photo: Michigan Dept. of Natural Resources
  • Leaf browning in a pattern unique to oak wilt. Browning starts from the leaf tips and there is generally a clear margin between live and dead tissue. There is no leaf curl like you might see in other diseases.
8 oak leaves lying side by side, all discolored brown from the top down and with a clear margin between brown and green tissue.
Photo: Monique Sakalidis, Michigan State University

How oak wilt spreads

A major vector for oak wilt spread is people moving infected wood. Anecdotally, people have brought innocuous loads of firewood onto their property, only to see all the healthy oak trees in the vicinity die of oak wilt within a few short years! Left to its own devices and natural pathways, oak wilt spreads relatively slowly.

There are two major natural pathways for oak wilt spread:

  1. Underground root grafts – an estimated 90% of oak wilt is spread this way. Larger diameter tree roots of certain species will graft together and allow the tree to share nutrients and other resources. Unfortunately, as oak wilt is a vascular disease, this phenomenon becomes a vector for .
Two thick roots attached together like an X.
Example of a root graft. Photo: Wood Geek, CC BY-SA 4.0

2. Spore movement by sap beetles/picnic beetles from sick to healthy trees. These beetles are attracted by the chemicals released by wounded oak trees. They land on the fungal mats and bring the spores to new, healthy trees.

Oval, black and red beetle with two spots on its back.
Photo: Michigan State University

How can oak wilt be stopped?

Unfortunately, a tree cannot be saved once it has been infected with oak wilt. Prevention is thus the best option for dealing with the disease.

  • Don’t harvest, prune, or otherwise harm oak trees from April to August, as the wounds will attract sap beetles; be aware of construction/unintentional tree wounding
  • Do not move firewood/wood! Burn it where you buy it
  • Spread the word!

What to do if you suspect oak wilt on your property?

REPORT suspicious symptoms immediately to any of the following:

Further Resources

Invasive Species Centrer – Oak Wilt

CFIA – Oak Wilt Fact Sheet

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