Black oak, bur oak, pin oak, red oak, shumard oak, swamp white oak, and white oak—these trees all have something in common. For one, they are all native oak trees in Ontario. Sadly, they are also all susceptible to oak wilt.
Ceratocystis fagacearum is the fungus behind oak wilt disease. You don’t want to see it come anywhere near any of your oak trees. This virulent fungal infection is a deadly killer with little that can be done about it. Infected trees can die within a year of becoming infected and treatment is non-existent. It should be noted that members of the white oak (white, swamp, bur) and red oak (black, pin, red) families react slightly differently. Red oaks tend to die within the year, whereas white oaks take longer to die, but can sometimes stand a chance at recovery. The overall prognosis is grim though.
So what exactly is oak wilt and how does it affect oak trees?
Good question. Let’s look at red oaks first.
For starters, the fungus attacks the outer sapwood of oak trees. When the tree recognizes the presence of the fungus, it cuts off the flow of water and nutrients to the area. This causes leaves to wilt and bronze. It starts at the top of the crown and branch tips, but quickly spreads throughout the tree with leaves continuously dropping throughout the process.
That’s not all though. The tree produces a pressure pad or fungal mat, as it is dying. As the mat grows, it puts pressure on the bark, until it splits. It also emits a fruity scent, which attracts insects that feed on it. Sticky spores attach to oak bark beetles and sap beetles, which are then transported to nearby trees, starting the cycle anew.
Oak wilt is even nastier though, as it comes with a two-prong attack. Not only does it spread via insects, but it also spreads via the tree’s underground root systems. Any nearby oaks with roots that touch are susceptible to the disease, making it that much harder to fight.
While white oaks can contract oak wilt, the spread of the disease is slower and not always fatal. Branches die off one at a time over the season, which can go on for several years. Fungus mats rarely occur, but a dark ring stains the xylem of affected branches.
What Can You Do?
We’re not going to lie. If your oak tree contracts oak wilt, the prognosis is grim. There are no treatment options. You might be lucky and see your white oak survive, but anything in the red oak family will quickly expire. The only option available to us is prevention.
- don’t transport oak products, be it firewood, infected nursery stock, or wood products (especially with the bark attached)
- remove diseased trees as soon as possible
- sever root grafts between infected trees and nearby healthy trees
- avoid pruning in the spring to limit wounds where infected insects can spread the disease (insects enter trees through open wounds)
- if a healthy tree is storm damaged, apply a wound dressing to minimize risk of infection
- ensure all equipment is thoroughly cleaned and sanitized between tree jobs
At present, Oak wilt has not been found in Canada, but it is in many US states, including Michigan. Being aware of what it looks like and how to prevent it is key in limiting the spread of this deadly disease. If we want to prevent oak wilt, we all need to be vigilant in stopping it.