How to Identify and Prevent Oak Wilt

A Deadly Fungus That Can Quickly Kill Oak Trees

Oak tree infected with oak wilt the year before, with some new growth about 25 feet up the tree​

rcmickel / Flickr / Courtesy of photographer

Oak wilt is a disease caused by a fungus. It starts with fall coloring out of season—the leaves are wilting and browning, and within a few weeks, the tree is dead. The good news is that oak wilt is preventable and treatable. The bad news is that it's such an aggressive disease, it's almost always too late to save the tree by the time a homeowner or gardener notices. At that point, there is nothing you can do other than remove it promptly to prevent the disease from spreading to nearby healthy oak trees. That’s why it’s so important to know the symptoms of the disease.

How to Identify Oak Wilt

The first sign of oak wilt is a rapid wilting and browning of the leaves in late spring to early summer, followed by the leaves dropping. This starts at the top of the tree, which is not easy to inspect in tall trees. Use binoculars to take a closer look.

The browning of the leaves starts at the margins of the leaf at the apex, which is the tip of the leaf. From there it progresses downwards along the margins towards the midrib and the leaf stem. The browned leaves drop to the ground.

Another symptom is vertical cracks in the bark with mat-like fungal spores underneath. The bark swells and eventually ruptures from the pressure created by the growing fungus.

In the following you might also notice sap beetles. While feeding on the sweet-smelling fungal mat, these beetles, about one-quarter to one-eighth of an inch, pick up the spores and thereby spread the disease further.

How Oak Wilt Gets Into a Tree

Sap beetles are attracted by wounds in a tree, such as breakage from a storm, pruning cuts, and involuntary mechanical damage. When a sap beetle feeds on an infected tree and then moves to a healthy tree with a wound, the disease is transmitted to the vascular tissue of the other tree.

The tree then tries to fend off the intruder by walling off its cells, which causes leaves to wilt and brown, and eventually killing branches and the entire tree. Oak wilt is a rapidly progressing disease that usually kills a tree within just a few weeks.

A second way how the disease can spread is through natural root grafts, which occur when roots of trees of the same species, such as two red oaks, grow together. Oak wilt fungus can move more than 50 feet through the root systems of interconnected trees. That makes the disease especially challenging to control once it’s in a neighborhood.

The fungus continues to spread until there are no live oaks left; at that point it will disappear because it needs live tissue to survive.

Affected Oaks

Oaks that belong to the group of red oaks (pin oak, black oak, northern red oak) are more susceptible than white oaks (white oak, bur oak, swamp white oak). One reason why the disease spreads faster in red oaks than in white oaks is because the root systems of red oaks tend to graft together so the fungus can spread more easily.

Red oaks are likely to die within a few weeks, whereas white oaks can hang in there for longer, a year or more.

How to Treat and Prevent Oak Wilt

There is no way to save an infected oak tree; the only way to deal with oak wilt is prevention.

Since the fungus enters the tree through wounds, avoid injuring oaks between April and August. That means no pruning during those months, and careful maneuvering around oak trees with your lawnmower and other power tools.

If an oak gets injured during that time period, you should support the tree in its healing by minimizing stress wherever possible. Cut the broken limb in question just outside the branch collar to preserve the tree's immune response and give it the best chance at recovery. Ensure that the tree has adequate water and nutrients. If the would is severe, or there are other trees in the area with oak wilt, a preventative treatment of a fungicide may be a good idea. (A certified arborist can make a recommendation for your tree and apply it via injection.)

Don't paint or dress the wound on the tree; it will heal best on its own. Trees can compartmentalize wounds, closing the area off from the rest of the tree and outside. If you dress the wound, it won't be able to compartmentalize. Applying a wax as a wound dressing may keep the oak wilt away, but but as soon as the wax wears off, the tree will be vulnerable to disease with no ability to protect itself; it is much harder for a tree to compartmentalize a wound that has been previously dressed.

Even if the wound does become infected, if the tree can successfully compartmentalize, it will starve any pests in the wound of oxygen, thus suffocating them.

If you have an oak tree that died from oak wilt, remove it promptly to prevent the disease from spreading.

Warning

Do not transport firewood of unknown origin—fresh firewood from infected trees can pose a danger to other trees because it might still contain the live fungus. As additional precautions, do not store fresh firewood near your oak trees, and cover the firewood tightly with tarps. The fungus cannot survive in dry firewood, and burning it cannot transmit the disease.

Unlike houseplants with root rot, you cannot sever the affected roots of a tree with oak wilt. Severing mature roots poses a safety risk and causes the tree to become unstable in high wind events. It could cripple the tree, or maybe even kill it. Only a certified arborist should excavate roots, and they use a highly specialized air tool that blows soil away and avoids damage to the tree.

That said, severing affected roots is an uncommon treatment for oak wilt. If you have a certified arborist look at the tree, it is likely they will apply a fungicide if the disease is caught early enough. A fungicide can also sometimes be used as a preventative measure as well.

Distinguishing Oak Wilt From Other Oak Diseases

Brown leaves on your oak tree does not necessarily mean the has oak wilt, it could also be anthracnose, scorch, chlorosis, drought, root stress, or a number of other diseases.

Article Sources
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  2. University of Minnesota Extension Office. “Oak Wilt in Minnesota.” Umn.edu. N.p., n.d. Web

  3. Bob Bricault, Michigan State University Extension. “Oak Wilt: Diagnosing and Preventing - MSU Extension.” Msu.edu. N.p., n.d. Web. 

  4. Erler, Emma and Ask UNH Extension. "Should I cover large pruning woulds with a tree wound dressing?" University of New Hampshire Extension Office. July 3, 2019. Web.