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. Once a tree has been infected, 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 are 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 100 feet per year 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 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, cover the spot with a special tree wound dressing or latex paint to seal it off. And do it immediately, as it takes sap beetles as little as half an hour to find the new spot and start feeding on it.

Do not transport firewood of unknown origin—fresh firewood from infected trees can pose a danger 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.

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

Unfortunately, if there are other oak trees around, limiting the spreading below ground is virtually impossible for homeowners because it requires extensive digging and trenching to sever the roots; in that case, you’d need to hire a professional excavator to do the job.

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.