Invasive Disease: Oak Wilt

Why is oak wilt a problem?

Oak wilt is capable of spreading quickly through oak woodlands and killing any oak species of any age – including the several-hundred-year-old oaks you cherish. An oak wilt outbreak in a high-quality timber stand may require immediate harvesting to capture the timber’s value before it is lost and the disease spreads.

Dying trees
Trees infected with oak wilt.

How do I recognize it?

Oak wilt is caused by a fungus that deprives the tree of water. The fungus is not visible, but the symptoms are obvious: the tree’s leaves wilt from lack of water and die. As the disease advances – sometimes quickly – the whole tree, or a cluster of trees, will show symptoms and begin to die.

Which trees are susceptible?

Oaks in the red oak group (black, red, pin, and others with pointed leaf edges) are most susceptible. After they’re infected, these trees drop their leaves rapidly (usually within a three-week period), most often beginning in late June throughout August.

Others will lose a portion of their leaves in September, and then rapidly lose all their leaves just after the leaves emerge the following spring.

Oaks in the white oak group (white, swamp white, bur, and others with rounded leaf edges) are less susceptible. Infected trees in this group will drop their leaves on one or more branches for several years in a row. In other words, trees in the white oak group take longer to die and show more chronic symptoms.

How does the disease spread?

The disease can spread from tree to tree underground through roots that are connected (called root-grafting). It is also spread by beetles that pick up the fungus on their bodies and transport it to wounds on healthy trees.

How do I prevent it?

In your yard, it’s best to avoid pruning, cutting, or wounding oak trees from April through July (or, more conservatively, April to October). In early spring, stop pruning when daytime temperatures reach 50 degrees F.

During a timber harvest, you should restrict the timing of the cut if oak is present in the area to be harvested or in nearby stands, including on your neighbor’s land.

Learn more about oak wilt by viewing: Oak Wilt in Wisconsin Forest Health Fact Sheet or Oak Wilt Management Options