Oak wilt

In August, I went up to my family cabin in northern Michigan. My grandparents built the cabin in the 1960’s and my family has been going up there ever since. The dry sandy soil of the northern upper peninsula of Michigan is home to trees such as bigtooth aspen, Jack pine, balsam fir, white pine, and several species of oak. In recent years, the large oak trees on the property have been dying off one by one to a fungal disease known as oak wilt (Bretziella fagacearum). This year, several northern red oaks (Quercus rubra) had to be felled to avoid them crushing the cabin. Death of trees due to oak wilt is an ongoing problem not only for our family, but for much of the eastern US.

Ideally, diseased oaks should be cut in fall or winter to avoid the spread of oak wilt

What is oak wilt?

Oak wilt is a fungal disease capable of killing oak trees in a matter of just a few months. It was first recorded in the US in the 1940s in Wisconsin, but rapidly spread and is now found in at least 24 states (1). While it is most prevalent in the midwest and Texas, the range of oak wilt appears to be expanding and observations occur in new counties every year.

Browning leaves that fall at unexpected times of the year may be a sign of oak wilt

Typically, the first signs of oak wilt are browning leaves that fall at unexpected times of the year. The fungus effects the vascular system which controls the flow of water and nutrients in the tree. You can often see a ring of dark discoloration under the bark in cross section. You may also see cracks appearing in the bark as fungal mats underneath the surface being to grow and bulge, usually in the spring. Trees can die as early as a few weeks after signs of oak wilt first appear (2).

Discoloration of vascular tissue (black stuff under the bark) in a diseased oak

Not all oaks are equally effected by oak wilt. In general, oaks of the North Eastern US are classified into two broad groups; red oaks and white oaks. Leaves of the red oak group (ex. black oak, pin oak, red oak, etc.) tend to have pointed tips sometimes ending in a bristle and their acorns take two years to ripen. White oaks (Ex. bur oak, swamp white oak, chestnut oak, etc) have leaves with rounded tips and acorns which ripen their first year. Red oaks are much more susceptible to death from oak wilt in comparison to white oaks, which can often survive infection. White oaks are better at containing the fungal infection cutting off the spread to other parts of the tree (3). White oak wood is often used in barrel making as it is impermeable to liquids. This is due to structures called tyloses which it forms in its vascular tissues. Tyloses block the passage of substances in the vascular tissue and help prevent the spread of fungus throughout the tree. Red oaks do not form tyloses and are more porous, so it is easier for the fungus to spread throughout the tree (4).

This white oak near our cabin has thankfully been untouched by oak wilt

How does it spread?

This fungal disease spreads through connections between the roots of trees or by insect vectors. Insects may be attracted by the fruity smell of the fungus and land on open wounds of infected trees, collecting fungal spores which may be carried by the insect to healthy trees. Because of potential spread from insect vectors, it is recommended not to trim or cut oak trees that are known to be infected in spring or summer when insects are most active. Trimming infected trees in fall or winter is preferred for avoiding the spread of this fungus. If an infected tree is damaged, you can paint over the wound with tree wound sealer to keep insects away.

Once a tree is infected with oak wilt, the best course of action to stop the fungus spreading to nearby trees through roots is by removing the tree and severing the root connections underground using a trencher (5). Healthy oaks can be treated with fungicide as a preventative measure (6).

Acorns are important food sources

Acorns are important food sources for many woodland animals like squirrels, chipmunks, and deer. Oaks also host a variety of insects, providing food and habitat for birds. Humans use the wood for timber, furniture, firewood, and many other things. North America has seen several drastic changes in forest compositions such as the die off of the American Chestnut due to chestnut blight or, more recently, the Emerald Ash Borer wiping out large numbers of ash trees in the eastern US. Hopefully, oak wilt does not effect our forests on the same level. By taking preventative measures highlighted above, we may be able to contain this disease.

Sources: 1, 2, 3, 4, 5, 6

More oak wilt photos:

Fungus on a dead tree
Felled oak
The buds on this branch never developed into leaves
This red oak seems healthy, but we will have to monitor it for signs of wilt

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