Oak Wilt Disease
Oak Wilt kills all species of our native oak trees:
- Bur oak (Quercus macrocarpa)
- Northern pin oak (Quercus ellipsoidalis)
- Northern red oak (Quercus rubra)
- Swamp white oak ((Quercus bicolor)
- White oak (Quercus alba)
The City of Maplewood works to control the spread of this epidemic disease through inspection, sanitation and education. The city contracts with a state-certified tree inspector to identify and mark oaks with oak wilt. When an oak with oak wilt is found, the property owner is sent a letter regarding the tree's condition and what needs to be done. Removal is usually required and other measures may be recommended. The city tree inspector works with property owners on how best to manage a stand of oaks that includes an infected oak.
Causes, Signs & Spread
Oak Wilt is caused by a fungus, Ceratocystis fagacearum. When the fungus invades an oak, it grows into the tree's water-conducting vessels (xylem). The tree responds by attempting to wall-off the fungus. These obstructions combined with the fungus block the effectiveness of the water movement in the tree. This blockage leads to the wilting and eventual death of the affected tree.
The 1st sign of oak wilt disease is the yellowing of leaves in an isolated branch in the upper canopy. The leaves dry out quickly and turn a dark green or bronze, beginning at the leaf edge and moving toward the midrib and base of the leaf. Leaves may fall or may remain on the dying branch. The branch progressively dies back. Red oaks and pin oaks usually die within a growing season, while bur oaks and white oaks may take several years to die.
Other diseases, pests, or drought can make an oak tree appear to have oak wilt. The city tree inspector can determine the cause of an unhealthy oak.
Oak wilt is spread by:
- Adult sap beetles (Nitidulidae family) and oak bark beetles (Pseudopityophihorus spp.) when they transport fungal spores from an infected tree to healthy tree
- The Ceratocystis fungus traveling underground through root grafts
In addition to natural spread, oak wilt disease spread is accelerated by transporting diseased firewood.
Spreading by Beetles
Live fungus remains in the diseased wood, even after the tree dies. Within a specific range of moisture, the fungus produces spores. The smell of a sporing mat is a fermenting "banana beer" scent. This sweet scent attracts sap beetles to the spore mat. Spores stick to the body of the beetle crawling in the spore mat. Later, when sap beetle adults are attracted to sap exposed by an injury to the bark of a healthy oak, they introduce Ceratocystis spores to the new oak.
Oak bark beetles can also spread oak wilt disease. When oak bark beetles lay eggs in oak wilt-diseased trees, fungal spores can adhere to the beetles' bodies. The adult beetles leave to feed in other oaks, transporting the spores which may infect the healthy oak. Also, the eggs that were laid in the diseased oak hatch into larvae that feed in the oak. After metamorphosing into adults, they carry fungal spores to healthy trees, where they feed and lay eggs.
Spreading through Roots
Oaks growing within about 50 feet of each other can develop a common root system as their roots graft together. Because of the root grafting, oak wilt fungus can travel through the diseased roots into healthy oaks.
Once oak wilt is established in a group of oaks, the primary spread is by the fungus traveling to healthy trees through their root grafts.
Managing Oak Wilt
Management of oak wilt spread includes these actions:
- Chemical Treatment: An injection of a systemic fungicide may help a healthy white, bur, or swamp oak resist oak wilt. The fungicide is injected around the base of the tree, and should be done by a licensed contractor. The treatment must be repeated every three years.
- Diseased Tree Removal: The first step in management is typically to remove the diseased tree. If the stump remains, it should be de-barked. The ideal time for removal is late fall or winter. If the City of Maplewood marks a diseased oak on private property, the property owner will receive a letter explaining what needs to be done. In almost all cases removal is required. If the marked tree is on public property, such as a park or boulevard, the city (or other government agency that owns the property) is responsible for the tree.
- Firewood Management: All diseased oak wood must be de-barked, chipped, buried, or burned to break the cycle of the infection.
- Root Graft Disruption: Oaks growing within about 50 feet of each other can develop a common root system as their roots graft together. Because of the root grafting, oak wilt fungus can travel through the roots into healthy oaks. Root graft disruption, or the severing of root connections, is done using a vibratory plow with a 5-foot blade. Plowing can stop the disease from spreading underground, but must be re-done every 3 years. Oak wilt fungus can remain alive in roots underground for at least 7 years. Signs of disease spread need to be carefully monitored.
- Timing of Pruning: Because exposed moist wood attracts sap beetles, oaks should not be pruned between April and October. However, if an oak is injured during a storm, a torn limb should be cut and the exposed wood should be sealed as soon as possible.
Storing & Burning Diseased Oak Firewood
Oak firewood can be stored only if the logs are de-barked. A fireplace log can retain enough moisture to enable the fungus in the wood to produce spores which can travel to other healthy oaks.
Preventing Oak Wilt
Fungicide injections can be given to an apparently healthy tree that is threatened by oak wilt. The injection is done by a licensed contractor and must be repeated every three years. The city tree inspector can help determine if the tree in question is a good candidate for preventive treatment.
Saving a Diseased
Although oak wilt is deadly to all oaks, trees in the white oak group (white oak, swamp white oak, and bur oak) have a chance of surviving oak wilt if a fungicide is injected as soon as symptoms of oak wilt are evident. Oaks in the white oak group have rounded lobes on their leaves.
Oaks in the red oak group (northern pin oak and northern red oak) are very susceptible to oak wilt. Fungicide injections are not effective once symptoms of oak wilt are evident. Oaks in the red oak group have pointed lobes on their leaves.
Please carefully weigh the pros and cons when considering injection. Injections are costly and must be repeated, they are not always effective, and they can have negative environmental impacts.