Brief History
Oak wilt is a fungal disease first identified in the early 1940s in Wisconsin, with records suggesting its presence in the United States as early as 1912. It was formally described in 1942 and recognized as a major threat to oaks by 1951, spreading through the Upper Mississippi Valley and beyond. The disease has affected regions from Texas and South Carolina in the south to Maryland, Pennsylvania, eastern Nebraska, and central Minnesota, with virtually all counties in Iowa reporting cases. Its distribution has remained relatively stable over the last 50 years, though incidence fluctuates locally. Likely originating from Mexico, Central America, or South America—where oak diversity is highest—it emerged alongside other major vascular diseases like chestnut blight and Dutch elm disease in the early 20th century. Factors like deforestation, fire suppression, and timber movement during railroad expansion contributed to its spread, particularly in the Midwest and eastern U.S., as well as Central Texas and Southern Ontario. It has not been found west of the Rocky Mountains or outside North America.
Science
Oak wilt is caused by the fungus *Bretziella fagacearum* (previously known as *Ceratocystis fagacearum*), an ascomycete in the class Sordariomycetes that invades the xylem (water-conducting tissues) of oak trees (*Quercus* species). The pathogen triggers the formation of tyloses—cell outgrowths that block water and nutrient flow—leading to wilting, dieback, and often tree death. Transmission occurs primarily through two modes: underground via root grafts, where interconnected roots (common among same-species oaks up to 50 feet apart) allow the fungus to spread between trees, and aboveground via insect vectors, mainly sap-feeding beetles from the Nitidulidae family (picnic beetles), which carry sticky spores from fungal mats on infected trees to fresh wounds on healthy ones. In southern regions, oak bark beetles may also play a role, but picnic beetles dominate in the Upper Midwest. Human activities, such as moving infected firewood, significantly contribute to long-distance dispersal. The fungus can survive in root systems for at least three years after a tree dies. Effects vary by oak group: red oaks (e.g., red, black, pin) are highly susceptible and die quickly (weeks to months), while white oaks (e.g., white, bur) progress more slowly and may partially recover. Ecologically, it impacts forests and urban areas, potentially threatening global oak populations if it spreads further.
Identifying Characteristics
Symptoms typically appear in late spring or early summer and include leaf discoloration (chlorosis starting at tips and edges, turning dull bronze-brown), wilting, and defoliation, often progressing from the tree's top downward. A sharp line often separates discolored from green tissue, with leaves dropping at various stages, including fully green. In red oaks, rapid crown dieback leads to death within 1-4 months or up to a year; white oaks show slower, scattered branch dieback over years, sometimes surviving with limited damage. L Internal signs include brown streaks or rings in sapwood when bark is peeled, or branches are cut in cross-section. Fungal spore mats (grey with white margins, 10-20 cm, under bark) form on infected red oaks but are rare in white oaks. Diagnosis should involve lab testing, as symptoms mimic other issues like drought, anthracnose, or insect damage; samples include symptomatic branches sent to clinics for confirmation.
Mitigation
Mitigation emphasizes prevention to avoid new infections and human-spread. Avoid wounding oaks during high-risk periods (April-July in the Midwest, February-June in Texas), when sap flow attracts beetles; if unavoidable, immediately paint wounds with shellac or non-toxic dressings (avoid asphalt-based). Sanitize pruning tools between trees. Handle firewood carefully: burn or process infected wood before April, cover unseasoned logs with thick plastic (edges buried) for solarization, and use certified heat-treated wood for transport to prevent long-distance spread. Diversify landscapes with native, adapted species to increase resilience, and encourage multiple oak types in forests. Public education on not moving firewood and early reporting is key. In unusually warm springs, risks may start earlier than April. Install root barriers in urban settings to block root grafts.
Oak Wilt Risk Status
The oak wilt status system uses three "risk" description periods: High Risk, Low Risk and Safe.
- High Risk – usually occurs during April, May, and June.
- Low Risk – typically occurs during July, August, September, and October
- Safe or No Risk – November, December, January, February, and March.
Management activities including pruning and tree removal should be avoided during High and Low risk periods to minimize the spread of oak wilt to susceptible trees. Click link below to confirm the current oak wilt status.
Current Oak Wilt Status
These three probabilities of risk are based on the following:
- First, is the fungus that actually causes the disease active?
- Second, is the beetle that carries the fungus to the oak tree active?
- Third, is there oak wilt disease in the area?
If all three criteria are met, then the transmission of oak wilt from one area to another is very likely.
Questions, consultation
Contact
Managing the Spread of Oak Wilt
Oak wilt management
There is no cure for infected trees, so management focuses on integrated pest strategies including sanitation, chemical treatments, and cultural controls to contain spread and minimize losses.
Sever root grafts
To stop underground spread, sever root grafts with mechanical trenching (4 ft deep, 50-100 ft from infected trees) or vibratory plows, often requiring removal of buffer trees.
Sanitation
Sanitation involves felling and disposing of infected red oaks (potential spore producers) before spring, through burning, chipping, debarking, or tarping to prevent beetle access.
Chemical Control
Chemical control uses propiconazole injections (a systemic fungicide) every two years for high-value trees. Injections are undertaken in spring preventively or therapeutically to delay symptoms in white oaks.
In forests with abundant disease, control may be impractical; instead, promote diversity by planting natives and protecting saplings. For residential trees, hire contractors and confirm diagnosis via lab tests.
If an oak wilt infected tree is identified
If an oak wilt infected tree is identified
If the City of Eagan Forestry staff identifies oak wilt infected trees on your property, the trees will be clearly marked and numbered with yellow paint. An oak wilt notification packet will be mailed to you containing the following documents:
- An official Notification Letter
- An aerial photograph identifying the location of the infected tree(s) on the property
- A copy of Eagan City Code 10.20 – "Shade Tree Disease Control and Prevention"
- A copy Minnesota Statute 18G.13 – "Local Pest Control"
Removing Infectious Oaks
Removing Infectious Oaks
Red oak trees infected with oak wilt this year may spread the oak wilt fungus overland to adjacent, vulnerable red oaks next spring. These trees have the potential to produce spore mats that contain oak wilt fungal spores. Sap beetles visit the spore mats and transport the fungus to adjacent trees. A tree that has the potential to produce spore mats is called a potential spore producer (PSP). Not all oak wilt infected red oaks are PSP trees. A standing PSP tree or uncovered wood, in a woodpile from a felled PSP tree, poses a threat to adjacent, healthy, red oaks next spring. All red oaks that were marked with a number and a yellow ring, June – September, will be re-inspected in late November by City of Eagan Forestry staff to determine the PSP status.
Proper disposal of a PSP tree:
- Cut the entire tree down in late autumn or winter leaving no more than a 2 – 4-inch high stump.
- All wood from the PSP oak tree must be promptly transported to an approved wood waste use site, kiln-dried or processed into lumber. The homeowner may burn infected wood on site. If the firewood is not used by the following spring, it must be completely covered and sealed to the ground with 4 – 6-mil thick plastic sheeting until the end of September of the following year while being stored on site.
The following steps are recommended for covering infected wood:
- Stack or pile all infected wood 3 inches in diameter or greater. The size of the pile does not matter, but it should be stable and not collapse. Trim branches that may poke holes through the plastic.
- Dig a 4-inch deep trench completely around the woodpile. If you cut down the tree and cover the wood during winter months, when the ground is frozen, seal the plastic to the ground by piling heavy objects around the base of the pile, then return when the ground is thawed to dig a trench and bury the plastic into the ground.
- Cover the woodpile completely with a single sheet of heavy (4 mil or thicker) plastic sheeting and lay the sheet edges in the trench. (Clear plastic is preferable.)
- Fill the trench with soil to COMPLETELY SEAL the woodpile.
- Keep the woodpile sealed until the end of September of the following year. At that time you may uncover the wood to complete drying.
Reimbursement Guidelines
Oak Wilt Management Activities Reimbursement
The City of Eagan offers monetary incentives to encourage residents to slow the spread of oak wilt disease on private property. The availability of funds is limited and will be dispersed in the order in which management activity invoices are received.
Reimbursement Request Procedure
Residents should submit a copy of a current, paid, itemized, invoice for any or all of the Oak Wilt management activities covered under the reimbursement program to the City of Eagan Forestry Staff as soon as possible following the completion of the activity.
Submit by mail or e-mail to:
3501 Coachman Point
Eagan, MN 55122
We encourage you to contact us with questions or comments by e-mail to the
Tree Contractors
Licensed Tree Contractors
We encourage residents to hire licensed tree contractors to safely remove and dispose oak wilt infected tress and debris. Visit the Licensed Tree Contractor page listing companies offering these services.