EXTENSION CORNER: Oak Wilt— what is it and what can we do?

Laura Bailey, Natural Resources Educator, Yates County Cornell Cooperative Extension & Regional Director of the Master Forest Owner Program, Northwest Region

In March 2020, the DEC announced that an oak wilt quarantine district had been established for the Town of Middlesex in Yates County. 

An oak tree dying from oak wilt.

MIDDLESEX – Oak wilt was first discovered in the U.S. in Wisconsin in 1944, but where it originated is still unknown. It currently occurs in 24 states in the Eastern U.S. and is not known to occur elsewhere.

Oak wilt was first detected in New York state in 2008, in Glenville, Schenectady County. In 2016, oak wilt was discovered in multiple locations on Long Island: Islip, Riverhead, and Southold in Suffolk County, and Brooklyn in Kings County. It was also detected in the Finger Lakes region, in Ontario County, during 2016. In 2018, there was another finding of oak wilt in Ontario County.

Oak wilt quarantine locations in the Town of Middlesex.

In March 2020, the New York State Department of Environmental Conservation (DEC) announced that an oak wilt quarantine district had been established for the Town of Middlesex in Yates County. Following confirmation of two oak wilt infection locations in 2019, a town-wide quarantine district was established, which restricts the movement of oak wood and firewood in an effort to prevent the disease from spreading.

What is oak wilt? Oak wilt is a disease that affects trees in the oak genus (Quercus) and is caused by a fungus (Ceratocystis fagacearum) that grows in the xylem, or water-carrying cells, of oak trees. The fungus prevents the flow of water and nutrients through infected trees, clogging the xylem cells and causing the leaves to wilt. Wilting usually starts at the outer portions of branches, located at the top of the tree crown, and progresses downward. Leaves begin to turn reddish-brown along the tip and edges, spreading toward the midrib and base of the leaf. This process occurs rapidly in trees in the red oak group, causing them to drop their leaves in the spring and summer months, which is not typical. Leaves may still be partly green when they drop. Substantial wilting and leaf loss can occur in as little as four weeks in red oaks.

Oak wilt is a fungal disease which can kill otherwise healthy oak trees within a matter of weeks.

What trees are most susceptible? The genus of oak trees is divided into two groups: red and white. Red oaks (including red, scarlet, pin, and black oak, etc.) are most severely affected by oak wilt, the disease being fatal to these trees, dying within a few weeks to six months. White oaks (including white, chestnut oak and bur oak, etc.) are less vulnerable, often able to survive years with the disease, and it is much less likely to spread to nearby trees in the white oak group.

How does oak wilt spread? Oak wilt is spread primarily through root-to-root connections between oak trees, which occurs more often among red oaks than white oaks. Oak roots that are in close proximity commonly graft together, which is beneficial for sharing nutrients, but unfortunately creates a pathway for spread of oak wilt disease. Spread of the disease is assisted by sap feeding beetles and bark beetles. Sap feeding beetles are attracted to freshly wounded trees, to feed on the sap. The oak wilt fungus creates fungal spore mats under the bark of infected oak trees, and in the last stages of disease, the bark splits open to reveal these mats. The mats emit a sweet odor that attracts sap beetles to feed, and as they feed, fungal spores attach to them. The beetles transport spores to uninfected areas (a fresh wound is required by the fungus to invade) and once infected, the disease can then spread underground through root grafts.

What can you do?

• Learn to recognize the symptoms of oak wilt and be on the lookout.

• Avoiding pruning, wounding, or cutting down oak trees in the spring and summer when spore mats are present and beetles are the most active. Prune only during September through March, and even later than September if possible.

• Be careful with the lawnmower and weed whacker around the base of trees.

• Put off construction activities around oaks until late summer or fall, if possible.

• If an oak becomes wounded, it should be sealed immediately with wound dressing. Latex paint works well. The beetles can potentially find fresh wounds in less than a half-hour.

• Only use local firewood, since oak wilt can also be transported in logs and firewood. Adhere to the N.Y.S. firewood regulation, which limits firewood movement to no more than 50 miles.

• If you suspect that an oak tree is infected with oak wilt, contact DEC Forest Health at 866-640-0652. For more information about oak wilt, visit DEC online at https://www.dec.ny.gov/lands/46919.html