Oak wilt disease


Some days it feels like we are gearing up for battle – a battle against the weeds and pests that seem to be conspiring against us. Canada thistle, emerald ash borer, slugs, powdery mildew, and wild garlic mustard seem to laugh at our attempts to impose order on them. Whether you are an agricultural producer or a home gardener, we all invest time, money, and sweat into controlling these problems and increasing productivity. Just last week, we learned of a new problem for woodland owners and homeowners – oak wilt disease.

Oak wilt is a serious and often deadly fungus of oak trees. Its origin is unknown. It was first discovered in Wisconsin in 1944 but is thought to have been in the United States since the 1800s. It is now found in 23 states, as far south as Texas, and east to New York and the Carolinas. According to the Ohio Department of Natural Resources Division of Forestry (DOF), there are 14 counties with confirmed cases of oak wilt and three with suspected cases. Unfortunately, decades ago land managers in three states used the fungal pathogen as an inexpensive way to control unwanted oaks and achieved a 98 percent mortality rate, a reason why the problem is so widespread and deadly.

Oak wilt is caused by a fungus that develops in the xylem, the water carrying cells of the tree. It blocks the flow of water and nutrients from the roots to the crown, causing the leaves to wilt and fall off. The fungus is spread above ground by beetles and below ground through tree roots. Infected red oaks develop spore mats under the bark to which beetles are attracted. These beetles move spores to uninfected areas by finding a healthy tree that has been freshly wounded and feeding on the sap. The beetles can spread the fungus to healthy red oak and white oak trees that can be miles away. The underground spread occurs when roots of nearby oaks graft or fuse to each other, creating a connection through which nutrients and the disease can move. It is thought that most of the spread of oak wilt is underground.

Ohio’s oaks are an important landscape tree as well as a significant contributor to timber production. Oak wilt affects all oaks but is most detrimental to the red oaks (which includes Northern red oak, black oak, scarlet oak, pin oak, and shingle oak). Some white oaks can live with oak wilt for several years before dying, and some may be able to overcome the disease; but be aware, oak wilt can kill white oaks, too. What to look for in identifying oak wilt:

• Symptoms usually occur in mid to late July and can last into late September

• Leaves begin browning from the tips and edges and works its way into the midvein

• Trees are defoliated or mostly defoliated by the beginning of August

• Groups of dead and dying trees are usually present

• Spore mats or fungal mats appear on dead trees in late spring and early summer

• Streaking will occur in the vascular tissue

Treatment options are very limited. Once a tree is infected with oak wilt, it cannot be saved. It is critical is that the disease does not spread to other trees, so early identification is essential! The Ohio State University Extension has a publication entitled, Oak Wilt, which you can download for free at ohioline.osu.edu/factsheet/plpath-tree-02. If you suspect your landscape tree might have oak wilt, please call a certified arborist for diagnosis and treatment strategies. The Ohio Chapter of the International Society of Arborists has a “find an arborist” tool on its website at www.ohiochapterisa.org. Woodland owners can find contact information for a DOF service forester, as well as consulting foresters, at forestry.ohiodnr.gov/woodlands/landowner-assistance/getting-started. The only way to get an official diagnosis is through testing of a fresh sample. The C. Wayne Ellett Plant and Pest Diagnostic Clinic in Reynoldsburg can perform this testing and details can be found at ppdc.osu.edu/submit-sample/landscape/wilt-diseases.

Upcoming events for the Delaware Soil and Water Conservation District can be found at soilandwater.co.delaware.oh.us, including Family Creeking Night on July 11 at Ruffner Park in Galena and the Delaware Farm Tour on July 20. You can also find us on Facebook and Instagram.


By Bonnie Dailey

Delaware Soil & Water Conservation District

Bonnie Dailey is deputy director of the Delaware Soil & Water Conservation District. For information, go to www.soilandwater.co.delaware.oh.us.

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