Avoid Oak Wilt: Stop Pruning Now

Pruning oak trees in spring and summer exposes them to a high risk of contracting oak wilt, a deadly disease that can infect and kill a range of oak species. Maintenance pruning should be stopped now until August.

Oak wilt is caused by the fungus Bretziella fagacearum (previously Ceratocystis fagacearum). When oaks are pruned or wounded during the spring and summer, the risk of transmission by beetle vectors that carry spores to newly wounded trees becomes very high. 

Scouting for fungal mats is important throughout the spring months. As temperatures rise, beetles that transmit the fungus become active. The beetles are attracted to the sweet odor that the fungus mats produce; they pick up the fungus and then transmit the disease as they are also attracted to the sap that oaks produce when pruned. 

Spotting oak wilt fungal mats

r fungal mats is important to do in early April and through the spring months. These mats will emit a fruity smell, and if the bark peels easily, look for blackened, raised, fluffy areas on the sapwood.T.W. Bretz, USDA Forest Service, Bugwood.org
Fungal mats form in red oaks that died the prior summer due to oak wilt.  Look for raised cracks in the trunk that emit a fruity smell.   Photo credit T.W. Bretz, USDA Forest Service, Bugwood.org

Look for cracks in the bark of oak trees, with a fruity smell, fungal mats (sign) form right under the bark of the tree trunk and limbs. Se these image fungal mat photo gallery. Have a scouting guide handy, such as The US forest service How to recognize common diseases oaks.

If you are not sure what you see is a fungal mat, the Plant and Insect Diagnostic Clinic at Iowa State University Extension and Outreach can help. Send good quality digital photos; review digital photography instructions and tips.

Red oaks are very susceptible to the oak wilt fungus and can die within four to six weeks during the summer. Fungal mats will only form on red oaks (pointy leaves) in the spring. White and bur oak (lobed leaves) are moderately more tolerant, and therefore no fungal mats will be present. 

Refrain from pruning all healthy oaks starting now and through August to keep your and your community oak trees healthy and strong.

See the US forest service resource page Oak Wilt in the Northeastern and Midwestern States, Iowa Department of Natural Resources Oak wilt page, plus these Iowa State resources Oak WiltVascular Wilts Testing.



Lina Rodriguez Salamanca Extension Plant Pathologist and Diagnostician

Dr. Lina Rodriguez-Salamanca is a diagnostician and extension plant pathologist with the Iowa State University Plant and Insect Diagnostic Clinic  (clinic.ipm.iastate.edu), a member of the National Plant Diagnostic Network (NPDN, ...

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