Forest Insect and Disease Newsletter

Oak wilt: prune oaks before April 1

Click to enlarge

map of Oak wilt infection center from aerial survey dectection 2005-2011

Oak wilt infection centers from aerial survey detection 2005 to 2011.

Oak wilt is an aggressive disease that affects many species of oak and kills thousands of oak trees each year in forests, wood lots and home landscapes in the eastern United States. Trees in the red oak group are more susceptible to oak wilt than trees in the white oak group, and often shed their leaves and start to die within a few weeks of the onset of symptoms. Black oak, northern pin oak and northern red oak belong to the red oak group. Trees in the white oak group, bur oak, swamp white oak and white oak, usually die slowly over one to many years.

The disease is caused by a non-native fungus that invades the water-conducting vessels of oak trees. Oak trees respond to the invasion by plugging their vessels, which causes leaves to wilt from the top of the crown downward. Oak wilt can easily spread from infested trees to healthy trees through root connections.

Ryan Blaedow, DNR forest health specialist for central and southern Minnesota, reminds Minnesotans to prune oak trees before April 1 to prevent new oak wilt infestations. "The best time to prune oak trees is during the winter months when the beetles that carry the oak wilt fungus are dormant," he says.

Beetle activity usually peaks between April and June; however, oak wilt can be transmitted any time during the growing season when it is warm enough for the beetles to feed on the sap of wounded oaks.

"The key to keeping oak wilt from spreading is to remove diseased trees before spore mats form and by preventing the transfer of spores to healthy trees," Blaedow says. "If your oaks need to be pruned, get the work done before April 1, otherwise wait until fall. Unfortunately, people who prune their oaks in the spring find their tree is dead by the end of the summer."

To prevent the spread of oak wilt, follow these management guidelines:

  • If possible, trim oak trees during the winter.
  • Avoid wounding oak trees between April and July when sap beetles actively feed.
  • Immediately treat wounds with pruning paint during spring and summer months to stop sap flow.
  • Remove diseased trees before the following spring to prevent spore mat development.
  • It is best to remove trees in the winter to avoid wounding neighboring trees.
  • Prior to removing dead and diseased trees, sever root connections to healthy trees by trenching around diseased trees with a vibratory plow equipped with a 5-foot blade.
  • Destroy infested wood or treat wood on site by debarking, chipping or drying the wood.
  • To dry wood properly, cover split wood with plastic and bury the edges for at least six months to kill the oak wilt fungus and any insects.

More information on oak wilt and how to prevent its spread.