Delay Trimming Oaks Until Late Summer To Slow Oak Wilt

Apr 10, 2019

Oak Wilt distribution in the state
Credit Wisconsin DNR

Though snow is in the forecast this week, a DNR tree specialist says don't prune oak trees until later this year.

Pruning and cutting oaks in spring and early summer leaves them vulnerable to oak wilt, which rapidly kills trees in the red oak group and weakens those in the white oak group. Any damage during this time, including broken branches caused by storms, exposes living tree tissue beneath the bark and provides an opportunity for the oak wilt fungus to infect the tree. Sap-feeding beetles introduce the disease by carrying oak wilt spores from infected trees or firewood to fresh wounds.

DNR urban forester Don Kissinger in Wausau says the tree dies as the vascular system plugs up, robbing the tree of nutrients it needs. Kissinger says beetles move the fungus one way, but an infected tree can affect another oak nearby..

"Once it begins in a tree, I would say about 90 percent of the transmissions that die are from root to root or tree to tree. Tree roots don't grow real deep, probably 80 percent of the trees roots are in the top foot and a half of soil. But they grow very wide. You can have trees 100-150 feet away from each other and conceivably though could have roots that are connected...."

Kissinger says property owners with oak trees shouldn't prune them until after the growing season, largely from now until July. Oak wilt and other diseases move easily on or in firewood logs year-round, so Kissinger says keeping firewood local, or purchasing Wisconsin-certified firewood, is important to protect trees.

Oak wilt has been found in all Wisconsin counties except Forest, Iron, Ashland, Taylor, Door, Kewaunee, Calumet and Manitowoc counties. Several of these counties contain the highest abundance of healthy and productive oak forests in the state.

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