DNR: Threat of oak wilt infection high with warm temps

With the warmer temperatures earlier this spring, the state Department of Natural Resources (DNR) is advising people to avoid trimming or pruning their oak trees for fear of promoting oak wilt, a fungal disease which kills trees by initially blocking their water conducting vessels.

It mainly affects red oak, northern red oak, black oak, and pin oak trees.

“It’s a lethal fungus disease that is transmitted to new wounds on oak trees,” said Michigan State University Arborist Dr. Dave Roberts

These wounds can be caused in a variety of ways, including storm damage, trimming and pruning, which poses a problem when performed during the warmer months of April through July. Under the right conditions, the oak wilt threat can extend into October.

Red oaks can die within a few weeks after becoming infected — a fact well-known by residents of the Axford Acres subdivision in Highland Township, whose trees fell victim to oak wilt after being pruned last April.

“The normal time-tested advice is to prevent oak wilt by not pruning or otherwise ‘injuring’ oaks from April 15 to July 15,” said Dr. Robert Heyd, Forest Pest Management Program manager for the DNR’s Forest Resources Division.

He said the spread of oak wilt occurs during this time of year as beetles move spores from fungal fruiting structures on last year’s oak wilt-killed trees to wounds on healthy oaks. The beetles are attracted to the odors given off by a tree after wounding, and then haphazardly spread the fungus on their bodies.

Sap beetles have the ability to spread the disease for “quite some distance,” according to Roberts.

These beetles have become active earlier because of the unseasonably warm weather and, as such, the transmission of oak wilt has already been seen statewide, including in Oakland County.

“Anyone who has lost trees to oak wilt knows not to cut trees from mid-April to mid-June,” Heyd explained. “But, with the warmer weather and the higher risk, the timeframe has moved up much earlier. Prevention efforts — not cutting and pruning — really need to start now.”

Another mode of disease transmission is through root grafts. However, that can only occur with trees that are sufficiently close to be root grafted together.

Transporting infected firewood can also spread the disease.

“With the transport of firewood and other tree-related activities, you have to assume the risk is present, whether you live in metro Detroit or Menominee,” Heyd said.

Roberts has several suggestions for preventing the spread of oak wilt, including:

• If trees are injured by storms during the warm season, the affected limbs should be cut immediately at the appropriate location and sealed with a barrier to prevent beetle transmission of the fungus;

• Trees not immediately affected can be treated with a fungicide trunk injection;

• To prevent further spread through root grafts, trenching between healthy and diseased trees may be an option;

• Don’t prune or remove oak trees during the warm weather season.

“Don’t let anyone prune oak trees during the warm season. What turned into a rather cheap pruning job will now result in many thousands of dollars in tree removals and property loss,” Roberts said.

For more information on the background, symptoms and prevention of oak wilt, go to http://michigansaf.org/forestinfo/Health/E2764-OakWilt.pdf.