Oak wilt, a disease lethal to red oaks, is caused by the fungus Bretziella fagacearum, which clogs the water-conducting vessels of infected trees, causing them to wilt and ultimately die. To prevent the spread of the disease, do not prune or damage oak trees from April to November.

Oak wilt, a disease lethal to red oaks, is caused by the fungus Bretziella fagacearum, which clogs the water-conducting vessels of infected trees, causing them to wilt and ultimately die. To prevent the spread of the disease, do not prune or damage oak trees from April to November.

Photo provided by the Michigan Department of Natural Resources


Officials warn to avoid wounding oak trees to prevent oak wilt

Disease is deadly, easily spread

By: Sarah Wojcik | C&G Newspapers | Published April 24, 2018

METRO DETROIT — Officials say that oak wilt, caused by an invasive fungus spread by beetles attracted to fresh-cut sap, is exploding through Michigan, and now is the peak time to take preventative measures to save oak trees.

Mid-April is the highest risk period for infections, and the best way to prevent the spread of the disease is to avoid pruning, wounding or damaging oak trees until they have lost their leaves for the winter.

“That risk period stays high into July, then the risk period level drops off a bit, but there’s still risk until we go into our cold winter months,” said Julie Stachecki, president of the Arboriculture Society of Michigan.

Ryan Wheeler, an invasive species biologist with the Michigan Department of Natural Resources, said the disease can weaken white oaks and kill red oak trees within weeks.

Red oaks have leaves with pointed tips and include black oak, pin oak, northern red oak and scarlet oak. White oaks have rounded leaf edges and include white oak and swamp white oak.

“Affected trees will suddenly begin to wilt from the top down, rapidly dropping leaves, which can be green, brown or a combination of both colors,” Wheeler said in a statement.

Stachecki said human behaviors are largely contributing to the exponential spread of oak wilt.

“It’s been in Michigan for many decades, but it’s been documented right now in 56 of our (83) counties,” she said. “One thing with oak wilt is that it requires an open wound on the tree to initiate a new infection center.”

Stachecki said any oak trees in the red oak family that become infected with oak wilt die within three to six weeks, regardless of the size of the tree.

“Not only does that tree die, the pathogen moves from one single oak tree that it came in through that pruning wound and moves through the trunk into the root system, and moves through a shared root system to the next oak tree and the next oak tree,” she said.

Stachecki said an oak tree’s root system extends in all directions for twice the length that the tree is tall.

“If you have a 50-foot tree, its roots can spread 100 feet in any given direction,” she said. “The epicenter continues to move outward over many years and many seasons until it runs out of oak trees to kill.”

The spores of the invasive fungus, Stachecki said, are spread by native beetles known as sap beetles or picnic beetles. 

She explained that the reproducing stage of the oak wilt fungus develops spore mats and pressure pads under the bark of the tree. The pressure pads pop the outer bark away from inner wood, which creates cracks in the bark. It is through the cracks that the beetles access the spore mats. 

“The beetles love the fresh sap smell of fresh green oak wood,” she said. “It’s been documented that the beetles arrive in 10 minutes.”

Unbeknownst to the beetles, she said, they bring the invasive spores to other pruned or damaged oak trees and infect them.

Sap or picnic beetles are approximately the size of a grain of rice, are oval-shaped and are brown in color with knobby antennas. Stachecki said they often can be found in wine or beer during picnics, since they are attracted to the fermented smell.

If an oak tree is wounded, she said there are preventative measures that people can immediately take to make sure it does not become infected with oak wilt.

She recommended sealing the exposed areas with pruning sealant — which is usually available at hardware stores, nurseries and landscaping retailers — or latex paint right away.

“The purpose of the sealant is simply to create a physical barrier,” Stachecki said, “so if a beetle arrives, the spores will not cross that physical barrier and get into the wound.”

Another way to prevent the spread of oak wilt is to not move firewood, especially if it comes into contact with oak wilt-killed leaves, because it can harbor the fungus, according to the Michigan Department of Natural Resources.

If you suspect that your firewood is tainted by oak wilt, the Michigan Department of Natural Resources recommends covering it with a plastic tarp and leaving no openings so that beetles cannot carry spores to healthy trees. Once the firewood dries out to the point where all of the bark loosens, the disease can no longer be spread.

To find out more information or if you suspect that your oak trees might have oak wilt, visit www.michiganoakwilt.org or www.pestid.msu.edu, or call (517) 355-4536.