By Monique Sakalidis, Michigan State University, Department of Forestry

Remember the no-prune dates of April 15–July 15 to reduce the chance of oak wilt infection.

Bretziella fagacearum (used to be known as Ceratocystis fagacearum) is a fungal pathogen that causes the disease oak wilt.

How did oak wilt come to the United States and how long has it been here?

Oak wilt was first recognized as an important disease in 1944 in Wisconsin, where in localized areas, over half the oaks had been killed. The fungal pathogen is thought to be native to the Eastern U.S. Difficulties in identifying the fungus led to a delay in recognizing the exact extent of its impact until the 1980s. More recent evidence suggests oak wilt is an exotic disease that arrived in the U.S. in the early 1900s. The fungus has not been reported in any other country other than the U.S., so its origin remains unknown. In Michigan, it was first reported in the 1970s.

Extent of range

In the U.S., oak wilt has been confirmed in 24 states, including 829 counties. The Michigan Department of Natural Resources has confirmed oak wilt in 56 Michigan counties. Oaks comprise about 10 percent of the forest in Michigan and oak wilt has the potential to impact the 149 million red oak trees across 3.9 million acres of Michigan forest land (private, state, local government and federal ownership).

Why is oak wilt a problem?

Oak wilt is a serious disease of oak trees that mainly affects red oaks. The disease also affects white oaks, but because they are somewhat more resistant (due to their better ability of compartmentalizing the fungus whilst maintaining a functioning water transport system), the disease progresses more slowly. Activities that result in tree wounding such as pruning, tree climbing spikes, nailing signs on trees, hanging lanterns on trees, tree barking and storm damage during the warmer months of the year can result in more new tree infections. Oak wilt causes devastating ecosystem damage and is also an aesthetic blight across the landscape.

Oak wilt symptoms

An infected tree is often first noticed due to a sudden drop or browning of leaves in the summer months (Photo 1A). Leaves may be brown, somewhat bronzed or partially green. Often, leaf tips and margins will be bronze or brown whilst the leaf base will remain green (Photo 2). There are other pest, pathogen and environmental problems that may cause similar symptoms and therefore it’s important that suspected oak wilt-infected trees are lab verified.

How it kills the tree

Once the fungus enters the tree via a spore coming into contact with a tree wound or via interconnecting root grafts, it grows throughout the water conducting channels of the tree—the xylem vessels. These vessels are eventually blocked by the fungus and structures produced by the tree, and this means water cannot be effectively transported and we start to see the “wilting” effects. Tree death in red oak is rapid and can occur within three to four weeks after initial appearance of symptoms.

Six to 12 months after the tree has died, the fungus will complete its life cycle and produce spore-containing mycelial mats (Photo 1D) on the dead tree. These mats form under the bark and, as they mature, produce specialized, non-spore producing structures in the center of the fungal mat called “pressure pads” that exert pressure outward to the bark, causing it to split (Photo 1B) and thus provides a route for insects to reach the mycelial mats. These mycelial mats have a distinctive odor that makes them attractive to a variety of beetles (Photo 1C) that will feed on the mat then fly to other mats or fresh tree wounds, through which the fungus then enters the tree and starts the infection process anew.

How it is spread

Spread of the disease is rapid and there are multiple ways the disease can be spread.

• Below ground by root-to-root transmission. Local spread of oak wilt occurs when the fungus travels through the interconnected roots of infected and healthy trees. This can account for up to 90 percent of new infections each year. This type of spread results in outwardly expanding pockets of dead trees (infection epicenters) in the landscape (up to 39 feet per year). One important management strategy when dealing with oak wilt is disrupting these root grafts via trenching or vibratory plows.
• Overland by insect transmission. Nitidulid beetles carry fungal spores from sporulating mats on infected trees to wounds on healthy trees, from which a new infection can develop. Overland transmission results in new infection centers. Removing the entire infected tree, including stump removal, and limiting activities that result in tree wounding is essential to reduce overland infection.
• Overland by firewood. Since mycelial mats develop on dead oak trees, they can also form on wood cut from infected oaks. Sporadic long-distance infections can result from moving firewood. Specific handling of firewood is mentioned below.

Cool and unusual facts

One way this fungus is spread is by sap-feeding nitidulid beetles, also known as picnic beetles, and, to a lesser extent, bark beetles. The mycelial mats smell like fermenting apple cider vinegar, red wine or even bubblegum.

Management actions and options

Because red oaks have no natural resistance to this disease, the only way to stop new infection is to prevent the spread of the fungus to new, healthy trees and locations, and reduce the fungal presence or inoculum load in known oak wilt-positive locations. This is done by reducing activities that cause tree wounding, disrupting root grafts that may have formed between healthy and infected trees, and by removing confirmed oak wilt-positive trees.

Help prevent the spread of oak wilt

• Do not prune oak trees during the warmer months of the year. Limit any activity that results in tree wounding or movement of cut trees, such as pruning, harvesting, thinning, utility line clearance and firewood. To prevent aboveground spread, trees should not be pruned from April 15 to July 15.
• Paint tree wounds with pruning paint as soon as they are made. Beetles have been known to find their way onto wounds within 10 minutes of pruning.
• Do not move firewood. If you cut oak down, either chip, debark, burn or bury it. If you cut it into firewood, cover the wood with a plastic sheet (minimum 4-millimeter thickness) and bury the edges of the plastic underground, making sure none of the plastic breaks. This needs to be left for six to 12 months until the wood has dried out enough—and therefore isn’t conducive to fungal growth—and the bark falls off.
• Report suspect trees to the Department of Natural Resources Forest Health Division by emailing DNR-FRD-Forest-Health@michigan.gov, calling 517-284-5895 or through the Midwest Invasive Species Information Network (MISIN) website or MISIN mobile app.
• Get a lab verification of oak wilt via the Michigan State University Diagnostic Services Clinic. Unless there is the presence of a mycelial mat on a dead tree, the presence of oak wilt must be lab-verified before any management options. See MSU Diagnostic Services’ specific sampling instructions.

Additional resources

• How to Identify, Prevent and Control Oak Wilt, USDA and U.S. Forest Service
• Oak Wilt in Michigan’s Forest, MSU Extension bulletin E3169
• Oak Wilt Research from the Sakalidis Lab at MSU
• Michigan Invasive Species website
• Oak Wilt fact sheet, Michigan Invasive Species
• Michigan Invasive Species Information Network (MISIN)
• MISIN Oak Wilt fact sheet
• MISIN mobile app

This article was published by Michigan State University Extension. For more information, visit http://www.msue.msu.edu. To have a digest of information delivered straight to your email inbox, visit http://www.msue.msu.edu/newsletters. To contact an expert in your area, visit http://expert.msue.msu.edu, or call 888-MSUE4MI (888-678-3464).