National Invasive Species Awareness Week: Oak wilt

MSU researchers recently were awarded a total of $1 million in grant funding to prevent and control invasive species in Michigan, including oak wilt.

A. Dying red oak B. Crack in bark indicating presence of pressure pad. C. Dark colored sporulating mat and pressure pad. Photo: Monique L. Sakalidis, Michigan State University
A. Dying red oak B. Crack in bark indicating presence of pressure pad. C. Dark colored sporulating mat and pressure pad. Photo: Monique L. Sakalidis, Michigan State University

National Invasive Species Week 2017 is Feb. 27 to March 3. Invasive species are plants, animals, and other organisms that are not traditionally found in a given location (in this case the Great Lakes) AND are having a negative impact of some kind, whether ecological, economic, social, and/or a public health threat.

To help bring awareness to these threats, Michigan State University Extension and Michigan Sea Grant are featuring articles on invasive species that have invaded or have the potential to invade Michigan’s environment. This article features the terrestrial invasive species is Ceratocystis fagacearum, a fungal pathogen that causes the disease oak wilt which is responsible for the widespread decline of oaks across the United States.

What Michigan is doing to battle invasive species: Michigan is committed to detecting and preventing the spread of invasive species throughout the state. The Michigan Invasive Species Grant Program has awarded more than $11 million in grants to local governments, nonprofits and institutions for management, education and outreach and innovative control methods. This funding includes the work of several Michigan State University researchers, who were awarded a total of $1 million in grant funding to prevent and control invasive species in Michigan. Their projects include work on two invasive species, hemlock woolly adelgid (HWA) and oak wilt. Another project focuses on improving an existing tool used to detect, identify and manage invasive species.

MSU Department of Plant, Soil, and Microbial Sciences, and Department of Forestry's Dr. Monique Sakalidis and her collaborators, including Dr. Deb McCullough, Dr. Bert Cregg and Dr. Jan Byrne, will focus on refining Michigan-specific oak wilt control measures. They will study the epidemiology, biology and population genetics of oak wilt. They will monitor fungal spore production and host tree susceptibility, and evaluate molecular methods to identify infected trees and the source of spread. They will also determine high-risk periods when insects are most likely to introduce the oak wilt fungus into trees. (Read more on the other two MSU research projects here.)

Species Name: Ceratocystis fagacearum is a fungal pathogen that causes the vascular disease, oak wilt (OW).

How it came to the United States and how long it has 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 United States. Difficulties in identifying the fungus led to a delay in the recognition of the exact extent of its impact until the 1980s. More recent evidence suggests oak wilt is an exotic disease that arrived in the United States in the early 1900s. The fungus has not been reported in any other country other than the United States, so its origin remains unknown.

Extent of range: In the U.S., OW has been confirmed in 24 states, including 829 counties. The Michigan Department of Natural Resources has confirmed OW in 56 Michigan counties. Oaks comprise about 10 percent of the forest in Michigan and OW 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 it is 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 vascular 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. OW causes devastating ecosystem damage and is also an aesthetic blight across the landscape.

Symptoms: An infected tree is often first noticed due to a sudden drop or browning of leaves in the summer months (Figure A). 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. There are other pest, pathogen, and environmental problems that may cause similar symptoms and therefore it’s important that suspected OW infected trees are lab verified.

How it kills the tree: Once the fungus enters the tree (either 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 (both 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 four weeks of infection. Six-12 months after the tree has died the fungus will complete its lifecycle and produce sporulating mats (Figure C) on the dead tree. These mats form under the bark and as the mats mature, they produce specialized, non-spore producing structures called “pressure pads” that exert pressure outward bark causing it to split (Figure B) and thus, provides a route for insects to reach the sporulating mats. These sporulating mats have a distinctive odor that makes them attractive to a variety of beetles 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. Radial transmission of OW from its center can occur. There are multiple ways the disease can be spread:

  • Root-to-Root transmission. Local spread of OW occurs when the fungus travels through the interconnected roots of infected and healthy trees. This type of spread results in outwardly expanding epicenters in the landscape (up to 39 ft./year). One important management strategy when dealing with OW is disrupting these root grafts via trenching or vibratory plows.
  • Overland 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. Removal of the entire infected tree (including stump removal) and limiting of activities that result in tree wounding is essential to reduce overland infection.
  • Firewood. Since sporulating mats develop on dead oak trees, they can also form on wood cut from infected oaks. Sporadic long-range infections can result from the movement of firewood. Specific handling of firewood is mentioned below.

Cool/unusual facts: This fungus is spread by sap-feeding nitidulid beetles, also known as picnic beetles and to a lesser extent, bark beetles. The sporulating mats smell like fermenting apple cider vinegar, red wine or even bubblegum.

Management actions/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 OW positive locations. This is done by reducing activities that cause tree wounding and by the removal of confirmed OW positive trees and disrupting root grafts that may have formed between healthy and infected trees.

What you can do to help prevent the spread:

  • Do not prune oak trees during the growing season. Limit any activity that results in tree wounding or movement of cut trees (pruning, harvesting, thinning, utility line clearance and firewood). To prevent above ground spread, trees should not be pruned from April 15th - July 15th.
  • Paint tree wounds with pruning paint as soon as they are made. Beetles have been known to find their way onto wounds within ten 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 make sure you cover the wood with a plastic sheet (min. 4mm thickness) and bury the edges of the plastic underground and make sure none of the plastic breaks. This needs to be left for 6-12 months until the wood has dried out enough (and therefore isn’t conducive to fungal growth) and the bark falls off.
  • Report sightings to the Department of Natural Resources Forest Health Division: or by phone at (517) 284-5895 or through the MISIN website or mobile app.
  • Lab verification of OW via the MSU Diagnostic Clinic. Unless there is the presence of a sporulating mat, the presence of OW must be lab verified before any management options.

Additional resources:

How to identify, prevent and control oak wilt

Oak Wilt in Michigan’s Forest Resource

Sakalidis Lab, Michigan State University:

Michigan Invasive Species

Oak Wilt Fact Sheet

Michigan Invasive Species Information Network (MISIN) Oak Wilt Fact Sheet

MISIN mobile app

Read the complete 2017 Invasive Species Series:



Hemlock woolly adelgid

Oak wilt

New Zealand mudsnail

Red swamp crayfish

Flowering Rush

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