Tag Archives: Oak wilt

Smart Gardening to prevent oak wilt

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Oak leaves showing oak wilt symptoms. Photo by Monique Sakalidis, MSU.

Prevent oak wilt!

By Monique Sakalidis, MSU Department of Forestry, and Ruth Dorando Marcy, MSU Extension

  • When possible, prune oaks in winter. Especially avoid pruning mid-April to mid-July.
  • Immediately seal wounds on oaks damaged during the growing season.
  • Don’t move firewood.

History and range of oak wilt

Oak wilt, caused by the fungal pathogen Bretziella fagacearum, was first reported in Michigan in the 1970s. It is now found in 56 counties with potential to impact at least 149 million red oak trees across over 20 million acres of Michigan land.

Trees at risk

Red oaks, those with bristle-tipped lobes, die rapidly from oak wilt within four to six weeks. Red oaks in Michigan include northern red oak, black oak, northern pin oak, pin oak, scarlet oak and shingle oak.


White oaks, those with rounded lobes and no bristles, or with large regular teeth, will slowly decline and likely die. White oaks in Michigan include white oak, swamp white oak, bur oak, chinquapin oak, chestnut oak and dwarf chestnut oak.

Symptoms and signs

Note the pointy, bristle-tipped lobes on this northern red oak. Bristles indicate the species is a red oak. Photo by Paul Wray, Iowa State University, Bugwood.org.

When an oak tree is infected, the fungus grows throughout the water-conducting xylem vessels and the tree responds by producing structures called tyloses to attempt to wall in the fungus. This blocks water flow through the tree, causing the wilt symptoms seen with this disease.


An infected tree is often first noticed when leaves suddenly drop or turn brown in the summer months. Leaves may be brown, somewhat bronzed or partially green. Red oak trees may be dead and defoliated within four weeks of infection.

This white oak can be identified by its rounded lobe leaves. Photo by Chris Evans, University of Illinois, Bugwood.org.

Since there are other pest, pathogen and environmental problems that may cause symptoms similar to oak wilt, a branch sample must be sent to a diagnostics laboratory for an accurate diagnosis. On trees that have been dead for six to 12 months, a mycelial mat may be visible underneath a bark crack. Presence of a mycelial mat or laboratory verification from a living tree that is showing symptoms are the only ways oak wilt can be confirmed.

How oak wilt is spread

Oak wilt spreads rapidly in multiple ways.

An oak with symptoms of oak wilt. Photo by Monique Sakalidis, MSU.
  • Overland transmission occurs when Nitidulid beetles (commonly called sap beetles) pick up spores while feeding on mycelial mats on infected trees then transfer them to fresh wounds on healthy trees. This creates new sites of infection (infection centers).
  • Underground transmission occurs when fungal spores move between connected roots (root grafts) of infected and healthy oaks. This type of spread outwardly expands pockets of dead trees (infection epicenters).
  • Moving wood from infected oaks can start infection sites because mycelial mats can form on wood cut from infected oaks, providing a source of infection.

Prevention

Prevent overland transmission of oak wilt by strictly following guidelines for safest pruning times and care of trees damaged during spring and summer. The safest time to prune is in the winter months when sap beetles are not active. The worst time is mid-April to mid-July when beetles are most likely present. Some beetles will be active throughout the summer and early fall. If a tree is damaged and pruning is required during the warm growing season, seal all wounds immediately—sap beetles can arrive within five minutes!

A sap beetle feeds on a mycelial mat. Photo by Wisconsin DNR.

Avoid underground transmission of oak wilt by severing root connections between infected and healthy oaks. Trenching equipment is used to cut a 5-foot deep line into the earth separating roots of dead and infected oaks from the unaffected oaks in the area. Trench lines are plotted using a formula that takes many factors into account and are best determined by a certified arborist or forester trained in oak wilt management. Injections of a fungicide with propiconazole as the active ingredient may offer protection of nearby valuable, healthy oak trees. This fungicide treatment will not protect against overland spread of oak wilt.

Root grafts are connected roots between trees where fungal spores can move from infected to healthy oaks. Photo by Ronald F. Billings, Texas A&M Forest Service, Bugwood.org.

To prevent the spread of oak wilt via firewood, dispose of wood from trees killed by oak wilt by burning it. If this is not possible, bury, chip or carefully seal wood from infected oaks under thick plastic. The plastic must cover the entire wood pile with the edges buried in soil to prevent beetle access. Once the wood is dry and the bark is loose or fallen off (about one year), it is no longer a source of infection.

Steps to take when oak wilt is suspected

A plastic tarp tightly covers wood from a tree killed by oak wilt. Photo by Julie Stachecki.

If oak wilt is suspected, take immediate action to get a positive diagnosis. Send a sample of a symptomatic branch to MSU Plant & Pest Diagnostics to confirm oak wilt unless a mycelial mat is discovered on a dead oak. If you find a mycelial mat on a dead oak—normally visible beneath a bark crack—that is sufficient for positive diagnosis. Once a positive diagnosis is obtained, consult a professional trained in oak wilt management as soon as possible. This disease will continue to spread unless management techniques are begun promptly.

Additional resources

More information and a free downloadable field guide to Michigan oaks: www.MichiganOakWilt.org


For more information on a wide variety of Smart Gardening topics, visit www.migarden.msu.edu or call MSU’s Lawn and Garden hotline at 1-888-678-3464.


Published August 2019. This publication is supported in part by the Crop Protection and Pest Management Program 2017-70006-27175 from the USDA National Institute of Food and Agriculture. Any opinions, findings, conclusions, or recommendations expressed are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture.


Download a PDF file of this article.


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).

Worried about oak wilt?

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Photo 1. Oak wilt symptoms. A) Dying red oak showing foliar wilt symptoms. B) Crack in the bark indicating mycelial mat presence. C) Nitidulid beetle visiting a mycelial mat. D) Gray spore containing mycelial mat and pressure pad. Photos: Monique Sakalidis

By Monique Sakalidis, Michigan State University, Department of Forestry


Species name


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 was 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 20 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, omewhat 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 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, meaning 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 the mats 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 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.


Once oak wilt is confirmed at a site either by the presence of mycelial mats or lab-based confirmation (from branch samples that show black streaks in the vascular tissue). Management decisions can be made and action taken normally between August and when the ground is frozen. Working when the ground is frozen will minimize wounding events that lead to overland spread as well as minimizing the chance of spores moving via root grafts (underground spread).


Management options once oak wilt is confirmed on a site vary depending on the accessibility of the site (steep versus flat), type of site (forest versus yard) and the number of trees affected. Management is primarily centered around disrupting root grafts that may allow infection to spread from diseased to healthy trees and removing and destroying infected material, including firewood (best to cover firewood until the bark has fallen off).


A great resource for specific management options can be found on the Minnesota Department of Natural Resources Oak Wilt Management web page or their downloadable flyer, and from Michigan State University Extension‘s Oak Wilt in Michigan’s Forest Resource.

Photo 2. Typical fallen leaves associated with oak wilt. Photo by Monique Sakalidis | MSU.

What you can do to 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 above-ground spread, trees should not be pruned from April 15 to July 15 during the highest risk periods. There may be some risk of spread outside of this date range, but at a reduced amount.
  • Paint tree wounds with tree-wound paint or latex-based 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 their online reporting tool.
  • 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

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).

Worried about oak wilt? (You should be)

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Photo 1. Oak wilt symptoms. A) Dying red oak showing foliar wilt symptoms. B) Crack in the bark indicating mycelial mat presence. C) Nitidulid beetle visiting a mycelial mat. D) Gray spore containing mycelial mat and pressure pad. Photos: Monique Sakalidis

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.

 

Photo 2. Typical fallen leaves associated with oak wilt. Photo: Monique Sakalidis.

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

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).