Oak Wilt

 

This disease of oak trees is caused by a non-native fungus known as Ceratocystis fagacearum.  The fungus invades the vascular system, or xylem, of a tree, which include the water conducting vessels that transport water up to the canopy of the tree from the roots. 

As a defense mechanism, an infected tree produces substances in the xylem in an attempt to block the fungus from spreading throughout the tree.  In reality, these substances actually clog the water-conducting vessels, preventing water from reaching the canopy (leaves) of the tree.  Eventually, most infected trees die as a result of an oak wilt infection.      

     

 


 

All types of oak trees found in Minnesota are affected by oak wilt.  However, some oak species are more resistant to the disease than others.  Generally, oaks can be placed into two groups, the red oak group (leaves with pointed lobes) and the white oak group (leaves with rounded lobes). 

The red oak group includes both red oak (Quercus rubra) and northern pin oak (Quercus ellipsoidalis) and is highly susceptible to oak wilt. Trees in this group can succumb to the disease in as little as a few weeks to a few months.  Once a tree in the red oak group is infected, no current treatment is available to save or “cure” it.

The white oak group, which includes bur oak (Quercus macrocarpa), white oak (Quercus alba), and bicolor oak (Quercus bicolor), is susceptible to oak wilt, but these species are able to withstand the disease for longer periods of time (usually one to several years).  Furthermore, if an infected white oak is identified and chemically treated early enough, it can often be preserved.

The images to the right offers comparisons of oak leaves.

                         


   

Symptoms

 

Oak wilt in the red oak group can be identified by the rapid wilting of the entire canopy.  As previously mentioned, an infected tree of the red oak family could be entirely defoliated within just a few weeks.  The wilting begins at the top of the tree and progresses downward.  As the wilt progresses, the tree begins to shed its leaves.  Individual leaves tend to wilt from the edges inward toward the midrib or stem of the leaf and from the tip down toward the base. 

Infected trees in the white oak group show similar symptoms; however, the process is much slower, often times over a period of several years.  Another possible sign of oak wilt is discoloration or streaking of the wood just below the bark surface.  However, this symptom is not always evident and should not be solely relied upon when trying to determine whether a tree is infected with oak wilt.

    

How is Oak Wilt Spread

 

The fungus that causes Oak Wilt can spread in one of two ways.  First, the fungus can be spread overland through various sap-feeding insects.  Infected trees in the red oak family have the potential of producing spore mats the year following infection (the white oak family of trees do not produce spore mats).  Certain insects are attracted to the fruity aroma produced by these mats.  As insects contact the spore mat, fungal spores can stick to their bodies; when these insects move to a freshly wounded oak tree to feed on the sap, the fungal spores can rub off and infect the new tree.

 

The second way that oak wilt spreads is through grafted (connected) root systems of individual trees.  It is not at all uncommon for oak trees of the same species (i.e. red oak to red oak or northern pin oak to northern pin oak) to have a “joint” root system with an adjacent tree(s).  If one tree is infected with oak wilt, the fungus can move through that tree’s root system and into the healthy tree’s root system, resulting in another infected tree.

 

 

Root Grafts.  Root graft transmission is the most common mode of infection.  Over 90% of all new oak infections are transmitted in this manner.  A root graft is formed when the roots of the same species meet and fuse together.  The disease is then able to move from the infected tree into the uninfected tree.

 

 

 

How to Manage and/or Treat Oak Wilt

 

There are several methods available for managing and/or treating oak wilt.  The most effective means of managing the spread of oak wilt is through root graft disruption.  This is accomplished with a vibratory plow or trenching machine that physically cuts through potentially grafted (connected) roots.

An example of vibratory plow is shown to the right.

 

 

          
 

Chemical injections can also be used to prevent new infections from occurring in red and northern pin oaks and to therapeutically treat trees in the white oak family if detected early enough (generally less than thirty percent [30%] canopy wilt).  Chemical treatment is accomplished by injecting propiconazole into the water conducting vessels of a tree, as shown in the picture to the left.

 

Often, this treatment will be used in conjunction with root graft disruption to provide healthy trees a better chance of survival.

 

Tree removal is also an important part of any management program.  Potential spore producing trees (trees that can produce the fungal spore mats) should be removed and properly disposed of no later than March 1 of the following year to prevent the possibility of overland spread of oak wilt.  If the wood is to be used for firewood, it should be: 

 

  • Cut and split the wood into firewood size pieces;
     
  • Cover the entire woodpile(s) with black 4 mil or thicker plastic sheeting;
     
  • Tape all seams or breaks in the plastic with duct tape or some other heavy-duty tape;
     
  • Bury the edges of the plastic around the woodpile(s) so the wood is 'sealed in'.  An airtight seal is crucial to disrupt the oak wilt fungus life cycle.
     
  • Keep wood covered in this manner from April 1 until October 1.  After October 1, the plastic can be removed as the wood should no longer pose a threat of spreading the oak wilt disease.

     

    One of the easiest and most important steps in preventing new occurrences of oak wilt is simply to avoid pruning oak trees April through July.  This is not only when sap is flowing readily but also when the insects that can transfer the disease from one tree to another are most active.