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Past Articles Library | Trees | What is Oak Wilt?

Oak Wilt (Ceratocystis fagacearum ) is a devastating fungus that kills oak trees in the Eastern part of the United States of America.  It was first identified in 1944.  Originally, oak wilt was thought to be native to the Eastern United States.  There is some evidence that it came to America in the early 1900s.  However, no exotic location has reported oak wilt, so its origin is unknown.  The disease has been much more prevalent since the 1980s, when many trees were wounded by homebuilders building in oak woods.

White oaks seem moderately immune to oak wilt.  This is probably due to the fact that when they are wounded, the form little caps called tyloses. These caps seal off the damaged vessels in the tree and prevent the fungus from entering them.  However, if a white oak does get oak wilt, it will die in one to several years.

Red oaks are the most susceptible to the fungus that causes oak wilt.  They can die as soon as three weeks after infection in some cases.   Recovery from oak wilt in red oak trees does occur, but it is rare.

Live oaks are moderately resistant to oak wilt.  However, because they have a habit of forming large colonies of cloned trees with interwoven roots, it spreads very rapidly when it does successfully colonize a live oak.

The oak wilt fungus is not known west of Texas.  Some studies have shown that the red oak is susceptible to the disease should it be introduced into their native habitat.

Oak wilt spreads two ways:  through the roots of one oak tree to another or through insect vectors. In a colony of oak trees, the roots grow together and form one continuous root.  If one of the trees in the clump has oak wilt, it will spread to all the other trees.  This is called an infection center.  It is not common for oak trees of different species to form a matted web of roots.  Depending on the species involved, roots one hundred feet away from an infected oak may be infected by that tree.

When an oak tree dies of oak wilt, the fungus in it may form fungal mats just under the bark.  As the mats mature, they they form specialized, non-spore-producing structures that exert outwood pressure on the bark.  Eventually, the mat causes the bark to split.  At this point, nitidulid beetles (sap beetles) can reach the mat.  They get fungal spores on themselves when they contact the mat, which produces a strong fruity or wine smell which attracts the sap beetles.

The sap beetles then fly to a healthy tree. Trees are often wounded by construction, storms, pruning incorrectly, and vandalism.  The wounds leak sap, which attracts the sap beetle. As the sap beetles feeds on the sap, it also brushes against the wound and leaves spores for oak wilt behind.  The previously healthy tree then becomes infected.  Red oaks and live oaks die rapidly, while it may take years for an infected white oak to die.  While not all sap beetles transmit oak wilt, there are approximately four that do.

Spore mats can usually form only during the spring, when the temperature and moisture are right.  In addition to the sap beetles that spread the fungal spores, oak bark beetles get the spores on them when they emerge from the infected tree and move to feed on the crown of healthy oak trees.  Oak bark beetles are considered minor vectors in the Central United States, but may be more important in the Appalachians.

The symptoms of oak wilt are different in white oaks, red oaks, and live oaks.  The symptoms for red oaks are: rapid leaf discoloration and wilting.  The first sign of trouble is  a visible off-green color shift on the leaves in the top part of the tree crown.  Shortly after this, leaves rapidly begin to wilt from the top of the crown.  The wilt continues to affect all of the leaves on the tree.  The individual leaves start to turn a bronze color.  Leaves fall off of the tree altogether as the infection continues to spread.  Generally, infected trees are defoliated  within a few weeks of the first symptoms.  Trees are often killed in groups or disease centers, when the infection travels through the grafted roots.

Sometimes the fungus is apparent as a brown substance plugging up the vessels.  In a cross section, it appears as a growth ring.  In a longitudinal cut, it appears of streaking of wood when the bark is removed from the tree.

White oaks suffer much the way red oaks do, but it may take several years for the fungus to kill the tree.  They tend to die slowly, as one branch at a time dies.

Live oaks may wilt and die rapidly or they may linger.  They generally die sometime within one to six months of infection.  Leaves develop yellow veins that eventually turn brown.  This is called veinal necrosis.  Affected leaves fall, and the tree crown progressively thins out until the whole tree is dead.  A small number of live oaks may survive oak wilt infection indefinitely while suffering varying degrees of crown loss.

Bacterial leaf scorch, anthracnose, decline, and infestation with twolined chestnut borer are often confused with oak wilt.  Treating these diseases and pests will not help a tree with oak wilt so proper identification of a tree is important to know how to treat it.  Foresters, arborists, or pathologists experienced with oak wilt can often identify it in the field.  They may take a sample from the tree, culture it, and see what grows in the culture.

There is no cure for oak wilt.  Once a tree is infected, the priorities shift to preventing the oak wilt from spreading to other trees.  The infected tree is removed and treated to kill any spores that are on it.  It is then chipped or cut up for firewood.

The best way to prevent oak wilt is to make sure you do not damage healthy oak trees.  Construction, heavy winds, or pruning should be avoided during the spring and early summer to make sure that the insects that carry the fungus do not spread it to a tree in your yard.

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