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Oak Wilt

Nitidulid beetles - major cause of the spread of oak wilt. Spore mat of tree infected with oak wilt.

Nitidulid beetles - one vector of oak wilt
US Forest Service

Fungal mat

Introduction

Oak wilt is a fungal infection affecting oak trees. All species of oak are susceptible with red oaks being particularly vulnerable. In red oaks, oak wilt is almost always lethal and death can occur in as little as one month. There is currently no known cure and the best way of dealing with oak wilt is to isolate and then destroy the affected trees. This disease has been discovered in 21 eastern US states with the heaviest damage occurring in the mid-west states surrounding the Great Lakes. However, oak wilt has been reported as far south as Texas.

Oak wilt is caused by a fungus (Ceratocystis fagacearum) which clogs the vascular system of oak trees preventing the flow of water and nutrients. Once infected, the entire tree literally wilts and dies.

What Trees are Affected

All oak trees are susceptible to oak wilt infection to a greater or lesser degree. Red oaks are the most vulnerable but the disease also attacks and kills white oaks and Texas live oaks.

Red oaks are more susceptible than members of the white oak family because they do not have the ability to produce the ‘tyloses’ or vascular plugs which white oaks create to contain damage due to breakage or disease. It must be noted, however, that in most cases, the white oak’s natural defenses only slow, not stop, the spread of oak wilt, allowing the white oak to survive, perhaps, for years instead of months. And it should be remembered that, even in cases, where a white oak survives an attack of oak wilt, the tree can remain a host for the fungus and give it a base from which to spread.

Symptoms

Oak wilt symptoms on the leaf of a white oak, northern red oak and Texas live oak.

Oak wilt symptoms on the leaf of a A) white oak, B) northern red oak, and C) Texas live oak
US Forest Service

Leaf damage is the earliest indication of oak wilt.  Leaves of infected trees begin to lose their green color, dulling and then browning or yellowing from the outer edges inward.  The leaves may appear to be water-soaked and wilting and may begin to curl around the mid-rib.  As the disease advances, leaves begin to drop, sometimes while still green, starting from the ends of the branches.

The speed of the progression of the disease depends upon the species of tree infected.  Red oaks can die from oak wilt in as short a time as a month.  Texas live oaks may survive as long as six months.  White oaks may survive as long as several years after infection. In a small number of instances, Texas live oaks and white oaks appear to have survived oak wilt infection.  However, it would be unwise to depend on this.

In red oaks, a ‘fungal mat’ may appear when large masses of fungal tissue break the bark.  These mats give off a distinctive odor which attracts different species of insects.

Unfortunately, particularly in the case of red oaks, by the time symptoms are noticed, it is already too late to save the tree.

Transmission of Oak Wilt

Oak wilt is spread largely in two ways, insect transmission and root transmission. 

Oak wilt life cycle.
US Forestry Service

 Insect transmission is generally by means of the sap (Nitidulid) beetle. These beetles are attracted to the damaged parts of trees where sap may be present and to the fungal mats created by the oak wilt fungus. The beetles transfer fungal spores attached to their bodies as they move from tree to tree.  This is of particular concern as they move from the spore mats (right) of infected trees to freshly damaged areas of healthy trees.

 Root transmission is by the underground root systems that often connect one oak tree with another. The fungus simply migrates from oak to oak by travelling through the vascular system of the roots in the same way that it spreads within the tree itself.

Control

There is, currently, no cure for oak wilt although a chemical inoculation may provide healthy trees with some degree of immunity.  At present, the best way of dealing with outbreaks of oak wilt is to minimize the risk of it spreading from tree to tree.

There is no guaranteed way of preventing the spread of oak wilt from one oak tree to another.  However, there are ways of making the transmission of the fungus much more difficult.

Destroying infected trees.  Diseased trees and trees which have died of oak wilt should be cut down and the wood burned, chipped or covered in plastic for six months with the edges of the plastic buried at least 6 inches in the ground.  Once the plastic is removed, the wood should then be dried.

Severing root networks.  To slow down or prevent root transmission of the fungus, root connections between diseased and healthy trees should be severed.  It is important to do this prior to the removal of diseased trees as the potential for spore transmission through roots is highest just after a diseased tree is removed.  Root severing is a task which may require specialized equipment and know-how.  This is a job best left to a tree maintenance specialist.

On properties with many oaks, the digging of ‘barrier lines’ might also be considered.  The construction of barrier lines involves the severing of roots on a broad scale to separate areas of infection from areas which are infection-free.  Once the barrier lines are established, consideration should be given to the removal of all oaks within an infected zone, even those trees which appear healthy.

Minimizing tree damage.  The transmission by insects of spores from diseased trees to wounds on healthy trees is one of the key ways oak wilt is spread.  It is important, therefore, to minimize pruning in the spring and early summer when the sap beetle is active and fungal mats most pronounced.  Some wounds, of course, are the result of storm damage or other factors.  

Strengthening your trees.  A healthy tree is less likely to become grievously infected than an unhealthy tree. Proper fertilization is very important (avoid quick-release Nitrogen formulations). 

Oak wilt is a dangerous disease, but, with proper tree maintenance, the threat to the beautiful and valuable oaks can be minimized.