How to control and diminish the spread of oak wilt

Two weeks ago I wrote about oak wilt, its detection, causes, how it's spread and which trees are susceptible to it. More than 35 native and exotic oaks may be stricken, with members of the red oak family being primary victims. Other trees, such as American and European chestnuts, chinquapin (chinkapin), tanoak and several varieties of apple, may also succumb to the deadly fungal disease.

Let's now look at control and ways of diminishing the spread of oak wilt. Currently, there is no cure, although a chemical inoculation may provide healthy trees with some degree of immunity. At present, the best way of dealing with outbreaks is to minimize the risk of it spreading from tree to tree. While no method of control is guaranteed, it is possible to make the transmission of the fungus more difficult.

On the front line is the destruction of trees that have been positively diagnosed. If you're not comfortable identifying a suspect tree as infected with oak wilt, contact your county agricultural agent or local city, county or DNR forester for a professional opinion.

What you don't want to do is cut down a mature, aging monarch by mistake. Nor do you want to prune oaks at the wrong time of year or implement improper control measures. Just a few words on this matter before we continue with oak wilt. During the past year, I have seen radical, severe mass pruning of trees along power lines not only in our area, but also across the state. As a trained arborist, I am appalled at the blanket approach and excessive nature of the pruning done in the name of right-of-way. Judging from the rise in my blood pressure, I can see this is a topic I need to cover in another column. Let it suffice to note, that improper and extreme pruning not only weakens a tree, but makes it more vulnerable to disease and death. With that said, let's move on.

Trees that 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 six inches in the ground. Once the plastic is removed, the wood should then be dried. A second line of defense is to slow down or prevent root transmission of the fungus. To do so, root connections between diseased and healthy trees should be severed. It's important 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 may require specialized equipment and know-how, so once again consult a tree maintenance specialist. Before you hire someone to do any work on your trees, be certain they are knowledgeable. Be well informed and ask professional arborists or foresters for recommendations of individuals or companies qualified to do control measures.

Don't be afraid to quiz a prospective candidate who may do the work for you and be sure the person who gives you satisfactory answers is the person who will be doing the actual pruning, severing or other necessary tasks. If you're uncertain about the answers you receive or the individuals are not comfortable with you asking questions, tell them, "I'll think about it and get back to you if I decide to use your services." In our area we have exquisite stands of oaks, we don't want to put the fate of these beautiful trees in the hands of incompetents.

On properties with many oaks, the digging of "barrier lines" may also be considered. The construction of barrier lines involves the severing of roots on a broad scale to separate areas of infection from areas that 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 that appear healthy.

A third tool in slowing down the spread of oak wilt, is to minimize 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. Therefore, it is important to keep pruning to a minimum in the spring and early summer when the sap beetle is active and fungal mats most pronounced. If pruning is unavoidable, the wound should be covered with tree paint.

Tear wounds caused by storm or wind damage should be "cleaned up" and the edges made as smooth as possible. Apply tree paint to the injured area. A great deal of information on this subject is available at the library, through the Extension Service and DNR and on the Internet.

On a final note, as if oak wilt wasn't devastating enough, a new pathogen,(Phytophthora ramorum) has appeared on the western horizon of the U.S. and in eastern Europe. Although clearly not limited to oaks, it is called Sudden Oak Death. It has been found, as of July 2002, in California black oak, coast live oak, Shreve oak, tanoak, rhododendron, California bay laurel, big leaf maple, madrone, manzanita, huckleberry, California honeysuckle, toyon, California buckeye, California coffeeberry and Arrow wood (in Germany, the United Kingdom and the Netherlands. Sudden oak death has now also spread into Oregon. Let's hope the Rocky Mountains serve as a total barrier to this destructive disease and that it never reaches the Midwest, where oak wilt has done its most damage.

Sources: USDA-Forest Service, TreeHelp.com