No Cure for Oak Wilt Disease
No cure for oak wilt disease available but suppression possible
Round Rock Leader
Saturday, October 2, 2004
By Emsud Horozovic, City of Round Rock Forestry Department
Recently I received several requests for assistance from residents suspecting they have oak wilt present in their trees. This article is an attempt to clarify the facts about the disease and therefore is written not for a scientific journal but rather to explain to Round Rock residents what to look for. Although we are fortunate that we do not have a lot of oak wilt in our city, it is a big problem in surrounding communities, particularly in Austin, where I use to work for years on oak wilt suppression. In Round Rock we do not have live oak lined streets. Therefore, it did not spread like an epidemic. We do have about five locations with oak wilt in town and that should not cause alarm but rather be considered a lack of a problem, so far.
Austin is losing one of its most important assets - the live oaks and red oaks that form a shady, green canopy over the city. These oak trees are being threatened by a contagious disease called oak wilt. Over the past 20 years, Austin has lost more than 10,000 high valued, residential trees to oak wilt. For both the home owner and the city as a whole, this loss is felt by increased utility bills, reduced property values, and a sense of devastation.
Oak wilt is caused by the fungus Ceratocystis fagacearum that clogs the water-conducting tissues of oak trees. The trees become dehydrated and in most cases die. Live oaks (Q. fustiformis and Q. virginiana) and Texas red oaks, particularly Spanish oaks (Q. texana) and Blackjack oaks (Q. marilandica) are especially vulnerable to oak wilt. White oaks, such as Bur oak (Q. macrocarpa) Chinkapin oak (Q. muehlenbergii), and Post oak (Q. stellata), are much less susceptible and mostly do not get oak wilt.
In the early 1940s, oak wilt was confirmed in Wisconsin. In Texas, cases of dying oaks were reported as early as the 1930s, but oak wilt was not considered responsible. It was believed that the fungus could not survive Texas’ high summer temperatures. Oak mortality in Texas was misdiagnosed, leading to years of improper treatment. Oak wilt was officially diagnosed in the 1960s, and has now been documented in 56 Texas counties. Twenty-two states have reported cases of oak wilt. Austin is probably the most heavily infected city in the nation.
Symptoms and Spread
There are three means of oak wilt spread: By beetles that feed on fungal mats from infected red oaks and carry oak wilt spores; through the roots, mostly on live oaks; and from diseased firewood.
Oak wilt on live oaks can be identified by a distinctive yellowing or browning along the veins of the leaf. Soon after these symptoms appear, the trees defoliate. Once the fungus enters the tree, most live oaks die between three months to a year.
Red oaks do not survive oak wilt infection. The initial symptoms of oak wilt on red oaks include the leaves turning pale green and then brown while remaining on the tree. Red oaks die between two weeks and several months after infection.
In the late fall and early spring, oak wilt fungal mats or masses of oak wilt spores are produced beneath the bark of diseased red oaks. The mats are sweet smelling and attract the sap-feeding nitidulid beetles. Oak wilt spores adhere to the insects’ bodies as they feed. These beetles are also attracted to the sap oozing from broken limbs or pruning cuts on healthy trees. While feeding on these trees, the beetles deposit the spores from the fungal mats and new infection centers are formed.
Oak wilt also spreads underground from live oak to live oak through common root systems. Root sprouting and root grafting are characteristic traits of live oaks. Several live oaks are capable of sharing one common root system. When one live oak becomes infected, all live oaks sharing the same root system have the potential for contracting oak wilt. The disease spreads through these communal roots systems at a rate of about 100 feet per year.
Prevention and Suppression
Currently, there is no cure for oak wilt. The only measures against this fungus are prevention and suppression.
The first precaution to be taken is to remove diseased red oaks before fungal mats can form. Another measure is to avoid pruning when the fungal mats are forming and the nitidulid beetles are the most active, during the fall and spring. If pruning cannot be avoided, it is important to immediately paint all wounds on all oak species at all times of the year. Pruning saws should be disinfected when pruning between different work sites. When pruning adjacent to oak wilt infested areas, disinfect tools between trees. After 48 hours, painting is not effective because the wounds are no longer attractive to the insects.
Infected red oak trees cut for firewood can also spread oak wilt. Diseased red oak logs can harbor the fungal mats full of oak wilt spores. Precautions to take regarding firewood include using wood other than red oak, burning only dry, seasoned wood, and keeping the wood pile securely covered with a clear, plastic tarp.
A fungicide called propiconazole (Alamoä) has given some relief to homeowners who have infected trees or who are in the immediate path of infection. The chemical is injected into the base of the tree, and is most effective as a preventative treatment. Although the fungicide can be used to save individual trees, it does not kill the fungus in the roots, and therefore, does not keep the disease from spreading from tree to tree.
A commonly used method of suppressing the spread of oak wilt is trenching. Trenching involves severing the common root systems of live oak trees with a rocksaw to a depth of three to four feet. Trenches are placed at least 100 feet from the front of the disease in order to prevent further underground spread of oak wilt. This method of control has proven to be 70 percent effective state-wide.
Very often people ask if they can plant new oak species on the site where oak trees died. Young trees, including oaks, can be planted on infected sites because the possibility of them getting root-grafted to old, infected roots is minimal to none.
For further detail information on Oak wilt please check out following web sites:
For Oak wilt assistance outside of Round Rock city limits please contact Texas Forest Service office in Austin at (512)451-2178.
And for the end just a reminder that live oaks are not true evergreen trees. They do recycle/drop their leaves every spring in March and April and then I get lots of phone calls from residents thinking they get Oak wilt. Keep a watchful eye on the foliar symptoms and let me know if I can help.
Forestry Manager Emsud Horozovic