Oak Wilt is Widespread

Oak Wilt is one of the most destructive tree diseases in the United States and is found in 25 states. In Central Texas, and especially the Hill Country, our vast forest of Live Oaks is being devasted. Oak Wilt currently affects 65 counties in Texas. Oak Wilt is caused by a fungus with the technical name of Ceratocystis fagacearum. This fungus invades and colonizes the water conducting tissues of healthy Oak trees. The reaction of the tree to this invasion results in the blockage of these water conducting tissues; ultimately resulting in severe die-back or tree death.

Oak Wilt Spreads Above and Below Ground

The disease can spread in two ways; via sap feeding insects (Nitidulid beetles) carrying spores on their bodies (overland transmission) and via interconnected roots among trees (root transmission). Other methods of transmission may be possible but have not been scientifically proven.

Overland transmission of Oak Wilt begins with an infected Red Oak (Shumard Oak, Spanish Oak, Blackjack Oak, Texas red Oak, etc.). When the tree dies, one or more fungal spore mats may form under the bark. The mat grows and expands causing the bark to crack open. The spore mat emits an odor that attracts Nitidulid beetles. The beetles enter the spore mat to feed and spores stick to the insect. These beetles then travel to other trees to feed. The insects are attracted to fresh wounds emitting sap. The Oak Wilt spores may then infect these other trees, starting a new disease center. Experiments have shown that under their own power these insects can travel a mile or more and can appear on fresh wounds in 15 minutes or less.

Root transmission is largely a problem for Live Oaks. The root system of one Live Oak is highly interconnected to neighboring Live Oaks. The fungus travels through the roots from one Live Oak to the next. The disease can spread in this manner from an infected tree at a rate of approximately 75 to 100 feet per year on average. Due to the high concentration of Live Oaks in Central Texas, root system transmission destroys large areas of Live Oak forest. Spore mats do not form on Live Oaks as they do on Red Oaks.

Contrary to popular belief simply removing dead trees either by cutting them down or bulldozing them, does not eliminate the root transmission of the disease. The only way to prevent root transmission is to completely sever the root system.

Identification of Oak Wilt

Confirmation that Oak Wilt is present in a particular spot can sometimes be challenging. However, a Certified Arborist that specializes in Oak Wilt can usually make a diagnosis in a fairly short period of time. A professional can make sure that other factors such as chemical poisoning, drought, construction damage and lightning strikes are not misdiagnosed. There are four primary ways to identify Oak Wilt; foliar (leaf) symptoms, patterns of tree mortality, fungal mats and laboratory tests.

The prominent symptom most commonly associated with Oak Wilt is the distinctive browning out of the veins in Live Oak leaves. This pattern that often reminds people of "fish bones" is called veinal necrosis. The veins in the leaf become yellow or brown and the leaf soon falls off the tree. Even after drying out and turning completely brown, the symptomatic leaf retains the distinctive darker veins. Unfortunately, veinal necrosis does not occur in red Oaks. Trees in this family defoliate in a manner similar to autumn with the leaves turning brown from the edges in and then falling off. Defoliation of a red Oak at an odd time of the year should always be investigated. In addition to veinal necrosis in Live Oaks, there are other types of foliar symptoms including interveinal necrosis (the reverse of veinal necrosis) or veinal banding, tip burn (where the end of the leaf turns brown) and margin burn (where the entire edge of the leaf turns brown). Major defoliation (leaf loss) of a Live Oak other than during spring (when they normally replace their leaves) should be investigated.

Live Oak Foliar (leaf) Symptoms
Veinal necrosis is the classic Oak Wilt foliar symptom. Symptomatic leaves tend to drop from the tree soon after the symptom develops. Look for these leaves on the ground as well as on the tree. The number of symptomatic leaves that a tree generates varies a great deal from tree to tree.
Veinal banding (interveinal necrosis) is a less common symptom. At times this symptom appears on trees that are having a less severe reaction to the disease.
Tip burn is where the end of the leaf dies. Insects and chemicals might cause this also, so be careful making a diagnosis on tip burn alone.
Margin burn is similar to tip burn but the area of brown includes the end and sides of the leaf.
The symptomatic patterns illustrated above persist in the leaf even after the leaf has dried out. Many times we look on the ground for the presence of fresh or old symptomatic leaves to help us determine if Oak Wilt is present.

A cluster of dead Live Oaks can be a symptom of Oak Wilt. This is especially true if the cluster continues to expand in size. Generally, Oak Wilt disease centers have completely dead trees at the center ringed by sick and dying trees further out followed by normal looking trees.

The presence of fungal mats (an example is shown to the left) on red Oaks is most definitely a sign of Oak Wilt. However, mats do not form on Live Oaks and they do not form on all infected red Oaks. The mats are commonly hidden from view under the bark and are therefore not immediately obvious.

Oak Wilt can be confirmed by growing the fungus in the lab. Samples from trees suspected of having the disease can be harvested and sent to the Texas Plant Disease Diagnostic Laboratory in College Station, TX. However, due to the delicate nature of the fungus it can be difficult to get a viable sample. The result is that there are a fair number of false negatives from the sampling process.

Management and Control of Oak Wilt

Once a positive identification of the disease has been made, there are three possible methods of dealing with the disease; do nothing, sever the root system between infected trees and healthy trees, and injection of fungicide into trees. Our staff of experienced professionals can help you balance a combination of all three possibilities by explaining the economics and aesthetics involved. We can design a management program based on your budget, the forest environment on your property and the realities of Oak Wilt.

Doing nothing may be an appropriate response in areas where there are a limited number of trees at risk and / or the value of those trees is minimal. Such a course of action might be taken where Oak Wilt has already claimed the high value trees in an area and the remaining specimens are in poor health or in a remote area.

Root trenching is appropriate where there is sufficient space and lack of existing buried infrastructure to correctly and economically install a trench. Click this link to go to our Root Trenching page.

Fungicide treatment is appropriate for high value trees that cannot be isolated by a trench. In general, injections work very well with pre-symptomatic trees and for smaller areas. Click this link to go to our Injection Page.

Prevention of Oak Wilt

Many new Oak Wilt disease centers are caused by human activity. We can all take steps to reduce the chance of starting a new Oak Wilt center.

Avoid pruning Oak trees between early February and late June.
If it is necessary to prune susceptible trees, or if trees are wounded, use a wound paint (pruning sealer) immediately. A new infection can be started quickly on a fresh wound (less than 15 minutes). Wounds are naturally sealed off by the tree after about a week. So painting old wounds is of no value. All wounds, regardless of size, should be painted.
Trench around active Oak Wilt centers to stop the spread of the fungus through common or grafted roots.
Watch trees in and around Oak Wilt centers for new infections. Take steps immediately to prevent additional spread.
Sanitize cutting equipment with a bleach solution (or Lysol® spray) before cutting on an Oak tree and before moving on to the next tree so as to avoid the possibility of cross contamination.
Avoid firewood infected with the Oak Wilt fungus, unless it has properly dried (12 months). Cover new firewood piles with clear plastic for one year. Make sure that the edges of the plastic are buried with soil to prevent insect migration out of the wood pile.

Oak Wilt is a serious problem. Call us before you work on your trees or if you suspect that you have a problem.