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 devastated. Oak Wilt currently affects 65 counties in Texas. Oak Wilt is caused by a fungus with the scientific 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.
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.
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.
This example shows how the veinal necrosis can be finely detailed throughout the leaf.
Old leaves showing veinal necrosis coexist with new leaves (not showing symptoms yet) put on in the spring.
This is an example of how veinal necrosis is typically presented on a leaf.
The tree these were collected from was generating a large number of symptomatic leaves.
Some Live Oaks turn a dark red color prior to dropping their leaves in the spring. Veinal necrosis on this leaf shows up bright red.
The veinal necrosis pattern is distinctive in old leaves.
Veinal banding or inter-veinal necrosis is shown here. The yellowing of the area between the veins is not common but really helps to illustrate the symptom.
When veinal banding occurs, the veins remain a normal green color and the remainder of the leaf turns pale.
This leaf has a combination of both VN and VB.
Margin burn is a symptom of oak wilt where the edge of the leaf turns brown. These leaves also have veinal necrosis.
This symptom is similar to margin burn, but only the ends of the leaf turn brown.
New leaves on Live Oaks often do not show symptoms. But when they do, you can see hints of VN and margin burn.
Sometimes a Live Oak will die from Oak Wilt very quickly. These trees turned completely brown in a matter of days. Often this type of quick death does not produce any of the symptoms shown above.
The normal progression of the disease in Live Oaks is extreme thinning of the canopy like that shown here.
Red oaks like this one die quickly from Oak Wilt. This tree has major portions of the canopy browning out.
One of the symptoms of Oak Wilt in red oaks is an apparent bronzing of the leaves as shown here.
Another common symptom of Oak Wilt in red oaks is tip burn.
Red oaks do not typically show VB. However, this example was observed in Marble Falls.
Shown is a fresh Oak Wilt fungal mat on a red oak. The insects that spread oak wilt are highly attracted to these spore mats.
Shown is the remnant of an Oak Wilt fungal mat on a long dead red oak. This spore mat is no longer an infection hazard because it has dried out.
The presence of fungal mats (examples shown above) 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.
A cluster of dead Live Oaks may indicate the presence of an Oak Wilt disease center. 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.
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 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. 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.
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.
1. Avoid pruning Oak trees between early February and late June.
2. 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.
3. Trench around active Oak Wilt centers to stop the spread of the fungus through common or grafted roots.
4. Watch trees in and around Oak Wilt centers for new infections. Take steps immediately to prevent additional spread.
5. 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.
6. 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 at 877-842-8733 before you work on your trees or if you suspect that you have a problem.