About Oak Wilt
Frequently Asked Questions
What is oak wilt and how does it affect trees?
Oak wilt is a major tree disease caused by the fungus, Ceratocystis fagacearum. This fungus plugs the water conducting vessels of certain oaks and greatly reduces the flow of water up the stem of the tree. Eventually, as more of the vessels become clogged, the tree will begin to wilt and most often die.
Infected live oaks usually die in three months to one year. Approximately 10 percent of the live oak population may survive for many years in various states of decline and never fully recover. Red oaks typically die very quickly, within two weeks to several months.
Where is oak wilt located?
Texas Distribution of Oak Wilt
U.S. Distribution of Oak Wilt
What types of trees can get oak wilt?
Oak wilt affects live oaks and members of the red oak family including: Spanish Oak, Shumard Oak, Water Oak, Blackjack Oak, Pin Oak, and others. However, trees in the white oak group such as Bur Oaks, Chinkapin (Chinquapin) Oaks, and Post Oaks are somewhat resistant to oak wilt
The two trees hit hardest by oak wilt in the Hill Country are Live Oaks and Spanish Oaks (red oak) or also known as the Texas Red Oak.
How does oak wilt spread?
Oak wilt spreads in two ways:
- Underground -- It is very common for oak trees to establish a system of interconnected roots. These connected roots allow the oak wilt fungus to move from tree-to-tree, often leading to patches of infected and dead trees. In fact, once one oak becomes infected, the disease can radiate outward, spreading to any oaks that share a common root system. This pattern of disease movement can be compared to the ripples of water that appear after dropping a rock in a pond. On average, oak wilt will move at a rate of 75 to 100 feet per year. In urban areas with numerous live oaks, the disease can move from one house lot to another each year.
- Above ground -- The disease can be spread by ( Nitidulid ) beetles carrying fungal spores from diseased red oak trees to fresh wounds on healthy oaks. These beetles are responsible for the long distance spread of oak wilt and are the cause of new oak wilt centers. When a red oak becomes infected with oak wilt, it can form a fungal mat under the bark. This fungal mat is the reproductive structure of the disease and produces millions of spores. When beetles feed on this sweet smelling mat, the spores stick to their bodies. The beetles also feed on the sap oozing from fresh wounds on healthy oaks During feeding, the beetles can deposit fungal spores onto a wound on a healthy tree and form a new infection center. Long distance spread can also occur when beetles are attracted to green, infected red oak firewood. If a beetle flies from a firewood fungal mat to a fresh wound on a healthy tree, infection can occur.
How can I tell if my trees have oak wilt?
In live oaks, leaves on infected trees show a characteristic pattern. The veins on the leaf will turn yellow or brown, while the rest of the leaf remains green. This very distinctive pattern is called veinal necrosis. Some leaves may turn yellow with only the veins remaining green (interveinal chlorosis), or the tips of some leaves may turn brown (tip burn). While these last two symptoms may indicate oak wilt, they can also be indicators of other problems like herbicide damage. Once symptoms appear, live oaks usually drop their leaves and die over a period of three months to one year. Some trees can survive for many years in various states of decline. When looking for symptomatic leaves, check both the crown (top) of the tree and on the ground underneath.
In red oaks, the time between the first sign of symptoms and tree death occurs much quicker than in live oaks. Red oaks die within several weeks to several months after the first symptoms appear. Leaves begin wilting, and soon after, the tips will begin turning reddish-brown or brown. This will continue until the leaves are entirely brown. Leaf symptoms usually develop on one branch and then quickly spread to the whole tree. . After the tree has died, the leaves will often remain on the tree for a short time. Red oaks infected with oak wilt do not survive the disease and the flare root injection is totally ineffective.
How can I prevent my trees from getting oak wilt?
The above ground spread of oak wilt from sap-feeding beetles is a critical factor in the establishment of new disease centers. Beetles can carry the deadly oak wilt fungus to fresh wounds. A wound can be a trunk scar (from a bulldozer, lawnmower, weedeater or caused by wild life), a torn root or even a pruning cut. Making proper pruning cuts and immediately painting the wounds significantly lowers the risk of oak wilt infection. Pruning paint or an inexpensive spray paint acts as a barrier to the beetles that are attracted to fresh wounds. Hire a professional arborist to prune your trees or delay pruning until the summer or winter months. During spring, from February 15 to June 15, our oaks are most susceptible to oak wilt infection.
A Spanish Oak or Texas Red Oaks that die of oak wilt during the summer or fall, should be removed immediately as during the cooler weather on into early spring, new spore mats will develop and the odor of fruiting spores, attract the beetle, and in-turn the beetle carries the spores on to a healthy tree, which will becomes infected.
It is also important to be careful with firewood. Firewood from red oaks infected with oak wilt can harbor fungal spores and/or beetles carrying the fungus. As a precaution, store firewood under clear plastic and bury the edges. Clear plastic traps the insects, and unlike black plastic, the beetles cannot use light holes to escape.
What if my live oaks already have oak wilt, or the disease is nearby?
A fungicide called propiconazole (Alamo TM) can be used by homeowners, ranchers and landowners who have newly infected live oaks or who are in the immediate path of infection.
If possible, live oaks and red oaks should be treated when the oak wilt is within 75 to 100 feet. Close monitoring of nearby oak trees is essential in identifying the oak wilt's presence and spread. It is best to inject trees before infected, as a preventive application, although there has been some success treating trees with therapeutic dosage rates, once they are infected. However, success diminishes rapidly the longer the treatment is withheld. The best candidates for injection are those which are immediately threatened, but are not yet showing symptoms. All live oak trees within a trenched area should be injected unless they have lost more than 20% of their leaves.
Although Alamo (TM) can be used to save individual trees, it is antagonistic to the fungus, though it is not a cure it will extend the life of the tree. The fungicide cannot totally destroy the fungus in the roots and therefore, does not keep the disease from spreading tree-to-tree.
A commonly used method of stopping the underground spread of oak wilt is trenching inorder to circumvent the oak wilt center. Trenching severs the interconnected root systems with heavy machinery to isolate diseased trees from healthy trees. Trenches should be placed at least 100 feet from the last symptomatic trees to contain the disease. Trenches must be a minimum of three to four feet deep. Existing utility trenches that are less than ten years old and more than three feet deep can be used as a barrier to disease spread. Approximately 70 percent of all the oak wilt trenches installed statewide have successfully stopped disease spread.
click on fig. to enlarge
In urban settings, cooperative action is required to stop the spread of the disease. An oak wilt suppression project usually involves multiple property owners, and benefits the whole neighborhood. Cost-share assistance from the Texas Forest Service Oak Wilt Suppression Project is available to ranchers, landowners, neighborhood associations who organize to install recommended suppression trenches.
What can I do if my red oaks contract oak wilt?
When red oaks become infected with oak wilt, they should be removed immediately to prevent the formation of the fungal mats. This is very important since the fungal mats are the reproductive structure of the oak wilt disease. Red oaks do not survive oak wilt infection. Diseased red oaks play a critical role in the above ground, long distance spread of oak wilt to new areas. Although long distance spread is less common than tree-to-tree spread, removing infected red oaks is an important part of managing disease spread. Red oaks that die in the late summer or fall and not immediately removed, the fungal mats will start to form during the cool of the winter and will become a full reproductive bloom in early spring when the beetle becomes most active and searching for food.
When homeowners are faced with infected red oaks, the trees should be cut, removed, and buried in a landfill. In remote areas, infected red oaks can be girdled and treated with an approved herbicide, or cut and left on the ground to dry out. Fungal spores cannot survive when there is a low moisture content in the wood. Preventive or therapeutic injections of Alamo is totally ineffective on Texas Red Oaks or Spanish Oaks due to the rapid progression of the disease within the tree.
Oak Wilt Assistance
Technical assistance is available to Texas Hill Country ranchers, landowners and homeowners from Arborist like ourselves or the Texas Forest Service. Oak wilt foresters and arborist can diagnose the disease and make recommendations for management and treatment. The Forest Service's oak wilt staff works with neighborhood associations, County Extension Offices to develop suppression plans to contain disease spread. Cost-share assistance is available to ranchers, landowners and neighborhood associations who undertake trenching projects. Half of the cost of trenching may be reimbursed by the Texas Forest Service.
For people living In the Texas Hill Country, contact your nearest Texas Forest Service office:
San Antonio - (210) 208-9306 - Mark Peterson firstname.lastname@example.org
Kerrville - (830) 257-7744 - Mark Duff email@example.com
Fredericksburg - (830) 868-7949 - Robert Edmonson firstname.lastname@example.org
Oak Wilt other FAQs
(frequently asked questions)
Avoid pruning your oak trees from February 15 to June 15. During this time of year, the spore-carrying beetles are most active and oak wilt spore production is at its peak. However, when you do prune your oaks, use proper pruning techniques and paint wounds immediately, no matter what the size, with a tree wound paint*. This will help prevent the beetles from transferring the fungal spores to the wound. Wound paint should also be used when any injury, such as storm or lawnmower damage, occurs to oaks. When pruning multiple oaks, and especially when pruning trees in an area with oak wilt, pruning tools should be disinfected with either Lysol TM spray, Consan 20 TM, (both contain ammonium Chloride, a triple action microbial disinfectant), or a 10% Clorox solution. (the most universal disinfectant but, Clorox is very corrosive). This serves as an added precaution against the transfer of the fungus.
* Painting tree wounds is recommended only for oaks susceptible to oak wilt.
Is it safe to mulch with or burn the wood from diseased trees?
Chipping or shredding the wood from infected trees to use as mulch is an acceptable method of utilizing the wood. The oak wilt fungus has certain moisture and temperature requirements in order to live. The pathogen is destroyed once the moisture content of the wood drops below 38 percent. Chipping or shredding allows the wood to dry out quickly, thereby killing the fungus.
Smoke from burning infected wood will not spread oak wilt. In fact, burning diseased logs actually kills the oak wilt fungus. The pathogen is destroyed at temperatures of 95 degrees F. However, diseased red oak firewood (from Spanish, Shumard, Blackjack, Water oaks, etc.) should not be stored on a homesite because the wood could contain fungal spores or insects which carry the spores and have the potential to infect healthy trees nearby.
Firewood from infected trees can be used if certain precautions are taken. If possible, find out where the firewood you are buying came from. Be extremely cautious of any oak firewood coming from an infected area or area you are unsure of. When buying oak, make every effort to buy only properly seasoned (dried) wood. Dry wood does not meet the specific moisture requirements of the oak wilt fungus. You can be sure that the wood is properly seasoned if the ends of the logs are cracking and/or the bark readily peels off the wood. It is also recommended to store red oak firewood under a sheet of clear plastic and tightly seal the edges of the plastic with soil or bricks. Doing so will prevent any spore-carrying beetles from escaping. It is also important to use clear plastic, as black plastic will reveal any escape holes to the beetles.
Note: These precautions need to be followed only for red oak firewood. If your not sure what kind of wood you are using, follow the above recommendations. Spore mats or fungal mats do not form on Live Oak logs and it is perfectly safe to burn as fire wood once it is air dried for one season.
Do I have to remove a tree killed by oak wilt?
Red oaks are the main concern for removal. On red oaks, the oak wilt spore producing fungal mats may form; these are responsible for the over land long distance spread of oak wilt. Therefore, once a red oak becomes infected, the tree must be removed immediately. The wood must then be chipped, burned, or buried. On live oaks, fungal mats do not form, so immediate removal is not necessary, nor is it mandatory as it is with red oaks.
When should I inject my threatened or infected trees
If possible, live oaks and red oaks should be treated when the oak wilt is within 75 to 100 feet. Close monitoring of nearby oak trees is essential in identifying the oak wilt's presence and spread. It is best to inject trees before infected, although there has been some success treating trees once they are infected. However, success diminishes rapidly the longer the treatment is withheld. The best candidates for injection are those which are immediately threatened, but are not yet showing symptoms. All live oak trees within a trenched area should be injected unless they have lost more than 20% of their leaves.