Frequently Asked Questions
Adapted from Common Questions About Oak Wilt (Texas Forest Service).
1. What is oak wilt?
OAK WILT is one of the most destructive tree diseases in the United States. The disease has killed more than 1 million trees in Central Texas. Oak wilt is caused by the fungus Ceratocystis fagacearum. The spores of the fungus invade and clog the tree’s water conducting system, called xylem.
2. Where is oak wilt a problem?
Oak wilt has been found in over 60 counties and in almost every city in Central Texas, as well as, Abilene, Midland, Lubbock, Dallas, Ft. Worth, College Station, Houston, and San Antonio. It can be a problem wherever live oaks tend to be the predominate tree. It does not matter whether they are transplanted or naturally grown. An individual tree’s age, size or previous health status does not make it more or less likely to contract or die from oak wilt.
3. Can my trees get oak wilt?
ALL OAKS ARE SUSCEPTIBLE!
White oaks - are the least susceptible. Very few have been identified with oak wilt in Texas. They generally survive for a number of years with the disease. Common White Oaks - Post, Bur, Chinkapin, Monterrey.
Red oaks - are the most susceptible. They typically die within 2-4 weeks of symptom appearance. Common red oaks are Spanish, Texas, Shumard, Pin, and Blackjack.
Live oaks - die in the greatest numbers, most often in expanding areas called Oak Wilt Centers.
4. How does oak wilt spread?
Oak wilt is spread by two methods, root transmission and by sap-feeding (Nitidulid) beetles. The fungus spreads from tree to tree through connected roots, either by root grafts or through a common root system.
Root Grafts, Common root system, and Nitidulid Beetle
The water conducting tissues in the roots need to be connected when a root graft is formed for oak wilt to spread to the next tree. This fungus does not live in the soil and can not be spread that way. Live oaks have a common root system where the trees in a group, or mott, have the same root system where all the water conducting tissues are connected, so the fungus can spread from tree to tree within that group of trees very easily. Different species of oaks can also form root grafts. This has been seen among live oak and red oak.
Live Oak Mott
When oak wilt does infect a red oak species these trees under the right conditions can form what is called a fungal mat. This is the second way the oak wilt fungus can spread. The red oak tree has to be infected with the oak wilt fungus in the fall and still be green. In the spring, these trees may form fungal mats.
Cracks in bark where fungal mat is formed underneath.
The fungal mats form underneath the bark and produce millions of spores. The bark is cracked which is caused by the pressure of these mats pushing against the bark of the tree. Fungal mats can be found all over the trunk and major branches of the tree.
Fungal mats have a very sweet smell like rotten banana’s or rip fruit. The Nitidulid beetles are attracted to sweet smells like rotten or ripe fruit so they are attracted to fungal mats. Nitidulids are about the size of the ball on the end of a straight pin. Nitidulid beetles feed and breed on fungal mats and as a result have the spores of the oak wilt fungus all over and within their bodies.
Nitidulid beetles feeding on fungal mat.
After the beetles feed and/or breed on mats they fly off in search of other food. Freshly wounded oak trees have a sweet smell and are also attractive to the beetles as a food source. If a contaminated beetle lands on a fresh wound on a healthy oak and starts to feed on the fresh wound, that tree can become infected with oak wilt. It does not matter how the wound was made, whether by chainsaw, wind damage, or a car bumper. Once established, the fungus moves from one tree to the next through common or grafted roots.
5. How quickly can oak wilt spread?
Because live oaks tend to grow from root spouts and can form root grafts very readily, all of the live oaks within a given area share a common root system. The pathogen can spread through this system at an average rate of 75 feet per year.
6. How can I tell if my trees have oak wilt?
Live Oak - Trees die within 6-12 months. Adjacent trees begin to drop their leaves and die a few months later. Live Oaks also show characteristic leaf symptoms including:
Veinal Necrosis - area around leaf vein turns brown rest of leaf is still green, found on the tree or on the ground. Veinal Necrosis is the most diagnostic.
Vein Banding – the leaf vein is a darker green then the rest of the leaf. This is symptom is seen before the veinal necrosis. These leaves are found on the tree.
Red Oak - symptoms are less distinct. Leaves turn pale green then brown usually remaining attached for a period of time. This can begin on one branch and quickly engulf the entire tree. Trees generally die within 4-6 weeks.
7. How can I prevent my trees from getting oak wilt?
Immediately paint all wounds on oaks to prevent contact with contaminated beetles.
Avoid wounding of oaks between the period of Feb. 1 - June 1.
Do not transport or buy unseasoned firewood.
Promptly remove and either burn or bury all red oaks that are dying or have been recently killed by oak wilt.
To protect from other diseases sterilize/sanitize all pruning equipment between trees.
8. How can I treat my trees if they have oak wilt?
There are two main recommendations that are generally given to treat oak wilt infection areas:
1. Stopping the spread through the roots
Measures can be taken to break root connections between live oaks or dense groups of red oaks to reduce or stop root transmission of the oak wilt fungus. The most common technique is to sever roots by trenching at least 4 feet deep with trenching machines, rocksaws or ripper bars.
Trenches more than 4 feet deep may be needed to assure control in deeper soils. Correct placement of the trench is critical for successful protection of uninfected trees. There is a delay between colonization of the root system by the fungus and appearance of symptoms in the crown. Therefore, all trees with symptoms should be carefully identified first. Then, the trench should be placed a minimum of 100 feet beyond these symptomatic trees, even though there may be healthy trees at high risk inside the trench. Trees within the 100 foot barrier, including those without symptoms, may be uprooted or cut down and removed to improve the barrier to root transmission. Tree removal should be initiated after trenching, starting with healthy trees adjacent to the trench and gradually working inward to include symptomatic trees.
Oak wilt centers are more easily suppressed when treated early, before they become too large. Untreated trees immediately outside the treated area should be closely monitored for several years. If the pathogen appears to have crossed the barrier, the same measures (new trenching and treatment of trees within the barrier) should be repeated while the diseased site is still small.
2. Fungicide treatment
The fungicide propiconazole (Alamo) can be used as a preventative to reduce oak wilt symptoms in live oaks when applied before infection. Limited success may also be achieved in trees with therapeutic injections during the earliest stages of infection. The fungicide is injected into the tree’s water-conducting vascular system through small holes drilled into the root flare at the base of the tree. Treatment success depends on the health condition of the candidate tree, application rate, and injection technique. Injection should be done only by trained applicators. Fungicide injection does not stop root transmission of the fungus. This treatment, therefore, is used best in conjunction with trenching or to protect individual high-value trees in situations where trenching is impractical.
9. When should I prune my oak trees?
New oak wilt centers are started when a contaminated beetle finds a fresh wound on a healthy oak tree. Insect populations in general increase during mild spring like weather and mild spring like weather is when fungal mats are most likely to form. So the most likely time a contaminated beetle will find your tree is when the weather is mild. The best time to prune is when a contaminated beetle is least likely to find your tree. The heat of summer and the cold of winter are when the beetle populations are the lowest and fungal mats are least likely to form. Therefore that is the best time to prune to prevent against an oak wilt infection.
The Texas Forest Service recommends that you always paint pruning cuts as soon as the cuts are made. When pruning cuts are made during the proper times of the year there still maybe a few contaminated nitudulid beetles in the area. If the pruning cuts are painted with a pruning paint or latex paint this will prevent these sapfeeding Nitidulid beetles from feeding and infecting your oak tree with the oak wilt fungus.
10. Can my trees get oak wilt from the smoke of burning diseased wood?
No, the fungus that causes oak wilt is very heat sensitive. The problem with firewood is the potential for storing contaminated wood. If the wood came from an oak wilt center and has fungal mats, then the beetles can fly off and infect the healthy trees in your yard. The best precaution is to always buy well seasoned firewood. If you cut down infected red oaks cover the wood with a clear plastic for one year. This will ensure that the fungus is killed by the summer heat. Once the wood has dried out to the point where the bark is falling off and the wood is cracked then the fungus that causes oak wilt is no longer viable (alive).
11. Where can I go for additional help?
Contact the Regional Forester nearest to you.