Oak Wilt: Frequently Asked Questions
By Matt Grubisich, Regional Urban Forester, Texas A&M Forest Service
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. The fungus Ceratocystis fagacearum causes oak wilt.
Oak wilt invades and clogs the tree’s water-conducting system.
WHERE IS OAK WILT A PROBLEM IN TEXAS?
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.
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 – Spanish, Texas, Shumard, Pin, blackjack
Live oaks die in the greatest numbers, most often in expanding areas called “Oak Wilt Centers.”
HOW DOES OAK WILT SPREAD?
Oak wilt spreads long distances with the aid of sap-feeding (Nitidulid) beetles. These beetles are about the size of the ball on the end of a straight pin.
They are attracted to fungal mats that form underneath the bark of diseased red oaks. The fungal mats produce spores and have a ridge down the center that lifts the bark creating a tiny crack. Insects are attracted to this opening because the mat has a fruity smell.
These mats can be found all over the trunk and major branches. The Nitidulid beetles after feeding on the mats fly off. If a contaminated beetle lands on a fresh wound on a healthy oak then that tree can become infected. It does not matter how the wound was made whether with a chainsaw, by wind damage, or the bumper of the car.
Once established, the fungus moves from one tree to the next through common or grafted roots.
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.
HOW CAN I TELL IF MY TREES HAVE OAK WILT?
Live Oak – Trees die within 2 – 4 months. Adjacent trees begin to drop their leaves and die a few months later…leaf symptoms: Veinal Necrosis – area around leaf vein turns brown rest of leaf is still green, found on the tree, or on the ground. Most diagnostic.
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 2 – 4 weeks.
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.
· Sterilize/Sanitize all pruning equipment between trees.
· 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.
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, rock saws, or ripper bars. Trenches more than 4 feet deep may be needed to assure control in deeper soils. The correct placement of the trench is critical for the successful protection of uninfected trees. There is a delay between the colonization of the root system by the fungus and the 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. The injection should be done only by trained applicators.
Fungicide injection does not stop the 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.
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 is 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 an oak wilt infection.
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. 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).
The Texas A&M Forest Service website is another great resource, texasforestservice.tamu.edu