Oak Wilt in Red Oak
(updated April 2020)
What is Oak Wilt?
Oak wilt is a vascular wilt disease caused by a fungus that infects the water-conducting tissues of oak trees and causes them to wilt and die. Good health does not improve resistance to this disease. If the oak tree is challenged by oak wilt, it can become infected. Prevention of Oak Wilt is easy. However, it is a very difficult disease to manage after it becomes established.
What causes Oak Wilt?
Oak wilt is caused by the fungus Bretzillia fagacearum formerly known as Ceratocystis fagacearum. In San Antonio and Central Texas, it is mostly a problem in Live Oaks and in Red Oaks such as Spanish, Shumard, and Blackjack. Live Oaks are a particular problem because of the common root system that develops from root sprouting. This connects all of the Live Oaks in an area with a common root system.
Oak wilt moves from location to location by the transport of a spore (a sort of seed) carried on the body of an insect. It travels from a spore mat produced on a Red Oak, to a fresh wound on another oak. The spore does not blow in the wind and cannot be carried by smoke.
Once oak wilt infects a Live Oak, it is moved from tree to tree through the common root system. It can also be carried from tree to tree through root grafts that can be formed between the same species. It is not uncommon for different species of oaks to form root grafts, allowing the disease to move between oak species.
What are the effects of Oak Wilt?
Typical leaf necrosis
Leaf symptoms of oak wilt on Live Oaks are distinctive. The veins of the leaf turn yellow or brown while the rest of the leaf is green. We refer to this symptom as veinal necrosis (death of the vein) and it is considered diagnostic, Other symptoms include tip browning and leaf margin browning. These are non-diagnostic symptoms that also occur with other conditions such as drought or chemical burn. Once leaf symptoms occur, Live Oaks generally die within three months to a year. A few Live Oaks can survive for many years in decline. Leaf symptoms are difficult for most people to determine including most tree care companies. As a result, these companies treat trees erroneously or remove trees that are not infected. Buyer, beware! Before you sign a work order, get a confirmation from the Texas Forest Service or from someone that TFS has certified (TOWC, Texas Oak Wilt Certified) or qualified (TOWQ).
Leaf symptoms in Red Oaks are much less distinctive, having none that are considered diagnostic. Red Oaks generally die within 30 days of the appearance of symptoms, leaving little time to react or confirm the disease. During winter and early spring, structures covered with spores can form just below the bark on infected Red Oaks. These structures are called spore mats and they are difficult to find. These spore mats produce the spores that are carried by insects to fresh wounds on other oak trees.
Laboratory tests can be used to confirm Oak Wilt using the sap wood of a suspected oak. Negative results are currently only about 70% accurate. Some labs offer DNA testing to confirm Oak Wilt that is less expensive and faster.
How can Oak Wilt be prevented?
Prevention of Oak Wilt is easy:
- Paint all pruning cuts and injuries on Oaks.
- Avoid pruning oaks during the spring.
- Do not bring contaminated firewood on to your property.
Paint or treat all wounds on Oaks within a few minutes to an hour. Treating Live Oak wounds after three days has no effect. You can use latex paint (our preferred choice), hair spray, Elmer’s glue, mud, shellac, you get the idea. Get the wound covered. The size of the wound does not matter, treat all pruning cuts. If you cannot treat it, do not cut it! Strangely, we do not recommend the use of pruning sealers (except to seal clay pots) because the sealer is too thick and can result in other problems. You can use the cheapest latex spray paint you can find and color is not important. We know of no benefit to painting wounds or cuts on other species of trees, so oaks are the only trees that need treatment.
When possible in San Antonio, avoid pruning oaks during March, April, and May. Those months generally have good conditions for the presence of spore and for high populations of sap feeding insects. That does not mean you cannot prune in those months to remove broken branches or branches that are hitting the roof of your house or car. If you must cut a limb, paint the pruning cut. If you have a choice or if you are doing routine pruning, avoid pruning in the spring. The safest time to prune is during hot weather from June through Thanksgiving or some years thru Christmas. It is also safe during cold weather when insects are inactive. Even during safe times, paint all pruning cuts regardless of size. If you cannot paint it, do not cut it!
The transport of firewood is responsible for many new oak wilt centers. During storage, insects can emerge from the transported wood and infect nearby trees. Or spore mats can form on fresh Red Oak during storage. Burning of the wood is not the problem as oak wilt does not spread in the smoke, and is killed by fire. Firewood cut from trees growing on your property is not a problem. The problem is wood brought in from other locations. A lot of firewood is coming from oak wilt centers where it is cheap or free and easy to obtain. If you can easily remove the bark with your fingers, the wood has cured long enough to be safe. The recommendation to cover suspicious wood with clear plastic to trap or kill insects is not wise. Do not purchase or transport firewood that is not safe. Just like Jurassic Park, life has a way of escaping. If you are going to cover the firewood with clear plastic, it is not safe and you should not buy it or transport it.
What are the Treatments for Oak Wilt?
Oak wilt is not an easy disease to manage once is gets on your property or in your neighborhood. Rarely, we are able to discover the infection in the first tree, before the disease has moved into the root system, and the total removal of that tree stops the disease. Once two or more oaks are involved, trenching usually is the only effective way to contain the disease center. Trenches are installed 100 to 150 feet beyond trees showing symptoms, completely enclosing the infection center. You can imagine the mess, destruction, and coordination problems to accomplishing this. Trenches need to be at least five feet deep. The purpose of the trench is to cut all root connections. The trenches can be filled as soon as they are made.
Fungicide injection or infusion can protect individual trees. Often, just a single injection is effective. If symptoms return after 2 years, the tree needs to be re-treated. If symptoms do not re-appear, the tree should be fertilized or mulched to help it recover from disease damage.
Injection has some worth as a preventive, but will not stop the movement of the disease through the roots into non protected oaks. Treatment success is not guaranteed. It is expensive and may need to be repeated about every two years. Unless you have a large budget, you need to be selective with the trees you choose to inject and you need to be prepared for some failures.
There are several new generic (less expensive) mixes with propiconazole that have the same percent active ingredient as Alamo. They are causing the cost of Alamo to come down dramatically (yay!).
Our experience with micro-injection has been absolutely dismal. Current research shows no success with micro-injection. We would all like to use this method because it is easy and neat. But, it does not work.
There are other oak wilt treatments being sold, miracle treatments using secret ingredients. None of them has shown any long term success. Most of them use fertilizer to make a tree look better for a short time and have no effect on oak wilt.
There is promise with Systemically Induced Resistance (SIR) using Trichoderma fungus obtained from whole ground cornmeal to enhance the effect of Propiconazole. Mix rates are one cup corn meal per gallon of water. Five gallons for a small tree, ten gallons for a medium tree, and fifteen gallons for a large tree. I recommend a cornmeal tea drench with all Oak Wilt injections.
The TFS Oak Wilt Suppression Project was designed to assist you with oak wilt management. They expanded this program and are training certified arborists to help them identify the disease and develop management approaches for individual property owners. The original training certified arborist as Texas Oak Wilt Certified (TOWC) and they were listed on their web site, www.texasoakwilt.org. Current training just initiated in 2017 qualifies the arborist as Texas Oak Wilt Qualified and requires retraining or re-qualifying every 5 years.
David M. Vaughan
Certified Arborist TX 0118