Answers about Oak Wilt

(updated April 2017)

Oak Wilt Tree

Red Oak with Oak Wilt

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 Ceratocystisfagacearum. 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?

Oak Wilt on Live Oak
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).

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 sticky 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.

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 hours. Treating 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 probably responsible for most 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. 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 injectionFungicide injection or infusion is limited in value and effect. The results of injection with propiconazole (Alamo) are random and varied. Basically, it is our only treatment option and it can extend the life of an infected tree on your property, sometimes upwards of ten years. Occasionally (rarely) we get a cure. It has some worth as a preventive, but will not stop the movement of the disease through the roots into non protected oaks. It is not reliable, predictable, or guaranteed. It is expensive and needs 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 limited or no success.

There are several new generic (less expensive) mixes with propiconazole that have the same percent active ingredient. We have little experience with these mixes and we have no confidence in them. They are causing the cost of Alamo to come down dramatically (yea!) and we have chosen to stay with Alamo.

Our experience with micro-injection has been absolutely dismal. We have observed several other companies using micro-injection and they have had absolutely no success. 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 corn meal Recipes are available at www.Dirtdoctor.como We are now including corn meal tea soil drenches (1 cup corn meal per gallon water) with all propiconazole injections and we are doing tea drenches on oaks that clients choose not to inject. First year results are encouraging, but this is very early in our research.

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, 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
TOWC 0061

Root Collar Problems

Girdling root“By far the most common cause of tree decline and death. It is responsible for most of the secondary pests and diseases we encounter”

The most likely cause of tree problems in the urban setting in our area is buried root collars. I seldom have a day in which I do not diagnose a root collar disorder. In my experience, it is by far the most common cause of tree decline and death and is responsible for most of the secondary pests and diseases we encounter. Into this root collar disorder category, I also group all the girdling problems such as girdling roots, girdling straps and twine, girdling wire baskets; and containers such as plastic pots, nylon bags, and felt bags that are not removed when a tree or shrub is planted.

The root collar is the area of a plant where the trunk or stem joints the roots. Tree have a noticeable flare at the root collar that we call the root flare and we want this flare to be visible and exposed to the air.

The trunk and root flare of a tree are not genetically programmed to resist constant soil moisture. Respiration occurs in this area of the tree (the exchange of oxygen and carbon dioxide) and constantly wet bark restricts or stops this gas exchange. Over a period of time, this restriction causes this area of the tree to function poorly, interferes with the downward movement of food to the roots, and eventually results in root dieback and reduced water uptake. These trees are more susceptible to disease infection, insect invasion and to stress caused by drought or compaction or irrigation excess.

Early symptoms of root collar disorders include yellowing of leaves, margin burn or necrosis of leaves, and dieback in the upper canopy. These are the same symptoms we see from water stress (both too much and not enough) and most of us react by watering the tree which usually makes the problem worse. Often, we see no early symptoms and we get the phone call that starts out with my tree died overnight. My tree was fine on Friday and was dead on Monday. That kind of rapid death is almost always caused by a root collar disorder. The tree exhausts its reserves, often looking OK, and dies overnight. The problem that led to that rapid death has been going on for years. And it is not uncommon for that overnight death to occur 1 to 3 years after the installation of an irrigation system.

Checking for a root flare is easy, you can see it. If it is visible, the problem is something else. If you can not see it, you have found the most likely cause of your tree’s problem. The soil or mulch needs to be removed down to the level of the root flare and out away from the trunk of the tree about 6 to 12 inches. The distance away from the trunk is not critical as long as the trunk and root flare are exposed to the air. You will find that the deeper you have to dig to reach the root flare, the further out from the trunk you will dig just so you have room to work. Most excavations are 6 to 12 inches deep but we have one tree that is doing well that excavated about 7 feet deep!

Do not use a shovel or pick to excavate around your tree. Do not scrape or scar the roots. Use a hand trowel or rock hammer. You can also excavate with water pressure; it’s messy but very effective for shallow excavations. We use an air spade which removes soil with air pressure, does not damage the roots, and allows us to stand while we excavate. You will appreciate the standing after you have tried to dig a tree by hand. As you excavate, you should correct girdling problems caused by tree roots or things that were left on the tree when it was planted. Sometimes, you can not correct all of the girdling roots and will need to return in a year or two to correct additional girdling roots.

Severely declining trees need to be removed. Do not waste your time and effort trying to save a hazardous tree.

The well you’ve dug should remain open. There are many options for edging or lining the well to help keep soil from falling into the well and to make the well attractive. If the well is a safety hazard, it can be filled with large, course gravel, but that gravel will need to be removed and replaced every 3 to 5 years. It is best to leave the well open, the root flare exposed to the air.

The excavated tree should be fertilized with a good organic fertilizer or a good slow-release fertilizer or one of the good organic sick tree treatments that are available at better nurseries.

You may need to water the tree but you should be very careful. Our native or adapted Texas trees are very capable of dealing with drought. They are not well adapted at dealing with too much water. Be very careful with how you water because your trees are usually better off with dry conditions. We normally recommend that you water to keep your grass, annuals, and shrubs healthy and do not irrigate established native trees.

In our experience, root collar disorders are by far the most common reason for the decline or death of our landscape trees. Proper planting and careful use of fill soils for leveling and for landscaping are important to preventing this disorder. If the disorder already exists, excavation to expose the root flare to the air can often save your tree.

David Vaughan is a Certified Arborist with Etter Tree Care.