Oak Wilt and the Injection Process

THE DISEASE AND ITS BIOLOGY:

Oak Wilt is a vascular wilt disease of Oak trees caused by the fungus Ceratocystis fagacearum. All species of Oaks are susceptible with some species being somewhat tolerant. Red Oaks, both Shumard and Spanish, are highly susceptible to the pathogen with Live Oak following a close second.

The disease in Red Oak is expressed in two primary ways. The first is an oily green appearance to sections of the canopy, which rapidly turn a tan color as the foliage dries on the tree. The other and less common expression is a heavy and rapid shedding of foliage with a variety of the above symptoms on fallen leaves. The shedding is fairly uniform throughout the canopy. There is no effective control for Red Oaks that express active symptoms of infection. However, injection with the fungicide Alamo has proven to be effective as a preventative. Red Oaks properly diagnosed with Oak Wilt should be removed as soon as possible as these species are responsible for the overland spread of the pathogen. There is more and more evidence that Red Oaks infected with the Oak Wilt fungus can be effectively treated if the infection has not progressed too far.

There are a variety of possible foliar symptoms observed in infected Live Oaks. The most common is termed a veinal necrosis or yellowing and browning of the main leaf vein and side veins (see photograph below). The foliage will usually fall once this symptom has appeared. It is possible to see leaves still on the tree with veinal necrosis.The defoliation in Live Oaks can occur at anytime of the year, but should not be confused with the normal process of leaf drop and flowering which generally occurs late February to late March. A small percentage of diseased Live Oaks can potentially be treated with the fungicide injection. However, it should be noted that the curative qualities of the fungicide are limited.

The disease spreads from tree to tree in two methods that we are aware of to date. The first and most common is root to root contact. It is well known that Live Oaks commonly form functional root grafts with other Live Oaks. Red Oaks appear to form these root grafts as well. Once the pathogen has entered a tree, it can potentially move from tree to tree quite readily and efficiently. Less common, but reported to occur, is root grafting between Live Oak and Red Oak. This phenomenon is not nearly as common, but the potential is there. This is why we will commonly recommend to inject all Oaks on the property for protective purposes.

The second method of disease transmission is overland spread. This occurs when fungal mats are produced on dying Red Oaks and visited by the suspected insect vector, the Nitidulid Sap Feeding Beetle.

It should be noted that fungal mats are produced primarily in the Spring or cooler times of the year and only form on Red Oaks, which are at a particular phase of the mortality process. Mats will be viable, that is producing viable spores of the fungus, for a short period of time. This time frame is usually 2 – 3 weeks at the most. Although new infection centers do form from time to time, it is not fully understood whether it is from the insect spreading the disease or possibly other methods of transmission such as infected pruning tools, squirrel damage during the Spring, or methods not yet discovered.

THE ROOT FLARE INJECTION PROCESS:

Oak Wilt

The root flare injection process is three fold. The first is to expose the buttress roots or root flare of the tree or trees being injected. This is accomplished with careful excavation of the base of the tree where plant material and soil are removed from around the base of the tree. The distance away from the trunk in which plant material and soil will need to be removed will vary according to the size or diameter of the tree being treated.

The second step is to physically perform the high volume root flare injection with the registered fungicide “Alamo”. This is the only material registered and labeled for this purpose. It is now available in a water-soluble formulation, which will have a distinct blue color. Anything else being used is ineffective or being illegally applied.

A series of holes will be drilled up and down the exposed root flares at about 4-6 inch increments. Proper location of these injection sites is critical for proper distribution of the fungicide. Inadequate distribution leaves trees poorly protected. A series of injection tees are inserted into the injection sites and connected with tubing to create a closed system. The system is usually attached to a pressure tank containing the fungicide solution and the injection process begins. Once the tree has taken up the allotted amount of fungicide, the system is broken down and removed from the tree.

The third step is to return and fill in the exposed area. There will be many instances where trees have been planted too deep to begin with, or fill soil has been brought in. In these cases we will commonly recommend to fill in with a large stone or leave the root flare exposed entirely. Each situation will differ and your Arborist will recommend what is best for the long-term health of the tree.

In general, Live Oaks and Red Oaks not expressing active disease symptoms are good candidates for preventive root flare injection with the fungicide. It appears that Oaks can be infected with the fungus up to a year before active symptoms are expressed. What this means is that an apparently healthy Oak adjacent to a diseased tree may already be infected and in this instance, uptake of the fungicide may be poor.

COMMONLY ASKED QUESTIONS REGARDING ROOT FLARE INJECTION WITH ALAMO:

Q: Why does the process seem expensive?

A: The cost of the process is primarily driven from the cost of the fungicide. This is dictated by the manufacturer. There is also quite extensive labor involved in properly exposing the root flare of a tree or trees in the injection process itself.

Q: Does it matter how much fungicide the tree receives?

A: It is critical that the tree receives a minimum of 10 milliliters of Alamo for every inch in diameter. This diameter measurement is usually taken about 4.5 feet from soil grade. We will commonly recommend 12 milliliters per inch if infected trees are being injected or high disease pressure exists.

Q: Which trees should I treat?

A: Your Arborist will generally recommend to treat all Oaks on the property, and certainly all that potentially could form root grafts. This will vary from site to site and will usually not include trees already showing active symptoms.

Q: How fast does the disease spread in a population of Oak trees?

A: The disease has been reported to move up to 100 feet per year in a solid stand of Oaks. This can vary greatly due to variables unique to each site.

Q: Should my neighbor treat their trees?

A: The best candidates for preventive injection are those Oaks out ahead of the active disease center. Many times we are called to the property with trees showing active symptoms of Oak Wilt. We ask the fungicide to do much more than it is designed to do when we are always treating trees adjacent to infected trees.

Q: Is this injection process guaranteed?

A: The process has no guarantee. We are however, much more successful when we are treating trees that, number one, are not showing active symptoms. And, number two, are some distance from infected trees. We have had very good results with high volume root flare injection to date. We still see failures from time to time, which is usually due to the tree already being infected which leads to poor uptake and distribution.

Q: Are there alternative methods for the prevention of Oak Wilt?

A: NO, there are currently other types of injection, such as “microinjection” which has no data to date that suggest any type of control or prevention. This process is strictly experimental and will take a number of years to provide adequate data one way or the other. DO NOT LET anyone tell you differently. This can be a confusing point because the manufacturer has packaged Alamo in a microinjection system. This was done primarily to increase revenue for the chemical company and offer what appears to be a quicker and easier method of injection. If an easier method becomes available that is equally successful and cost effective with repeatable results, we will be the first to implement such a method.

Q: If I keep my trees healthy will they be less susceptible to infection by the Oak Wilt fungus?

A: No, it is unfortunate that this is not the case. We are dealing with an aggressive vascular wilt disease and a tree, which is genetically predisposed to infection by the fungus. To draw an analogy, the Chicken Pox Virus does not infect children, which are unhealthy. The healthiest children will still get the infection at some time or another because humans are genetically predisposed to the Chicken Pox Virus.

Q: Will the root flare injection ever need to be repeated?

A: Once trees are injected, it appears they are protected for about 2 years. If disease pressure is high, that is to say the Oak Wilt center is still active next door or across the street, we may recommend to retreat the following year. If we are treating trees with active symptoms and the symptoms reappear the following season, we will commonly recommend to retreat. Retreating high value or unique or historic trees annually as a preventative is not unusual.

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