What is Oak Wilt?
Oak Wilt is caused by a fungus (Ceratocystis fagacearum) that affects nearly all species of oaks and is particularly aggressive when contracted by Red Oaks. This disease is a major problem in the eastern and central United States, including the greater Madison area and all of southern Wisconsin.
The Oak Wilt fungus kills the tree by infecting the vascular system (xylem). This elicits an immune response from the tree which then begins to plug the "tubes" in the xylem in a attempt to stop spreading the disease. This stops the flow of nutrients and water and results in either tree death in the red oak subgenus, or dead branches in the white oak subgenus.
All oaks are susceptible to Oak Wilt, however the Red oak subgenus, including Pin oak, Scarlet oak and Black oak are more susceptible. Trees in the red oak group generally die rapidly, usually within weeks or months after infection. Treatment will not save an infected red oak. Trees in the White oak subgenus, such as Burr oak and Chinkapin oak can generally be treated for Oak Wilt and will recover within a year of treatment.
There are two modes of transmission of the Oak Wilt pathogen:
- After a Red oak dies, the oak wilt fungus will produce gray patches of fungal mats which attract the Picnic beetle and Bark beetle. These beetles feed on the stick sap oozing from the mats and in the process pickup Oak Wilt spores on their bodies. These beetles are then attracted to fresh wounds in healthy trees caused by storm damage or improperly timed pruning during the growing seasons. The beetles then feed on the healthy oak and Oak wilt fungus Ceratocystis fagacearum is then transferred to the once healthy tree, where the infection begins again.
- Most commonly, root grafts between oaks in close proximity (roughly 50' of each other) result in disease transmission. The trees will graft a part of their roots vascular system together and the pathogen will then be able to cross unimpeded from tree to tree.
Oak Wilt symptoms in Red oaks will typically appear in the upper canopy and progress inward and downward within a few weeks. At first, there may only be a single branch that dies, showing leaves that often bronze, or turn tan or dull green, starting at the tips or outer margins. The leaves will turn brownish and prematurely drop mid-summer.
Oak Wilt symptoms in White oaks are similar but progress more slowly. Generally, a single isolated branch will begin to "flag" and leaves will turn brown. If left untreated for several seasons the dead areas will generally spread out from the area around the initial infection.
With Red oaks, there is not much we can do if the tree is already infected, but in the case of white oaks, we must be sure to make a positive diagnosis by sampling and submitting the specimen to the UW tree lab for proper diagnosis since other factors or pathogens can cause similar dieback. Some examples include two-lined chestnut borer, soil compaction, storm damage, cankers, root rot or armalaria, and grade changes.
Oak Wilt Treatment
Red oaks must be removed as no treatment is available once the tree is infected. It is very important that the tree is left standing until the dormant season to reduce the risk of the pathogen spreading through any root grafts. Because of the dire results of infection, preventative/therapeutic treatments are a must for high-value Red oaks.
White oaks can be treated by tapping into the vascular system with a specialized pump setup that injects a liquid fungicide, Propiconazole, throughout the trunk and canopy. This will generally stop the progression of the disease and protect from reinfection for two years from the time of injection.
This treatment process is similar to an I.V. one might receive in the hospital. First, we determine the proper dosage:
- Measure the tree at 4' 6" to determine trunk diameter.
- Visually inspect the 'fullness' of the crown.
- If the crown is reduced in any way, the chemical dosage may be reduced appropriately.
With the precise dose calculated, we start drilling the correct amount of injection sites in the root flare. The injection system (or I.V.) is plugged into the holes, filled with the appropriate dose of a chemical and pressurized enough to allow a gentle flow of chemical into the vascular system of the tree.
The benefits of this "I.V." or direct injection method as opposed to other less-effective soil injected methods are many:
No chemical is released into the environment.
All of the prescribed chemical is delivered to the tree with no waste.
Adjacent plants will not be affected.
Beneficial soil microorganisms are left unharmed.
There is no chemical exposure to pets or children.
Oak Wilt Prevention
Many neighborhoods are lined with irreplaceable 80-200+ year old oak trees. Just like in modern health care, it is often better to be proactive than reactive. This is why a health assessment of your trees is necessary. If appropriate, therapeutic treatment of propiconazole fungicide may be prescribed to assure and infection can never take hold.