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Tree Diseases
Oak Wilt
How the Disease Kills Oaks
In the Twin Cities we see a lot of the oak wilt fungus, Ceratocystis fagacearum, it enters an oak either through a root graft or a fresh wound. We have learned that once inside the tree, the fungus grows in the water carrying vessels of the tree and spreads throughout the roots, trunk, and branches. We know that as a mode of defense, the tree attempts to stop the spreading fungus by using gummy substances, called tyloses. These tyloses clog the water vessels within the tree, but do not stop the fungus. The lack of water flow causes leaves to wilt rapidly and fall to the ground. The oak essentially shuts off the water supply and dies from drought.
How is the Disease Spread?
Overland Infections
Most overland infections occur between late April and mid July. Fungal spores transported on the bodies of sap feeding nitidulid beetles initiate these infections. The spore pads only grow beneath the bark of diseased red oaks. Nitidulid beetles feed on the spore pads which are exposed when the pads form pressure pegs cracking the bark open. The contaminated beetles transport the spores to fresh wounds on healthy red and white oaks.
Root Graft Infections
Root graft infections account for approximately 90% of the spread of the disease within an infection center. Oaks within 50-60 feet of one another often will have a common root system due to root grafting. Rarely do trees of different species graft roots. The root grafts allow the oak wilt fungus in a diseased oak to travel within the water carrying vessels, through the root grafts, into a healthy oak.
Symptoms
We know that symptoms of oak wilt can be difficult to recognize, with the understanding that Red and white oaks have many insect and disease symptoms that will mimic the symptoms of oak wilt. Also, red oaks and white oaks show different symptoms of oak wilt. The most visual symptom of oak wilt in red oaks is the rapid progression of wilting leaves from the top of the tree downward. Olive-tan, curled leaves are typical of an infected oak. White oaks display a much different progression of wilting symptoms. In most cases, minor wilting begins to occur in isolated branches. The leaves are slightly curled and sections of the leaves will be discolored. After successive years of infection, a diseased white oak will slowly wilt and die. Severe infections can cause a white oak to completely wilt and die within a year.
How We Control Oak Wilt:
Root Graft Barriers
Root graft connections must be cut before the diseased oaks are removed. Failure to do so can spread the disease even faster. The most common form of root severing that we use by a vibratory plow or a trencher. Before any plowing is done, Gopher State One must be contacted for all utility locates.
Sanitation
Once we have isolated the infection site, removal of the diseased oaks is crucial. Removal of diseased red oaks is very important since oak wilt spore pads may form beneath the bark. Picnic beetles are attracted to the scent of spore pads, which they will feed upon. For this reason, diseased wood must be destroyed or covered by black plastic between the months of April and August.
Oaks that have been dead for more than one year do not produce spore pads and removal of the trees is unnecessary unless they present a hazardous situation.
Injections
We have developed a deep competency in utilizing the fungicide Alamo (propiconazole) for suppressing oak wilt fungus in currently diseased white or bur oaks. We inject the chemical into the root flares below grade via the macro injection process.
The process has proven very effective for white and bur oaks. The process is not effective on infected red and northern pin oaks. However, Alamo (propiconazole), injections have proven themselves very effective for protecting red and northern pin oaks from root graft infection transmissions. These transmissions come from adjacent trees infected with the fungus.
Contact Rainbow to have a certified Arborist evaluate your site. Use our knowledge and experience to save your threatened oaks. Together, we can fashion a plan that works.
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