Recognizing Oak Wilt Symptoms

Oak Wilt Disease

Oak wilt is a serious systemic disease of oak trees. Recognizing oak wilt symptoms is important to managing the disease. Not all oak trees are equally susceptible, but all oak trees can be damaged.

Red oaks are most susceptible and can wipe out large sections of red oaks in worst case scenarios. Trees of any age or size can be affected.

(Photo by William M. Ciesla, Forest Images)

How the Disease Enters Trees and Evidence of Wilt Disease

Oak Wilt Leaf Symptoms

The disease is caused by a fungus that is carried to trees on wood beetles. The fungus must enter the tree through an injury, pruning cut, or insect boring damage.

The fungus, Ceratocystis fagacearum, hitches a ride on the back of insects, especially oak boring beetles that feed on the oak tree sap. Wood boring beetles and their larvae are some of the greatest enemies of trees. The fungus enters into the vascular system below the bark where it is spread through the tree. (Photo by D. W. French, Forest Images)

The first symptoms will show up on the leaves in the upper portions of the canopy.

Once in the vascular system, the fungus spreads and blocks the uptake of water and nutrients causing those branches to die. Eventually the whole tree can die.

Diseased trees will give a strong smell of fermented fruit. This is the smell produced by the living and actively growing fungus. If you don't smell fermented fruit right away, try kicking the fallen leaves under suspected trees. This will often release the spores into the air and you may get a whiff of the fungus.

The disease can also spread through root grafts, as the roots from infected trees graft into nearby roots from healthy oak trees. Trees with root grafts will share water and nutrients, but will also share wilt disease.

Oak wilt may be scattered widely throughout an area or state. It may be found heavily in one county and completely absent in the next. It depends on a lot of factors, but largely where there are concentrations of insects and stressed trees. Many wood boring insects will attack stressed trees first, but that is not a hard rule. Some insects will attack healthy trees.

Brown Streaks is Oak Wilt Symptoms in Branches

For further evidence of oak wilt symptoms take a knife and remove the bark of small infected branches. If there are dark streaks in what should be light colored wood, this may evidence of oak wilt symptoms. Keep in mind that dark streaks are also symptoms of other vascular diseases as well.

(Photo by Sandra Jensen, Forest Images)

Oak Wilt Symptoms in Red Oaks

Red oak trees can die is as little as a few months from first infection to death. Once the disease has spread through the canopy, the tree will rapidly decline.

The disease symptoms cause the leaf to take a strong two color appearance. Like in the photo, part of the leaf is brown and the other part is green as the leaf declines.

At other times the leaves looks similar to having an iron deficiency. In iron deficient leaves, the veins are yellow, while the rest of the leaf is green. It looks like a variegated leaf, but the oak wilt symptoms will increase and the leaf, branch and tree eventually dies.

Oak Wilt Symptoms in White Oaks

Oak wilt is less serious in white oaks. In white oaks, the disease often affects solitary branches, but may not spread throughout the tree. In white oaks, the infected branches can be removed and that may cure the tree. However, weak or stressed white oak trees can still be killed. Keeping your white oak trees healthy during stressful periods may give the tree an edge over oak wilt.

What to Do If you Suspect Oak Wilt

On white oaks, trim infected branches in an attempt to stop the spread of the disease. You need to work quickly. The sooner you trim out the affected branches showing oak wilt symptoms, the better your chances of halting its progress.

Red oaks, live oaks, etc, once the disease has started, is difficult to control. The tree will likely die and will certainly die if nothing is done. Trimming out infected branches may slow the progress if caught quickly. Injections may be helpful.

Using Fungicides

If you hear oak wilt is in the area, you can contact an arborist to inject the tree with fungicides labeled for oak wilt. If injected before the infection occurs, the tree may be spared. If the tree is infected, it will slow the oak wilt symptoms down. Fungicides will probably not cure the disease, but may greatly help. Spraying the trees won't work. They must be injected.

Fungicides work better as a preventative than a cure. The fungus clogs the xylem that transports water to the canopy, which is also the way the fungicides travel throughout the tree. Therefore, if the fungicide is already in the tree when the fungus contacts the tree, the fungicide will work better.

If the tree dies, do not transport wood out of the area. Cut down the tree and burn it if possible.

Preventative Insecticides

Injecting insecticides designed as a preventative can be your best friend. Getting an arborist to inject your oak trees at the beginning of the growing season will protect the tree all year. As soon as insects feed on the tree, they also ingest the insecticide. Injections are much safer than spraying and are a great way of getting the insecticide inside the tree.


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Did You Know:

Oak Wilt is a systemic disease spread by insects that can quickly kill certain oak trees. Red oaks are especially susceptible.

Find out what can be done to help your oak trees.


Delay Spring Pruning

Pruning branches releases sap that attracts boring insects. To help prevent wilt disease infection, do not prune oak trees from February to June. These are the main flight months for insects that carry the fungus.