Oak Tree Root Rot

Oak Tree Root Rot

Oak tree root rot can be caused either by the Armillaria fungus or the Phytophthora fungus. Both of these damage the roots of the tree and slowly kill it by spreading the rot to the rest of the tree.

If the disease is not discovered in its early stages, it may be almost impossible to salvage the tree.

In this article, we will discuss the most common causes of oak tree root rot, as well as some other common diseases of oak trees.

What causes oak tree root rot?

Armillaria root rot

Armillaria root rot is one of the most common oak tree diseases, and is also called the oak tree root fungus. It is spread through the rhizomorphs on the roots of the infected plant. When these rhizomorphs touch the roots of a healthy plant, it will also become infected. Moist conditions are ideal for the fungus, so they will target the roots of an overwatered tree that are constantly sitting in soggy soil.

You may not be able to catch the fungus in its early stages because it progresses slowly, meaning the visible symptoms will appear quite some time after the tree was initially infected.

It may take years for the tree to start looking weakened and affected by the disease. Other times, the tree may look completely healthy one day and suddenly ill after a couple of weeks.

One of the symptoms you need to watch out for is the presence of yellowish, fan-shaped mycelium between the bark and the wood. There could also be brown or black structures resembling strings on the surface of the roots. If there are large, honey-colored mushrooms at the base of the tree in the fall or winter, they could also be a sign that your oak tree has Armillaria root rot.

To confirm your suspicions, look for the crown of the tree’s root system and scrape off some bark from that area. If you can see the signature tissue between the bark and the wood, the tree probably has oak tree root fungus. Remember that the absence of mycelium tissue does not mean the tree does not have root fungus; you may just be checking the roots a little too early in the infection when it has not yet reached the crown of the root system.

Unfortunately, there is no known effective treatment for Armillaria root rot in living trees. The fungus can also lay dormant in the soil around the dead tree’s roots. You can try to control it by digging up as much of the old tree’s roots as you can.

As mentioned above, the fungus likes moist conditions, so avoid overwatering; also avoid pouring too much water over empty areas with no plants to water. Air-dry the soil as much as you can, and use physical barriers such as root collars to contain the centers of infection. Try not to plant any new trees in the same area and keep the roots of nearby healthy plants from reaching that soil if you do not want them to become infected.

Phytophthora root rot

This disease is also caused by a soil-borne pathogen. Like most fungi, it thrives and reproduces in warm, moist soil.

If the roots of an oak tree are overwatered or if it has been raining for several weeks, they could drown and die, making them vulnerable to these opportunistic fungi. Once the roots are infected, they will start to rot and produce bleeding symptoms on the tree’s trunks.

Another reason your oak tree may have root rot is if you planted a lawn around its base. The base of the tree should be allowed to dry out, especially during the summer, and the lawn will retain the soil’s moisture for longer than necessary, creating ideal conditions for root root.

If you place too much mulch near the base of the tree, the same thing happens. Make sure that, if you mulch the soil around the tree, you start a few feet away from the base so that the roots are able to breathe.

Infected oak trees will have yellowing, sickly leaves, and you will notice the branches dying, until eventually the entire tree is compromised and will die.

If you suspect Phytophthora root rot, you may need to consult a professional and task them with applying the proper treatment if they believe they are still able to save the tree. 

What are some other common oak tree diseases?

Oak wilt

This is a disease that can affect all types of oak trees, but the rate at which the disease affects the tree can depend on the type. It is also caused by a fungus, which travels through the tree’s vascular system. Red oak trees tend to be worst affected by oak wilt.

Insects can cause the spread of the fungus when they carry it to entry holes on the tree, such as wounds on the tree’s surface. One of the earliest signs of the disease is brown necrosis in the tree’s veins. The leaves may also start to fall off at an accelerated pace, despite still looking green and vibrant.

The disease starts on one limb and makes its way toward the rest of the tree. Spore mats will develop under the bark, and spores will be carried to nearby trees by insects and the wind.

Unfortunately, this disease is difficult to treat and you will need to consult a professional regarding your course of action. Often the only thing you can do is destroy and dispose of the tree so that it does not spread the infection to other trees.

Bacterial wetwood

This disease, also called slime flux, is not as damaging to oak trees as the other aforementioned diseases.

The pathogen is an anaerobic bacterium that disrupts the tree’s membranes, filling the microspaces with water and causing leakage of substances and increased concentration of certain elements. They usually infect wounds and tree or root stubs.

The infection is not usually treated; it is best to simply leave it alone.


Oak tree root rot is exacerbated by overwatering, or when the soil is constantly wet for one reason or another. When the tree’s roots are left to stand in soggy soil for extended periods, they can drown and die and will then become susceptible to pathogens that cause root rot.

The two most common causes of root rot in oak trees are the Armillaria and Phytophthora species of fungus.

If you suspect an infection, consult a professional in case there is a chance of saving the tree, but usually if the tree has been infected for some time, it may be better to destroy it and dispose of it properly in order to avoid the spread to healthy plants.

Image: istockphoto.com / Vitalii Livadnyi