Several fungi can attack the oak tree, and some fungi attack only certain species of oak. However, many diseases or insect infestations manifest similar symptoms. For example, browning of the leaves of the canopy of an oak tree could be a symptom of oak root fungus, oak anthracnose or an insect manifestation, so further investigation is required to determine which disease is causing the problem.
A plant's structural cellulose, hemicellulose and even lignin can be destroyed by decay fungi. This decay is not seen on the outside of the oak tree. You will only be aware of this decay if the bark has been removed through injury or if there is an open wound somewhere on the tree. The destruction of the plant's structure makes the diseased tree dangerous due to its instability/structure decline. A decaying tree cannot support the weight of its limbs and branches, and can easily be toppled over during a storm.
Identification of Wood Rot Fungi
Wood rot fungi are identifiable by their shape, color, and the formation of their fruiting bodies that appear on the tree. The fruiting bodies are called conks or brackets, and they can be found at wounds in the bark, by branch scars and often by the root crown. Fungi are divided into three groups: white rots, brown rots and soft rots.
Oak anthracnose is a fungal disease that attacks the leaves and new growth of the oak tree. Fungal spores (Discula quercinia) infect the tree, which result in irregularly shaped brown spots on the leaves, creating a browning of the canopy. This disease is likely to show up when weather conditions are warm and wet, and the lower branches are most severely affected due to higher humidity levels. Oak anthracnose is cosmetic in nature; it does not kill the tree. One precautionary measure (to prevent the spread of the spores) is to prune oak trees in dry weather. They should not be pruned in April, May or June. Chemical sprays are not effective in combating oak anthracnose.
Oak Root Fungus
Oak root fungus, also called Armillaria mellea, attacks coniferous trees. This fungus creates a white rot, which can be seen when the bark is removed. It appears below the soil line, between the bark and trunk, and also between the wood of the roots and the interior of the roots. Mushrooms, a symptom of oak root fungus, appear at the base of the tree after it rains. During wet conditions the fungus grows rapidly. To slow down the decay the affected area needs to be exposed to the air and sunlight so that it can dry out. This can be accomplished by removing enough soil at the base of the tree to expose the decaying trunk and roots. Once they are exposed the sunlight and air will remove the moisture, thus slowing down the decay.
Conk is yet another fungus that can attack the oak tree as well as a large number of ornamental trees. It is also known as Ganoderma applanatum. Wounds provide the entryway for this disease, and the fungus can kill the sapwood or cause white rot of the sapwood. The conks are generally at the ground level and appear in a semicircle that can be from 2 to 30 inches wide, and up to 8 inches thick. (It is called artist's conk because if you cut the conk/mushroom off the tree and turn it over, you can draw or write on it.)