Oak trees are part of the Quercus genus, which contains more than 450 species of plants that are hardy in a variety of climates around the world. Oaks are prized for their thick, tall trunks and their hardwood, which is used for a variety of projects in the construction and furniture industries. The presence of yellow fungus on your oak tree may indicate one of a few fungal decays that produce yellow fruiting bodies on or around infected tissue.
Sulfur fungus (Laetiporus sulphureus) causes brown heart rot of living oak trees, but will also attack decaying trees. One of the most serious decays of oak trees, sulfur fungus enters the tree through wounds in the bark and dead branch stubs, and decays the wood inside the trunk and branches. This results in reduced moisture and minerals to the stems and leaves, and it can lead to structural problems for the tree. The fleshy, moist conks produced by this fungus range from 2 to 12 inches wide, and are bright orange-yellow on the upper surface and red-yellow on the underside. The conks don’t appear until years after the onset of decay and often indicate extensive rot inside the tree.
Entering your oak tree through openings or punctures in the bark, oyster mushroom (Pleurotus ostreatus) decays heartwood and sapwood, producing a white rot disease. It causes the infected wood to turn white and flaky, losing its original rigidity. Decay is most severe when the wounds are large, and leads to the formation of shelf-like yellow and cream-colored mushrooms that range from 2 to 8 inches wide. These mushrooms are produced annually and indicate decay that extends up to 10 feet in either direction beyond the location of the mushrooms.
Oak Root Rot
Oak root rot, also referred to as armillaria root rot, is most often caused by the fungus Armillaria mellea, but may be caused by several other fungi in the armillaria genus. It enters your oak tree via dark root-like structures called rhizomorphs, which are found on the surface of infected roots. Armillaria root rot is a serious disease of oak trees in North America, causing a white root rot that results in discolored foliage, wilting and death of new shoots and stems. Yellow or tan mushrooms often form at the base of infected trees following fall and winter rains.
Preventing Fungal Infections
Fungal infection and decay is usually a problem of old, large oak trees, and can be difficult to manage once infection is present, because no chemical controls have proven effective. However, you can do a number of things to prevent infection or to limit the extent of the damage. Ensure the health and vigor of your oak trees through proper cultural care, and minimize the opportunity for fungi to enter the tree by protecting your oak from injury. When young oak trees are pruned to form a good structure, don't remove large limbs, which leave large wounds, when the tree is older. Cut away diseased or dead limbs, and make the cuts just outside the branch bark ridge. This leaves a collar of cambial tissue around the cuts to encourage rapid healing.
- UC IPM Online: Wood Decay Fungi in Landscape Trees
- The Neighborhood Association of Southwestern Williamson County: Oak Tree Diseases
- North Carolina State University Extension: Root and Butt Rot of Oaks
- US Forest Service: Heart Rots of Central Region Hardwoods
- University of Minnesota Cooperative Extension: Disease Management Recommendations for Trees and Shrubs
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