A Rotting Oak Tree

Armillaria Root Rot

• A rotting oak tree could be facing an onslaught of Armillaria root rot, commonly called oak root rot, or shoestring root rot, and caused by the pathogen Armillaria melea. The disease is characterized by a collection of white, fungal mats found under the bark, in addition to honey-colored mushrooms that take up residence at the base of the tree. Black, shoestring-like structures are also found under the bark, near roots or in the soil.

Fungal Mats

• The fungal mats often creep up the trunk several feet from its base and may also extend outward upon the earth for several feet. When the wood of the oak is dead or dying, the mat will glow in the dark. The shoestring fungi, called rhizomorphs, possess a white core with a black outer coloring, spread throughout the soil and penetrate the roots of new trees, passing on the oak root rot fungus.

Other Symptoms

• Along with the fungi growing on the trunk and surrounding earth, the oak tree will exhibit signs of general decline, including a loss of vigor, with the growth of new leaves stunted, a yellowing of leaves and dieback of branches. The tree may wilt when under stress, like during periods of drought. It is most susceptible to damage when weakened, with healthy, mature trees endowed with the ability to protect themselves by walling off the infection.

Oak Wilt

• Oak wilt, a disease caused by another fungus, is also capable of causing the oak tree to appear to rot, as this disease enters through the root system of the tree and initially manifests in the crown of the oak. Leaves will begin to wilt and droop, often browning at the edges, as the fungus works its way down the tree. Large groups of leaves -- both yellowing and seemingly healthy, green foliage -- will eventually prematurely drop to the ground. Oak wilt is extremely lethal and will generally kill a tree within one year of initial infection.