White Rot Fungus on Oak Trees
White rot fungus is unique among microorganisms in its ability to metabolize non only large amounts of lignin in wood, but also the cellulose itself. Thousands of related species of this type of micro-organism can degrade specific cell wall components of oak and other hardwoods, leaving behind pockets of white cells that consist almost entirely of cellulose. Brown rot usually doesn't affect the cellulose in the wood, but the varieties of fungus that causes white rot may even degrade the cellulose in tree cell walls.
Vulnerable Oak Species
Old growth of willow oak and water oak in urban landscapes and park areas are particularly vulnerable to white rot, experiencing steady increases in root and butt rot attacks in the late 20th century, especially along the East Coast of the United States. These oaks normally live 65 to 80 years and even longer, especially in protected parks and urban green spaces. As they age, oaks become more susceptible to root and butt rot. Infestations can spread to white oak, scarlet and red oak, Shumard oak, post oak and black oak. These longer-lived species may be vulnerable to the spread of white rot fungus from their shorter-lived cousins.
White rot fungus, particularly armillaria mellea, invades the heartwood in living trees. These types of fungi are normal members of the forest community, aggressively decomposing woody debris on the forest floor. They enter the trees through dark, rootlike rhizomorphs on the surface of the roots. If the soil is damp or soggy for an extended period, the growth of the fungus is stimulated. Once the tree is fully infested, the fungi consume cell material, leaving white mottled patches in the wood.
Sadly, the first symptom that owners of oak trees see is a blown down tree after a windstorm. This may be the first and only indication you get that your oak tree has white rot. Sometimes, the tree's foliage becomes sparce or limbs start dying, but these symptoms aren't necessarily caused by white rot. The damage may start a few inches into the base of the tree and extend deep into the tap root depending on the species of fungus that has established itself there. If you peel back the bark you will see whitish or straw-colored crumbly, dry wood. Look for horzontal and vertical fissures in the wood that is typical of white rot.
Watch for signs of white rot in your oak trees, particularly for the reproductive structures or basidiocarps of the fungus. Conks, shelf or bracket fungi, woody in texture and shelf-like without stems, may appear near the base of the tree. Look, also, for the presence of mushrooms attached to the tree, especially around the base and attached to roots. Check the tree trunk, 3 to 6 feet from the base. Basically, the more basidiocarp, the worse the infection. The more of the circumference of the tree that is girdled by basidiocarps like shelf and bracket fungus or mushrooms, the more severe the problem likely is. Also, if you find white, fan-shaped mats of mycelium (fungal tissue) underneath the bark of the tree on the roots or at the base of the tree or blackened rhizomorphs on the roots beneath the soil, you can be pretty sure you have white rot going on in the root system.
Controlling White Rot
White rot grows most rapidly in wet, boggy soil. The process can be slowed or stopped by removing soil from around the base of the tree and allowing the base to dry. Find a way to drain the soil. You may have to re-route small channels that funnel rainwater away from the base of your oak trees. Whether attempting treatment or prevention, reducing moisture in the soil around the base of the tree is job one. Fungus can also enter the tree through wounds, so any bark damage you find near the base should be sealed and treated. By the time basidiocarps appear, it may be too late. Sometimes, all you can do is remove the tree to prevent it from infecting other nearby trees with the disease.
- University of Minnesota; Microorganisms Causing Decay in Trees and Wood; Robert A. Blanchette
- UC Davis; Wood Decay Fungi In Landscape Trees; March 2003
- NCSU Agriculture and Life Sciences; Root and Butt Rot of Oaks; L.F. Grand et al; April 1991
- NIH; Screening Wood Decayed by White Rot Fungi; Robert A. Blanchette; September 1984