Honey Oak Fungus

The Culprit

• Honey oak fungus (Armillaria mellea) survives in the soil or on previously infected stumps for years. It's present as black, cordlike growths, or rhizomorphs, capable of spreading up to 20 inches per month, according to the University of Illinois Extension's Integrated Pest Management website. After reaching a healthy oak, the rhizomorphs secrete a corrosive, enzyme-filled gel that partially dissolves the cell walls of the tree's tiny feeder roots. The fungus then travels between the cell walls and moves through the roots and trunk. Active rhizomorphs continue inhabiting large oak stumps long after their hosts have died. Honey oak fungus also attacks trees already dead from other fungal diseases.

At-Risk Plants

• Honey oak fungus targets nearly 700 plant species, including many widely grown ornamental shrubs as well as shade and fruit trees. Those on sites with poor drainage, polluted air, or excessive shade as well as those located where recently cleared woodlands or orchards once have an increased risk of infection. So do trees and shrubs with root or bark damage and those weakened from drought, flooding, insects or other diseases.

Effects

• As honey oak fungus spreads gradually through an oak's feeder roots, the tree slowly loses its ability to absorb soil moisture and nutrients. It enters a gradual decline that accelerates as the root loss progresses. Initial symptoms include slowing, stunted growth, dying twigs and branches and overall weakening. The oak's leaves take on their fall colors prematurely. If you suspect your oak has the fungus, peel back its lower-trunk bark. The presence of spongy, saturated sapwood covered with white, fanlike mushroom-scented strands, or mycelia, indicates honey oak fungus infection. The sapwood of a tree with the disease's signature clusters of honey-brown mushrooms emerging from a tree's trunk is in the final stages of decay.

Protecting Your Trees

• Planting on sites where honey oak fungus has attacked other trees or is likely to exist, such as recently cleared woodlands or orchards, invites an infestation. If you're clearing an area, remove and dispose of all root remnants measuring 1 or more inches around. Give the trees good drainage and plenty of sun.

Treating Infected Oaks

• No fungicides control honey oak fungus. If you diagnose a tree before the disease has penetrated a significant portion of its roots or root collar, removing the root collar's soil so it can air dry. This may be enough to halt the fungus' progress. Removing severely infected trees along with as much as possible of their root balls and surrounding soil eliminates them as fungal sources, but may not be practical with large specimens.