Varnish fungus rot, caused by the fungus Ganoderma lucidum and (unvarnished) fungus rot, caused by G. applanatum infect the roots and lower trunk (butt) of many deciduous trees and some conifers. They attack the lower heartwood, and at advanced stages damage the structural integrity of the host tree, often resulting in windthrow (the potential to be uprooted or broken by wind). Maples, oaks, and honeylocusts are particularly susceptible, although ashes, elms, and many other deciduous trees and some conifers can be attacked.

Symptoms and Diagnosis

Trees affected by fungus rot may exhibit yellowing, wilting, or undersized leaves and dead branches. Tree vigor may decline as the decay of the sapwood advances. The first visible sign of infection is often the formation of fruiting bodies (single or in clusters) on the lower trunk and exposed root areas. The fruiting bodies are conks:  shelf-like in appearance and up to 14 inches wide. The upper surface of varnished fungus rot is typically red-brown with a white edge, shiny, and with a lacquered appearance. Conks of the unvarnished fungus rot are brown with a white edge weathering to grey. Both have a white, porous surface (when fresh) on the underside. Young trees as well as older, larger ones can be killed by this disease. Unfortunately, by the time the conks are noticed, it is too late to reverse the infection. The rate of decay can lead to death in as little as 3 to 5 years from the time of infection and appears to be determined by tree vigor, which is often influenced by environmental stresses.

Life Cycle

New spores released from the conks are dispersed throughout the summer during humid periods and infect open wounds on root flares and lower trunk areas of susceptible trees. The spores germinate, and the infection advances to attack the sapwood of major roots and the lower tree trunk. The amount of decayed wood increases year after year, resulting in dangerously soft, spongy wood in the part of the tree that serves as its anchor. The conks are annual; new conks may be produced each summer and fall, after which they die and deteriorate.

Integrated Pest Management Strategies

1. Proper planning.  Appropriate species and cultivar selection to match the right tree for the right site will make it easier to keep trees healthy for their entire life.

2. Proper tree maintenance. Subsequent good cultural practices (planting, fertilizing, watering, pruning, etc.) will help to maintain the health and vigor of any tree.

3. Avoid damage to tree trunks and roots. Even small wounds from mowers and trimmers can allow infection by decay fungi. Avoid damaging all deciduous trees, both young and old.

4. Contact an arborist. It may be wise to contact a certified arborist if this fungus is present. Find a certified arborist in your area using this website provided by the International Society of Arboriculture

5. As soon as possible, remove trees that exhibit conks on the lower trunk and exposed root areas. Large trees with severe internal rot may fall with little warning, injuring people and causing extensive property damage. Consult a certified arborist for the removal of large trees.

Organic Strategies

All of the recommended IPM strategies are strictly organic approaches.