Understanding Ganoderma Wood Rot in Trees

Ganoderma applanatum found on White Oak

Few diseases are as destructive and misunderstood as Ganoderma root and butt rot. This fungal pathogen is responsible for the slow decline and eventual structural failure of many mature shade trees in Northwest Indiana. While some fungi decay only dead wood, Ganoderma species actively invade living trees, weakening them from the inside out and creating serious risk concerns for property owners.

What is Ganoderma?

Ganoderma is a wood decay fungus belonging to a large genus of shelf fungi that produce distinctive conks (fruiting bodies) on the lower trunk or root flare of infected trees. These conks are often varnished or reddish-brown with a white edge when fresh, and they harden over time into a woody texture. The most common species affecting trees in our region is Ganoderma applanatum (often called artist’s conk), though other species can be found as well.

How the Fungus Infects Trees

White rot in wood of Engelmann Spruce

The fungus typically enters through wounds in the roots or lower trunk, such as those caused by lawn equipment, construction, or poor pruning practices. Once inside, it colonizes the heartwood and structural roots, decomposing the lignin that gives wood its strength.

This leads to white rot, a type of decay where both cellulose and lignin are broken down, leaving the wood spongy and stringy. As decay progresses, the tree may still appear outwardly healthy while significant internal structural loss occurs below ground or at the base of the trunk.

Signs and Symptoms

Ganoderma infections develop slowly, often taking years before symptoms become visible. Common signs include:

• Reduced vigor and thinning canopy

• Premature leaf drop or dieback in upper limbs

• Cracking or bleeding at the base of the trunk

• Mushroom-like conks forming near the base (most visible in late summer to fall)

• Sudden failure of major roots or trunk during storms, even when the canopy appears healthy

Because the conks are the fruiting bodies of the fungus, their presence means the internal decay is already advanced.

Risk Implications

Trees infected with Ganoderma are often structurally compromised. In advanced stages, the decay in the buttress roots or trunk base significantly increases failure potential, particularly under wind or saturated soil conditions. For this reason, risk assessment by a qualified arborist is critical to determine the extent of decay and whether mitigation or removal is necessary.

Diagnosis and Evaluation

Diagnosis begins with visual inspection for conks and basal decay indicators. However, because internal decay may not be visible from the surface, advanced diagnostic tools—such as sonic tomography or resistance drilling—can be used to measure the extent of internal wood loss. These tools help quantify the percentage of sound wood remaining and guide management decisions.

Management and Prevention

Unfortunately, once Ganoderma colonizes a tree, there is no curative treatment. The focus shifts to prevention and risk mitigation:

• Avoid wounding roots and lower trunks during mowing or construction

• Improve soil health through aeration and organic fertilization to support root vitality

• Monitor high-value trees annually for changes in vigor or new conk formation

• Remove and replace severely decayed trees with non-susceptible species if structural integrity is compromised

When removing infected trees, it’s important to grind out and dispose of infected stumps and roots to reduce the fungal inoculum in the soil before replanting.

Why It Matters

Ganoderma-related decay is a growing concern as many of Northwest Indiana’s mature oaks, maples, and locusts age in landscapes where soil compaction, root disturbance, and construction stress are common. Recognizing and addressing this disease early can prevent property damage and preserve valuable trees through proactive care and risk assessment.

If you’ve noticed conks or unusual growths at the base of your trees, it’s important to have them evaluated by a certified arborist. NWI ArborCare provides tree health assessments and decay detection services using both visual and advanced testing methods to determine the extent of internal decay and recommend safe, effective management options.

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