Fungal growth on trees, often called “tree mold,” represents diverse organisms interacting with their host. This growth is not automatically alarming, but it requires inspection, as the danger depends entirely on the specific fungal species and its context. Many outdoor fungi are natural decomposers, while others are aggressive pathogens or producers of harmful compounds. Understanding the type of fungus present is the first step in assessing the risk to human health, pets, and the tree’s structural integrity.
Identifying Common Types of Tree Fungi
Fungi are categorized by their feeding habit, which helps gauge their threat to a living tree.
Saprophytic Fungi
Saprophytic fungi feed exclusively on dead organic matter, such as dead branches, fallen leaves, or the non-living wood at the center of a trunk. These species, often appearing as mushrooms or shelf-like brackets (conks), do not attack living tissue. However, their presence signals that internal decay may already exist within the host tree.
Parasitic Fungi
Parasitic fungi pose the greatest threat because they actively colonize and extract nutrients from living wood and bark. Examples include canker-forming fungi, which create sunken, necrotic areas on the bark, and root-rotting species like Armillaria. The appearance of a parasitic fungus on a trunk or limb indicates an active infection.
Superficial Growths
A third category is superficial growths, such as sooty mold, often mistaken for a serious infection. Sooty mold is a black, velvety coating that grows on honeydew, a sugary excretion left by feeding insects like aphids or scale. It does not penetrate the tree bark. Lichens, a symbiotic partnership between a fungus and an alga, are also non-parasitic and harmless, using the tree only as an anchor.
Health Risks to Humans and Pets
The primary risk tree fungi pose to humans and pets is the inhalation of microscopic spores. When fungi are disturbed, they release vast quantities of spores into the air, triggering allergic responses in sensitive individuals. Symptoms include hay fever-like reactions, asthma attacks, or hypersensitivity pneumonitis, which results from an immune reaction deep within the lungs.
A more severe, though rare, risk is systemic infection, which primarily affects individuals with compromised immune systems. While most people inhale spores without issue, an immunocompromised person may develop a serious infection if the fungus colonizes body tissues.
Direct physical contact during yard work increases exposure to mycotoxins, which are toxic secondary metabolites produced by some fungi. Mycotoxin exposure is a concern for pets and young children who may ingest toxic mushrooms growing near the tree. Many outdoor mushrooms are poisonous, and accidental consumption can lead to severe gastrointestinal distress, neurological damage, or liver failure. Any unknown fungi should be treated as potentially toxic and kept out of reach.
Structural Impact on Tree Health
Fungal presence severely undermines a tree’s physical integrity by initiating decay in the wood structure. Heart rot is a common form of decay where fungi enter through a wound and decompose the heartwood, the non-living center of the trunk. This process hollows out the tree, increasing the risk of structural failure in high winds despite a healthy external appearance.
Fungi are classified by the wood components they break down, leading to distinct decay patterns. White rot fungi decompose both lignin and cellulose, resulting in wood with a soft, stringy, or spongy texture. Brown rot fungi primarily target cellulose and hemicellulose, leaving behind brittle, dark-brown lignin that often cracks into cubes. Brown rot is often more destructive to a tree’s bending strength.
Some fungi also cause cankers, which are localized areas of dead bark and underlying cambium tissue. A canker that completely encircles a branch or trunk is called a girdle, cutting off the flow of water and nutrients and leading to the death of the part above the infection. A visible fruiting body, such as a large conk, often signals extensive, advanced internal decay and a high risk of the tree or a major limb falling.
Safe Management and Removal
The first step in managing tree mold is determining if the fungus is parasitic or indicates structural decay. If a mushroom or bracket fungus grows from the root flare or main trunk, or if the tree exhibits cankers or significant dieback, call a certified arborist for a professional risk assessment. Arborists use tools like resistographs to measure internal decay and advise on saving or safely removing the tree.
For minor or superficial growths, homeowners can manage the issue by focusing on sanitation and improved air circulation. Pruning away infected or dead limbs is standard practice, but sterilizing pruning tools with a bleach solution or alcohol before and after each cut is important to prevent spreading fungal spores. Reducing dense foliage or thinning surrounding shrubs helps the bark dry quickly, creating an unfavorable environment for fungal growth.
When interacting with unknown outdoor fungi, especially during removal or cleanup, wearing appropriate personal protective equipment (PPE) is a necessary precaution. An N95 or N100 respirator mask prevents spore inhalation, mitigating the risk of allergic or respiratory reactions. Gloves should be worn to avoid direct skin contact, and hands must be washed thoroughly after handling the material to prevent accidental ingestion of mycotoxins.