Several fungi that grow on or inside trees can cause serious illness in people, ranging from chronic lung infections and allergic disease to skin infections and, in rare cases, fatal poisoning. The risk depends on how you encounter the fungus: breathing in spores released from infected bark, handling contaminated wood with a cut on your hand, or mistakenly eating a toxic species all carry different consequences. Most healthy people walk past tree fungi every day without trouble, but certain species, certain exposures, and certain health conditions can turn a harmless-looking bracket or patch of bark mold into a genuine medical problem.
Lung Infections from Inhaling Tree-Borne Spores
The clearest example of a tree fungus directly threatening human health is Cryptococcus gattii, a yeast-like organism found in the decaying wood of tree hollows, particularly in certain eucalyptus species. People become infected by inhaling airborne particles of the fungus, which can settle deep in the lungs and, in severe cases, spread to the brain and spinal cord. For decades this was considered a tropical concern, mostly studied in Australia, where researchers noted a high incidence of cryptococcosis in communities with heavy environmental exposure.1PubMed. Cryptococcus neoformans variety gattii That changed in the late 1990s when C. gattii appeared on Vancouver Island in British Columbia, thousands of kilometers from any tropical eucalyptus grove.
Research on that Canadian outbreak found something striking: human infection rates rose and fell in step with the intensity of tree harvesting in the surrounding area. At distances of about seven to ten kilometers from settled areas, tree-cutting activity and human C. gattii cases were strongly correlated. As annual harvesting increased between 1999 and 2003, so did infections; when harvesting tapered off after 2007, cases dropped as well.2PubMed Central. Forest Disturbance and Disease: Exploring the Effects of Tree Harvesting Area on Cryptococcus gattii sensu lato Infection Risk, Vancouver Island, Canada, 1998-2014 The implication is that disturbing colonized trees can liberate fungal cells into the air and carry them toward nearby communities. You do not need to be immunocompromised to develop C. gattii disease; unlike many fungal infections, this one has struck otherwise healthy people, though underlying host factors seem to matter in ways that are not fully understood.
Maple Bark Disease and Hypersensitivity Pneumonitis
A very different kind of lung problem can come from a mold called Cryptostroma corticale, the organism behind sooty bark disease in maples. When an infected sycamore maple’s outer bark peels away, it exposes dense masses of brownish-black spores underneath. Breathing in those spores can trigger a condition known as maple bark disease, a form of hypersensitivity pneumonitis. This is not an infection in the traditional sense; it is an aggressive immune overreaction in the lungs. Symptoms resemble COPD, allergic asthma, or a stubborn flu-like illness, and in chronic cases, the inflammation can scar lung tissue.3PubMed Central. Sooty bark disease of maples: the risk for hypersensitivity pneumonitis by fungal spores not only for woodman
This used to be considered a niche occupational disease, mainly affecting forestry workers and sawmill employees. But sooty bark disease in maples has surged in parts of Europe during recent warm, dry summers, meaning the spores are increasingly present in urban parks and residential neighborhoods where sycamore maples are common landscape trees. The exposure risk is no longer limited to professional woodworkers.4Frontiers in Public Health. Impact of climate change on wood and woodworkers—Cryptostroma corticale (sooty bark disease): A risk factor for trees and exposed employees Handling or cutting diseased maple firewood in a poorly ventilated space is a particularly risky scenario for anyone, not just people with a history of respiratory illness.
Skin Infections from Contaminated Wood
Not all tree-fungus hazards arrive through your lungs. Sporotrichosis, sometimes called “rose gardener’s disease,” is caused by Sporothrix schenckii, a fungus that lives on decaying wood, soil, hay, and sphagnum moss. It enters the body through a break in the skin, so the classic scenario is getting scratched or pricked while handling old lumber, bark, or thorny plants growing in contaminated mulch. After an incubation period that can last days to weeks, a slowly enlarging nodule develops at the wound site, sometimes ulcerating. In a common pattern called lymphocutaneous sporotrichosis, new nodules appear in a line up the arm or leg, following the path of the lymphatic vessels draining the original wound.5PubMed Central. Sporotrichosis: an overview and therapeutic options
Sporotrichosis is most prevalent in tropical and subtropical regions, but cases occur worldwide wherever the fungus inhabits the soil and woody debris. Gardeners, landscapers, and anyone who regularly handles raw wood or compost are at greatest risk. The infection responds to antifungal treatment, usually itraconazole taken for several months, but a delayed diagnosis is common because the slowly spreading nodules are often mistaken for bacterial infections or insect bites. The practical takeaway: wearing gloves when handling old wood, mulch, or sphagnum moss is a cheap and effective precaution.
A Common Bracket Fungus That Can Colonize Human Airways
Schizophyllum commune is one of the most widespread fungi on the planet, found on dead and decaying wood on every continent except Antarctica. If you have spent any time walking through woods, you have almost certainly seen its small, fan-shaped, whitish-gray fruiting bodies on fallen branches. For most people it is completely harmless, but in clinical settings it has emerged as an uncommon but real cause of respiratory disease. A review of globally reported cases found that out of 71 documented infections, about two-thirds were bronchopulmonary, roughly a third involved the sinuses, and the remaining few were infections in other body sites. Altogether, about 94% of S. commune infections targeted the respiratory tract.6PubMed. Schizophyllum commune as an emerging fungal pathogen: a review and report of two cases
The infections it causes range from allergic bronchopulmonary mycosis, an immune-mediated condition that resembles severe asthma, to fungal balls that lodge in the lungs or sinuses.7PubMed Central. Pulmonary infection caused by Schizophyllum commune: a case study Sinus infections from S. commune have been reported in people with no obvious immune problems.8PubMed Central. Schizophyllum commune a causative agent of fungal sinusitis: a case report Cases remain rare, but clinicians have become more aware of this organism in recent years, partly because improved laboratory techniques are catching infections that would previously have gone unidentified. For everyday purposes, you are unlikely to be harmed by walking past S. commune on a log, but the fungus is a reminder that even the most common wood-decay organisms are not entirely benign under the right circumstances.
Fatal Poisoning from a Tree-Dwelling Mushroom
When people think of deadly mushrooms, they usually picture species growing from the ground. But one of the most dangerous fungi known grows directly on wood. Podostroma cornu-damae is a bright red or orange, finger-shaped fungus found on dead hardwood in parts of East Asia, particularly Japan and Korea. It produces trichothecene mycotoxins, a class of poisons that damage rapidly dividing cells throughout the body. Forensic analysis of a fatal case identified satratoxin H in the victim’s blood and stomach contents, the first time a trichothecene mycotoxin was confirmed in human biological samples from a suspected P. cornu-damae poisoning.9Forensic Science International. Identification of trichothecene-type mycotoxins in toxic mushroom Podostroma cornu-damae and biological specimens from a fatal case by LC–QTOF/MS
Clinical reports describe a grim progression in severe cases: skin peeling on the palms and soles, a dangerous drop in all blood cell counts, overwhelming infection, and multiple organ failure leading to death. In one report of two patients who consumed the fungus, one died while the other survived after a month of intensive supportive care.10PubMed Central. Two cases of mushroom poisoning by Podostroma cornu-damae The fungus is sometimes confused with edible cordyceps species or other red fungi by inexperienced foragers. This is a narrow geographic concern for most readers, but it underscores a broader point: a fungus growing on a tree is not inherently safer than one growing on the ground, and visual identification mistakes with wild fungi can be lethal.
Allergic Reactions and Asthma Triggered by Fungal Spores
Even when tree fungi do not cause outright infection or poisoning, the airborne spores they release are among the most common biological triggers for allergic disease and asthma. Outdoor fungal spores tend to be more diverse and abundant than indoor ones, and they have been linked to hospitalizations for acute asthma attacks.11PubMed Central. Impact of Fungal Spores on Asthma Prevalence and Hospitalization While not every allergenic spore comes from a tree-associated fungus, many of the most clinically relevant genera, including Alternaria, Cladosporium, and Coprinus, thrive on decaying wood and plant litter found beneath and around trees.
A study of childhood asthma hospitalizations found that exposure to Alternaria, Cladosporium, Coprinus, and several other fungal spore types was significantly associated with hospital admissions, with measurable lagged effects lasting up to three days after exposure. Children who were already sensitized to Cladosporium showed even stronger reactions.12Journal of Allergy and Clinical Immunology. Associations between outdoor fungal spores and childhood and adolescent asthma hospitalizations In urban parks, allergenic spore concentrations peak during the summer months, making July and August the riskiest time for people with fungal sensitivities to spend extended time outdoors.13Aerobiologia. Allergenic fungal spores in the air of urban parks If you have asthma or known mold allergies, being aware of these seasonal patterns can help you time outdoor activities or adjust medication with your doctor.
Occupational Risks for People Who Work with Wood
People who cut, saw, strip, or transport timber face a concentrated version of the hazards described above. Wood-processing workers are routinely exposed to fungal bioaerosols that most people encounter only fleetingly. A study of sawmill workers in India found that about a third of those tested had detectable fungal colonization in their sputum, with genomic analysis identifying Candida species in their respiratory samples.14PubMed Central. Fungal contamination of the respiratory tract and associated respiratory impairment among sawmill workers in India While Candida is a common commensal organism that does not always cause disease, its presence at elevated levels in the airways signals heavy fungal exposure and potential respiratory compromise over time.
The maple bark disease discussed earlier was originally described as an occupational illness among loggers and sawmill workers, and that population remains the most at-risk group. Arborists, landscapers, and municipal park workers who deal with storm-damaged or drought-stressed trees are also in a higher-exposure category. Proper respiratory protection, particularly when cutting visibly diseased wood or working in enclosed spaces where dust and spores accumulate, is the most straightforward way to reduce risk. Open-air chainsaw work poses less danger than indoor milling of the same wood, because spore concentrations disperse quickly outdoors.
Tree Fungi Sold as Supplements
Some of the tree fungi making headlines are not wild hazards but products people deliberately consume. Chaga (Inonotus obliquus), a hard, charcoal-colored growth found on birch trees, has become hugely popular as a health supplement, marketed for immune support and antioxidant benefits. But chaga is exceptionally high in oxalates, and case reports have documented acute oxalate nephropathy, a form of kidney damage, in people who consumed chaga in large quantities. In these cases, oxalate crystals accumulated in the kidney tubules, and patients required dialysis.15PubMed Central. Chaga mushroom-induced oxalate nephropathy that clinically manifested as nephrotic syndrome: a case report16Frontiers in Pharmacology. Chaga mushroom: a super-fungus with countless facets and untapped potential
Reishi (Ganoderma lucidum), another widely sold tree fungus, carries a different concern. Because fungi absorb minerals from their substrate, reishi grown in certain regions can accumulate potentially toxic elements like arsenic and chromium. A risk assessment of reishi from one production area in China found that while overall exposure risk was relatively low for the general population, arsenic and chromium concentrations warranted further analysis to determine how much was in the more dangerous inorganic forms.17PubMed. Risk assessment of potentially toxic elements exposure through ganoderma lucidum consumption in the population of Longquan, China The takeaway is not that these supplements are universally dangerous, but that “natural” and “from a tree” are not the same as “safe in unlimited doses.” People with kidney disease should be cautious with chaga, and sourcing matters for reishi and other wood-grown fungi because heavy metal content varies dramatically by growing region and substrate.
When a Plant Pathogen Jumps to Humans
One of the stranger entries in the medical literature involves Chondrostereum purpureum, a fungus best known for causing silver leaf disease in fruit trees. It is a workhorse organism in forestry, even used commercially as a biological herbicide to prevent regrowth of cut stumps. For most of its known history, it was considered a strict plant pathogen with no relevance to human health. Then a documented case emerged of an otherwise healthy person developing a paratracheal abscess, a pocket of infection next to the windpipe, caused by C. purpureum. The patient was not immunocompromised, which made the finding even more unusual.
This appears to be a genuine case of a fungus crossing the kingdom boundary from plants to animals, something mycologists have long considered extremely rare. It does not mean that every gardener pruning a plum tree is at risk of throat infection. But it demonstrates that the wall between plant pathogens and human pathogens is not as absolute as once assumed, and that novel fungal threats can emerge from unexpected directions.
Contact Dermatitis from Handling Fungi
Even noninfectious, nontoxic encounters with tree fungi can cause skin problems. Allergic contact dermatitis from handling wild mushrooms is a recognized phenomenon, though it tends to be mild and self-limiting. Symptoms typically develop one to two days after skin contact and include redness, swelling, and itching at the contact site. The reaction usually fades within about a week without treatment. Most documented cases involve the mucilage layer on the cap surface of certain species, and the reaction pattern suggests a delayed allergic sensitivity rather than a chemical burn or irritation.
For most foragers, this is an annoyance rather than a health emergency. But it can be confusing when the rash appears a day or two after a mushroom hunting trip and the person does not connect the two events. If you regularly handle wild fungi and notice recurring unexplained rashes on your hands, gloves are a simple fix.
Climate Change Is Reshaping the Threat Landscape
Many of the hazards described above are not static. Rising temperatures, shifting rainfall patterns, and extreme weather events are thought to be driving fungal pathogens to adapt to warmer conditions and expand into regions where they were previously absent.18PubMed. Climate change: shifting boundaries of fungal disease in Europe and beyond The spread of Cryptococcus gattii from tropical to temperate zones is one example. The explosion of sooty bark disease in European maples during recent heat waves is another. As tree species undergo drought stress, they become more susceptible to fungal colonization, which means more fungal spores in the environment for people to encounter.19PubMed Central. Wake-up Call: Rapid Increase in Human Fungal Diseases under Climate Change
Compounding the problem is growing antifungal resistance. Azole antifungals, the most important class of drugs for treating human fungal infections, are also widely used in agriculture, timber preservation, and antifouling coatings. This overlapping use has accelerated the evolution of resistant fungi in the environment, meaning that when an opportunistic tree fungus does cause human disease, the standard medications may be less effective than they used to be.20Science. Worldwide emergence of resistance to antifungal drugs challenges human health and food security The intersection of climate-driven range expansion and shrinking treatment options is something infectious-disease researchers are increasingly worried about, even if it has not yet translated into everyday clinical crises for most people.