- Cladosporium is one of the most abundant mold genera in both outdoor and indoor air worldwide, with over 700 described species.
- It is a leading cause of allergic sensitization and is strongly associated with asthma exacerbations, particularly in late summer and autumn when outdoor spore counts peak.
- Unlike many molds that require significant water damage, Cladosporium can establish in relatively mild humidity conditions — making it the most commonly encountered indoor mold in routine air sampling.
- Its characteristic dark olive-green to brown-black colonies are found on a wide variety of substrates including textiles, wood, HVAC systems, and window condensation zones.
- Cladosporium rarely causes disease in healthy individuals but is a persistent and significant indoor air quality concern due to its allergenic potential.
Cladosporium is the mold most likely to be detected in any indoor air sample taken anywhere in the world. Its spores dominate outdoor fungal aerosols across temperate climates, peak in summer and autumn, and infiltrate indoor environments with ease. For allergists, indoor air quality professionals, and building scientists, understanding Cladosporium — its biology, the conditions that support it, and its effects on human health — is a foundational requirement.
Taxonomy and Classification
Cladosporium belongs to the order Capnodiales within the class Dothideomycetes — one of the largest and most ecologically diverse classes of ascomycete fungi. The genus was first described by Link in 1816. Molecular phylogenetics has substantially reorganized the genus in recent decades; many species formerly classified as Cladosporium have been reassigned to related genera, though the core genus remains large.
- Kingdom: Fungi
- Phylum: Ascomycota
- Class: Dothideomycetes
- Order: Capnodiales
- Family: Cladosporiaceae
Over 700 species have been described, though this number has shifted considerably with molecular revision. The most clinically and environmentally relevant species include C. cladosporioides, C. herbarum, C. sphaerospermum, and C. halotolerans.
Morphology and Identification
Cladosporium colonies are typically slow to moderately fast growing, with a characteristic olive-green, brown, or dark gray-green coloration. The surface texture ranges from powdery to velvety, and the colony reverse is usually dark. The genus is named for its distinctive branching chains of conidia, which resemble a tree (clados = branch in Greek).
Microscopically, Cladosporium is identified by:
- Conidia in branching chains: Dry, darkly pigmented spores borne in acropetal chains (youngest at the tip)
- Conidial scars: Prominent dark hilum scars at the point of attachment between conidia
- Conidiophores: Erect, branching, septate, and darkly pigmented (dematiaceous)
- Spore shape: Variable — spherical to ellipsoidal, often with a slightly thickened, darkened wall
The dark pigmentation of both the hyphae and conidia is due to melanin — a characteristic shared across the broad group of dematiaceous (melanized) fungi, to which Cladosporium belongs.
Ecology and Habitat
Cladosporium is one of the most ecologically versatile mold genera. It functions primarily as a saprotrophic decomposer of plant material but also colonizes living plant surfaces as an epiphyte and can cause disease in weakened plant hosts. Key habitats include:
- Leaf surfaces and plant debris: Cladosporium is among the dominant fungi on senescing leaves and plays a significant role in litter decomposition in temperate forests.
- Soil: Present in agricultural and natural soils globally.
- Indoor surfaces: Wood, textiles, leather, painted surfaces, window frames and seals, and HVAC components are all susceptible substrates.
- Refrigerators and cold environments: Some Cladosporium species are psychrotolerant and grow at temperatures near 4°C, enabling colonization of refrigerated food and cold storage areas.
- Marine environments: Several halotolerant species thrive in high-salinity environments.
Seasonality
Outdoor Cladosporium spore counts show strong seasonal variation in temperate climates, peaking in mid-summer to early autumn and declining sharply with the onset of cold weather and frost. This seasonality corresponds closely with epidemiological data showing increased asthma hospitalizations during peak Cladosporium season in sensitized individuals.
Moisture Requirements
One of the reasons Cladosporium is so prevalent indoors is its relatively modest moisture requirement. It can establish and grow at water activity values of approximately 0.85–0.88 aw, which corresponds to surface relative humidity above 85–88%. This means it can colonize in mildly damp conditions — condensation on window frames, slightly damp bathroom walls, poorly ventilated spaces — without requiring the sustained heavy wetting needed by more hydrophilic species like Stachybotrys chartarum.
This low moisture threshold makes Cladosporium the first mold to appear in buildings with ordinary humidity management problems, and a common finding even in buildings without overt water damage.
Health Effects
Allergic Sensitization
Cladosporium is one of the most important allergenic mold genera in temperate climates. Population studies consistently show sensitization rates of 5–15% in the general population, and substantially higher in asthmatic and atopic individuals. Sensitization is measured by skin prick test or specific IgE antibody testing.
Asthma Exacerbation
In sensitized individuals, Cladosporium spore exposure is a documented trigger for asthma attacks. Studies from the UK, Australia, and North America have shown statistically significant associations between days with high outdoor Cladosporium spore counts and increased emergency department visits for asthma. The mechanism involves IgE-mediated hypersensitivity — the immune system mounts an allergic response to inhaled conidia and fungal antigens.
Rhinitis and Other Allergic Symptoms
Beyond asthma, Cladosporium sensitization presents as allergic rhinitis (runny nose, sneezing, nasal congestion), conjunctivitis, and urticaria in susceptible individuals. Symptoms typically worsen during peak spore season outdoors.
Infections
Cladosporium is not considered a primary pathogen in immunocompetent individuals. It occasionally causes superficial infections (tinea nigra — a skin discoloration condition caused by a related species) and has been implicated in rare cases of onychomycosis (nail infection) and sinusitis in immunocompromised patients. Systemic or invasive Cladosporium infection is very rare.
Indoor Control and Remediation
Because Cladosporium establishes in relatively mild moisture conditions, prevention focuses on:
- Maintaining indoor relative humidity below 60%
- Ensuring adequate ventilation in bathrooms, kitchens, and other moisture-generating spaces
- Addressing condensation on windows and cold surfaces through improved insulation or ventilation
- Regular cleaning of HVAC components, particularly drain pans and coils
Surface growth of Cladosporium on non-porous materials (tile, glass, metal) can be cleaned with appropriate antifungal cleaning solutions. Growth on porous materials (wood, drywall, grout) requires more aggressive intervention, and extensive growth on porous substrates typically requires removal of the affected material.
Frequently Asked Questions
Is Cladosporium the most common indoor mold?
Cladosporium is consistently among the most frequently detected mold genera in indoor air samples worldwide, alongside Penicillium. Its abundance reflects both its prevalence in outdoor air (which infiltrates buildings) and its ability to grow indoors on a wide range of substrates at relatively modest moisture levels. In most buildings without active water damage, Cladosporium dominates indoor fungal aerosol counts.
Is Cladosporium dangerous?
Cladosporium is not considered a high-risk mold for healthy individuals in terms of infection. Its primary health impact is as an allergen — it is one of the most common causes of mold-related allergic sensitization and asthma exacerbation in temperate climates. For immunocompromised individuals, it can occasionally cause opportunistic infections, but invasive disease is rare compared to Aspergillus fumigatus.
What does Cladosporium look like?
Cladosporium colonies typically appear olive-green, brown, or dark gray-green, with a powdery or velvety surface texture. They are often seen on window frames and seals, bathroom grout, and damp wood. Under the microscope, Cladosporium is identified by its branching chains of darkly pigmented, ovoid to ellipsoidal conidia with visible attachment scars.
Why does Cladosporium peak in summer and autumn?
Outdoor Cladosporium spore counts peak in summer and early autumn because the species is strongly associated with decaying plant material — particularly senescing leaves and plant debris that accumulate at the end of the growing season. Warm temperatures and dry wind conditions in late summer favor mass spore release. Counts fall sharply after the first frost.
Can Cladosporium grow in a refrigerator?
Yes. Some Cladosporium species are psychrotolerant — capable of growth at temperatures near 4°C. They can colonize refrigerator seals, drip trays, and food stored in conditions of high humidity. This is one of the few common mold genera with genuine cold-tolerance, which distinguishes it from the majority of indoor molds that require temperatures above 10–15°C to grow actively.
