Understanding Different Types of Mould and What Your Testing Results Mean

When you receive a mould testing report, the results will identify specific mould genera found in your property — groups such as Aspergillus, Penicillium, or Cladosporium. Laboratory microscopy identifies mould to genus level based on spore morphology and structural characteristics, which is the standard for assessment reporting. But what do these names actually mean? Each genus has different moisture requirements, different health implications, and tells a different story about what is happening in your building. This article is the genus identification leg of the Scaada NZ laboratory guide — it explains the common mould genera found in New Zealand homes and workplaces, and what their presence tells you. For how to read the metrics and condition ratings on your test report, the field patterns we see in NZ lab work, and what (and what not) to do about a positive result, see the companion articles linked at the end.

Why Mould Type Matters

Not all mould is equal. Different genera require different moisture levels to grow, produce different allergens and toxins, and indicate different underlying problems in a building. A report showing Cladosporium on a bathroom ceiling tells a very different story from one showing Stachybotrys behind a wall lining. The genera identified determine:

• The likely moisture source and severity of the underlying problem
• The health risk to occupants, particularly vulnerable individuals
• The appropriate remediation approach and urgency
• Whether the mould is actively growing or settled from elsewhere

An important point many people miss: mould spores can be viable (living) or non-viable (dead) — and both can cause adverse health effects. The allergenic proteins and immune-stimulating compounds in mould cell walls remain active regardless of whether the spore is alive. This is why even dry, apparently inactive mould can still trigger respiratory symptoms and allergic reactions when disturbed.

Mould and Building Materials

Moulds evolved to break down organic matter in nature, and many modern building materials provide ideal food sources. MDF, plywood, OSB, and chipboard are particularly susceptible — their low lignin content, high cellulose composition, and permeable structure make them easily digestible. Plasterboard paper facing, timber framing, carpet backing, and even some adhesives can all support mould colonisation once moisture is present.

This is why mould damage to building materials is permanent — the fungal enzymes break down the substrate itself. Cleaning the surface does not reverse structural degradation that has already occurred in porous materials.

Water Activity: Why Different Moulds Grow in Different Conditions

The single most important factor determining which mould genera can grow on a surface is water activity (a_w) — a measure of the free water within a substrate that an organism can use to support growth, measured as a fraction from 0 (completely dry) to 1.0 (pure water). Different mould genera have evolved to exploit different moisture niches, which is why the genera identified in your report are a direct indicator of the moisture conditions in your property.

Mould scientists classify fungi into three broad groups based on their water requirements:

Xerophilic Fungi — Primary Colonisers (a_w below 0.80)

These moulds can colonise materials at relatively low moisture levels — even conditions that might not feel damp to the touch. Known as primary colonisers, they are the first to appear when moisture rises and can establish growth within 48 to 72 hours of conditions becoming favourable. They are the most common indoor moulds worldwide.

Aspergillus conidiophore and spores under microscopy

Penicillium conidiophore under phase contrast microscopy

Chrysosporium conidia on a textile fibre under microscopy — observed in NZ lab work, particularly on organic substrates after flooding

• Aspergillus — One of the most frequently identified genera in indoor environments. Produces extremely small "dry spores" (under 5 μm) that are easily airborne and respirable, penetrating deep into the lungs. Aspergillus fumigatus and A. flavus are the most common cause of invasive mould infections worldwide and carry a high mortality rate. Some species produce aflatoxins, which are associated with liver cancer. The (1→3)-β-D-glucan polymer in its cell wall is a potent immune stimulant, meaning even dead spores trigger inflammatory responses with cumulative exposure
• Penicillium — Equally common indoors, often found alongside Aspergillus. Also produces small, easily airborne "dry spores" that can exist in large numbers. Found in soil, damp building materials, and spoiled food. Some species produce mycophenolic acid — a potent immunosuppressant also used pharmaceutically. Rarely causes direct infection, but chronic inhalation exposure contributes to Hypersensitivity Pneumonitis (HP)
• Wallemia — A less commonly reported but distinctive xerophilic genus that thrives on very dry substrates. Often found on stored materials and dust. Its presence can indicate chronic low-level humidity issues
• Chrysosporium — A keratinophilic xerophile that tolerates low water activity and grows readily on organic substrates such as dust, wool fibres, feathers and paper. We see Chrysosporium more frequently in our NZ samples than across our Australian datasets — likely because New Zealand indoor dusts carry a higher proportion of organic material. We have recorded it colonising the sarking paper of NZ homes after flood events, which is unusual for a typically xerophilic genus and a useful flag that organic-rich substrates have remained wet for extended periods

What this means for your report: Finding Aspergillus and Penicillium dominant indoors — particularly at concentrations above outdoor levels — suggests elevated humidity or condensation conditions, even if no visible water damage is present. As primary colonisers, these genera are the "early warning" moulds that can establish within days of a moisture event.

Mesophilic Fungi — Secondary Colonisers (a_w 0.80–0.90)

These genera require more moisture than the primary colonisers and are secondary colonisers — they appear after moisture conditions have persisted long enough for the environment to become more established. They are often associated with intermittent dampness, condensation cycles, or moderate water ingress.

Cladosporium sphaerospermum — budding conidia in branching chains, the classic diagnostic morphology of this genus

Alternaria (Ulocladium) spores under microscopy

Scopulariopsis conidial chains under microscopy

• Cladosporium — One of the most abundant outdoor and indoor moulds globally, commonly found on plant material, window frames, bathrooms, and poorly ventilated spaces. Has a small spore size that disperses easily. Generally considered lower risk than Aspergillus, but can cause infection in rare cases — affecting lungs (55% of reported cases), superficial sites (28%), and deep tissues (15%). Commonly found in both air samples and on damp building materials, and often the dominant genus in outdoor air, making it an important reference point when comparing indoor versus outdoor results
• Ulocladium (Alternaria) — A significant allergen trigger, particularly associated with IgE-mediated hypersensitivity reactions including allergic rhinitis and asthma exacerbation in children. Produces larger spores (10–40 μm) that deposit in the upper airways, though fragments can be respirable. Some species produce mycotoxins including alternariol and tenuazonic acid. A strong indicator of water damage when found growing indoors. In our lab work, Ulocladium reads as a stronger water-damage signal in Australian samples than in NZ samples, where Chrysosporium tends to play a more prominent role on organic substrates
• Scopulariopsis — Produces distinctive chains of lemon-shaped, thick-walled conidia borne on brush-like conidiophores. In our New Zealand laboratory work we encounter this genus comparatively rarely — Aspergillus and Penicillium tend to dominate the same indoor niches, and Scopulariopsis is frequently misidentified as one of these by laboratories not looking closely enough at conidial shape and wall texture. We have recorded it during inspections and analysis in environments of sustained high humidity (above 75% RH) and following indoor flood events, where it most often appears on fabrics as a fine white powdery growth — and fabric samples remain the media on which we most frequently confirm positives for this genus in New Zealand. S. brevicaulis is a recognised cause of onychomycosis (nail infection) and can act as an opportunistic pathogen in immunocompromised individuals
• Bipolaris — Less commonly identified but associated with moderate moisture. Can cause allergic fungal sinusitis in sensitised individuals

What this means for your report: Secondary colonisers indicate more sustained moisture — not just elevated humidity, but likely condensation accumulation, slow leaks, or poor drainage. Ulocladium in particular is a reliable water-damage indicator genus. Their presence suggests the moisture problem has been ongoing long enough for conditions to progress beyond what primary colonisers alone would indicate.

Hydrophilic Fungi — Tertiary Colonisers (a_w above 0.90)

These moulds require near-saturated conditions and are tertiary colonisers — they need continuously wet materials over an extended period to establish. Stachybotrys, for example, typically takes 7 to 12 days of sustained saturation to colonise a surface. Their presence on building materials is a strong indicator of prolonged water damage, active leaks, or ongoing moisture intrusion — not merely elevated humidity.

Stachybotrys chartarum (black mould) spores under microscopy

• Stachybotrys chartarum — The notorious "black mould". Requires continuously wet, cellulose-rich substrates (plasterboard, timber, paper) for weeks to establish. Produces Trichothecene mycotoxins that inhibit protein, DNA and RNA synthesis. Critically, Stachybotrys does not aerosolise easily — its spores are sticky and heavy, so it is not often detected in air samples. This is why surface sampling is recommended on all visible mould when Stachybotrys is suspected. Its presence always indicates serious, prolonged water damage
• Chaetomium — Another tertiary coloniser often found alongside Stachybotrys on severely water-damaged materials. Requires long-standing water. Produces mycotoxins (chaetoglobosins, chetomin) that are highly allergenic even at low doses and impair lung cilia function. A definitive indicator of major water intrusion

Chaetomium perithecia under microscopy

Acremonium — phialides and conidial clusters under microscopy (red arrows indicate diagnostic spore-bearing structures)

• Fusarium — Associated with very wet conditions. Can cause opportunistic infections (fusariosis) in immunocompromised individuals. Some species produce trichothecene mycotoxins
• Trichoderma — Requires high moisture. Produces "wet spores" that are heavy and not commonly found on air samples unless the material has dried out or been disturbed. Produces microbial volatile organic compounds (MVOCs) that contribute to the characteristic musty odour associated with water-damaged buildings
• Acremonium — A slow-growing tertiary coloniser often found in chronically wet areas such as showers and kitchens. Will not typically be airborne. Its presence indicates long-standing, unaddressed moisture problems

What this means for your report: Tertiary colonisers in your results indicate serious, sustained moisture conditions. These genera take days to weeks to establish — their presence means the water problem is not recent or minor. If Stachybotrys or Chaetomium are identified, the building has experienced significant water damage that requires both professional remediation and source repair as a priority.

Beyond identification — continuing the guide

Identifying which genus is growing is only the first answer your test report gives you. The full picture also requires understanding the metrics and condition ratings on the report itself, knowing the patterns we see in New Zealand buildings specifically, and knowing what to do (and what not to do) about a positive result. Those three threads are covered in detail in the companion articles below.