Identifying mold in a petri dish comes down to a handful of visible traits: colony color, texture, growth speed, and the shape of the colony’s edges. While lab professionals use microscopy and DNA sequencing for definitive identification, you can narrow most household mold cultures to a likely genus just by observing what grows on your agar plate over five to seven days.
Key Traits to Observe on Your Plate
Before comparing your colonies to any reference, you need to know what to look for. Every mold colony has a set of macroscopic features you can assess with the naked eye or a magnifying glass.
- Top color: The pigment visible on the surface of the colony. This can change as the colony matures, so note color at day three, day five, and day seven.
- Reverse color: Flip the plate over and look at the underside of the colony through the agar. Some molds produce pigments that diffuse into the medium, giving a different color on the reverse.
- Texture: Colonies can be powdery, velvety, cottony (fluffy), or granular. Powdery surfaces usually mean heavy spore production. Cottony growth indicates abundant aerial filaments.
- Growth rate: Measure colony diameter at consistent intervals. Fast growers cover a standard petri dish in under a week. Slow growers may reach only one to two centimeters in the same period.
- Margin shape: The outer edge of the colony may be smooth and circular, irregular and feathery, or lobbed with finger-like projections.
Common Mold Colonies and What They Look Like
Penicillium
Penicillium colonies typically appear blue-green to green on the surface with a white outer margin that advances as the colony grows. The texture is usually velvety or powdery, and colonies grow at a moderate pace. The reverse side is often pale yellow or yellowish-brown. Penicillium is one of the most common indoor molds, frequently found on food, wallpaper, and damp building materials. On agar, its green pigmentation is distinctive and tends to develop within three to four days.
Aspergillus
Aspergillus is a large genus with many species, so colony color varies widely, from green to yellow-green to black depending on the species. The texture ranges from powdery to granular. Colonies often have a defined circular shape with a relatively even margin. On tryptone soya agar, Aspergillus species display green-to-black pigmentation. One distinguishing feature: many Aspergillus species grow quickly at room temperature and at slightly warmer temperatures (around 37°C), which sets them apart from Penicillium, which prefers cooler conditions.
Cladosporium
Cladosporium produces colonies that are dark olive-green to black on the surface with a black reverse. The texture is velvety to powdery, and colonies tend to grow slowly compared to Aspergillus or Penicillium. The dark pigmentation can make it easy to confuse with other dark molds, but Cladosporium colonies often have a slightly raised, dome-like center. This is the most common outdoor airborne mold and frequently shows up on plates exposed near windows or HVAC vents.
Stachybotrys (Black Mold)
Stachybotrys chartarum starts as a white cottony colony that turns blackish-green as it matures. The texture shifts from cottony to powdery once spore production ramps up. Colonies grow rapidly under the right conditions, reaching 2.5 to 3 centimeters in diameter within about four days on malt extract agar at 25°C. A brown pigment may diffuse into the surrounding agar, giving the reverse side a dark appearance. This mold requires high moisture to grow, so finding it on your plate likely means the sample came from a water-damaged area.
Fusarium
Fusarium colonies are characteristically fluffy and white, sometimes developing pink, purple, or salmon-colored pigments as they mature. Growth is typically fast, and the cotton-like aerial growth can be dramatic, filling the plate with loose, airy filaments. The reverse side often turns red or burgundy in older colonies. Fusarium is commonly associated with plant material and soil but can appear in damp indoor environments.
Alternaria
Alternaria forms flat, dark gray to olive-brown colonies with a velvety to woolly texture. Growth rate is moderate. The colony margins are often irregular and feathery rather than smooth. The reverse is typically dark brown to black. Alternaria is a very common outdoor mold and a frequent allergen, so it often appears on plates exposed in kitchens, bathrooms, or near entryways.
Quick Comparison by Color
- Green, powdery or velvety: Penicillium or Aspergillus. Penicillium tends toward blue-green; Aspergillus leans yellow-green, though overlap is significant.
- Black or very dark green: Cladosporium, Stachybotrys, or a dark Aspergillus species. Cladosporium is velvety and slow-growing. Stachybotrys starts white and darkens over days.
- White and fluffy: Fusarium, Mucor, or an immature colony of nearly any species. Wait a few more days to see if pigment develops.
- Gray-brown, flat: Alternaria is the most likely candidate, especially if margins are irregular.
- Pink or salmon tones: Fusarium or sometimes Trichoderma. Both produce distinctive non-green pigments that stand out on a plate.
Choosing the Right Agar
The medium you use affects what grows and how colonies look. The two most common options for home mold testing are Sabouraud Dextrose Agar (SDA) and Malt Extract Agar (MEA).
SDA uses glucose as its energy source along with a peptone supplement, making it broadly supportive of fungal growth. It was originally formulated to grow skin-related fungi but works well for most environmental molds. MEA relies on malt extract and maltose, which some environmental molds utilize less efficiently. For general household mold sampling, SDA tends to support a wider range of species. However, MEA can produce more characteristic pigmentation in certain genera, which is useful for visual identification. If you can only pick one, SDA is the more versatile choice.
Why Visual ID Has Limits
Colony appearance gets you to a genus-level guess, but it rarely confirms a species. Many molds look nearly identical on agar, and the same species can appear different depending on temperature, humidity, light exposure, and the type of agar used. A Penicillium colony on MEA may look distinctly different from the same Penicillium on SDA. Definitive identification requires examining spore structures under a microscope or sending a sample for molecular analysis.
Color can also be misleading in mixed cultures. If your plate has multiple mold types growing close together, colonies may overlap or one fast-growing species may suppress another. Isolating individual colonies by transferring a small piece to a fresh plate gives cleaner results for comparison.
What to Do After Identifying Mold
If you cultured mold from your home environment, the specific genus matters less than you might think for deciding next steps. Regardless of the type or amount, mold should not be growing indoors. The priority is finding and fixing the moisture source, whether that’s a leak, condensation, or groundwater intrusion. Mold is always a symptom of a water problem, and no amount of cleanup will matter if the moisture remains.
For small areas (generally under about 10 square feet), you can handle cleanup yourself following the EPA’s guide on mold remediation in homes. For larger problems or mold in HVAC systems, walls, or other hard-to-reach areas, hiring a qualified mold remediation contractor is the more practical path. The presence of Stachybotrys or heavy Aspergillus growth in your samples is worth taking seriously, as both are associated with respiratory problems in prolonged exposures, but the response is the same: control moisture, remove the mold, and fix what caused it.