Mold or mycelium? How to identify mushroom contamination at every stage

Mushroom contamination identification by eye relies on four clues: the stage of the grow, the growth's color and texture, its smell, and how fast it spreads.

Our main sources are Penn State Extension and the American Mushroom Institute (AMI), which write for commercial button-mushroom farms, plus the hobby suppliers GroCycle and North Spore. We checked them on 21 October 2026. Our photos were identified by eye, not in a lab.

Which contaminant is it? A quick ID table

The most common mushroom contaminants, per GroCycle and North Spore, are Trichoderma (green mold), cobweb, bacteria, and other molds such as black pin mold, Penicillium, Aspergillus and orange bread mold. A dash means none of our sources describes it.

The descriptions come from Penn State Extension (green mold, bacterial blotch), GroCycle, AMI and North Spore — affiliate link: we may earn a commission at no extra cost to you. GroCycle says Aspergillus can be hard to tell from other molds, so we toss that whole row. The CDC says people with weakened immune systems or lung diseases are most at risk from Aspergillus (CDC).

Is it mold or mycelium? Start with the 24-hour check

Healthy mycelium is typically white to off-white, while most molds change color as they make spores. Some species color up on their own: shiitake blocks brown, or "bark", before they fruit (Field & Forest), and pink oyster can be pale pink.

So isolate a worrying jar or bag, photograph it, and look again in 24 hours, then daily. A spot that changes color or outgrows the mycelium around it is likely mold; no change isn't an all-clear. If spores already show color, skip the wait and bag it where it sits.

That wait is our routine, not a sourced rule. Green mold and black pin mold start white (GroCycle); AMI says green mold may start grey (AMI).

North Spore says mushroom mycelium looks "more rope-like" and stays tight on the substrate, while Trichoderma rises from it. For a blue or dark stain, gently wipe a Q-tip over it: "contamination will transfer to the swab while bruising will not" (North Spore — affiliate link: we may earn a commission at no extra cost to you).

What looks like contamination but isn't?

Bruising, exudate, fluffy mycelium and pins often worry growers, but they're usually normal.

Look-alike	What it looks like	Why it isn't mold
Bruising	Blue stain where mycelium presses on the bag	May be natural on some mycelium, says North Spore; try a Q-tip wipe
Exudate	Amber or yellow droplets	Natural, but may mean it's fighting a rival (North Spore)
Fluffy aerial mycelium	Raised white fluff	Stays white; fluff that greys or fills the jar in a day or two suggests cobweb
Fuzzy feet	White fuzz where stems meet the block	Mycelium from stale, wet air (Martha tent guide); cobweb is often greyer and spreads
Pins	Small knots	They're baby mushrooms; if they stop growing, see why pins stall
Leathery mat (overlay)	A thick skin on oyster blocks	North Spore's kit FAQ says oyster mycelium can form one
Yellowing lion's mane	Yellowing spines	North Spore's kit FAQ says it yellows when it gets too wet or too dry; pick it before it yellows, and see our lion's mane problems guide

What does contamination look like at each stage?

Contamination shows as odd colonies on agar, wet or sour grain, fast patches in bulk, and fuzz or spots on fruit:

• Agar: colonies of a different color or texture.
• Grain spawn: wet, grey, slimy or sour kernels, or green or black patches. Our pink oyster grain spawn guide covers the smell test.
• Bulk while colonizing: patches that outrun the mycelium, or green breaking through white.
• Fruiting: cobweb over pins and caps, and blotch on caps. Pins that dry and stall are a pinning problem.
• After a soak: drain off standing water (North Spore help center), since caps that stay wet invite blotch. Our humidifier guide covers keeping mist clean.

Trichoderma vs mycelium: how do you spot green mold?

Trichoderma first grows as dense white mycelium that can pass for mushroom mycelium, then turns green as it makes spores. Penn State warns that "apparently normal spawn runs can give way to large patches of green Trichoderma sporulation".

Cobweb mold vs mycelium: how do you tell them apart?

Cobweb mold is a loose, cottony growth sitting on the substrate or mushrooms, often greyer than mushroom mycelium, which grows into the substrate.

Penn State describes "weft-cotton-like mycelium (usually white; sometimes gray, pink, or yellow)" (Penn State Extension). GroCycle calls it "light gray and not white". If grey wisps carry "tiny black pinheads", GroCycle says it's black pin mold (Rhizopus, or black bread mold) instead, so toss it unopened.

Penn State and AMI call cobweb mold Cladobotryum; GroCycle and North Spore call it Hypomyces rosellus, whose asexual stage is Cladobotryum dendroides (Journal of Fungi, via PMC).

Cobweb treatment is where the sources disagree: hobby suppliers spray 3% hydrogen peroxide on an early patch, and GroCycle says it "will not harm your mushroom mycelium". No extension page we checked says that. Spray only the patch; otherwise keep peroxide off blocks, pins and fruit (Martha tent guide). Farm guidance (Penn State, AMI) covers the patch with a damp paper towel, then salts it. Either way, switch the fans off first: AMI says cobweb spores are "very readily airborne" (AMI).

What does bacterial contamination look like?

Bacterial contamination shows up as wet, slimy, grey, sour-smelling patches in grain, and as yellow-to-brown spots on caps that stayed wet.

Wet-spot endospores can survive sterilization, so GroCycle and North Spore advise soaking grain 12 to 24 hours first: the soak wakes them, and sterilizing then kills them. Both suggest scooping out a small patch; we toss the jar.

Penn State's blotch spots turn from "pale yellow" to "golden yellow or rich chocolate brown" on caps that stay wet "4 to 6 hours or longer". To prevent it, GroCycle advises enough air exchange that water doesn't sit on caps. Heat adds to the risk (temperature guide).

Should you toss or salvage a contaminated jar, bag or block?

Toss any jar, bag or block showing green, black or orange spores (GroCycle, North Spore); we also toss sour ones, and any other mold from the table. We salvage only agar, one small, early cobweb patch, or a block with blotched caps, which we dry.

• Agar: move a clean wedge from the side farthest from the contamination to a fresh plate.
• Liquid culture: see our syringes and buyer's guides.
• Grain: out unopened, as our pink oyster guide explains.
• Blocks with sporulating mold: bag them where they sit, following our Martha tent guide.
• A treated cobweb patch: watch the block alone through one more flush. That's our practice, not a sourced rule.

How do you clean up contamination safely?

Clean up contamination with an N-95 respirator, gloves and eye protection, which the EPA calls "the minimum PPE" for disturbing mold (EPA).

Wipe shelves and tools with 3% drugstore hydrogen peroxide, never 35% "food grade". The CDC's toxic-substances agency warns that vapors from solutions over 10% "may result in severe pulmonary irritation" (ATSDR). Even 3% can irritate airways, ATSDR notes, so air the space once the moldy item is out.

The EPA adds that "it is necessary to clean up mold contamination, not just to kill the mold". After green mold, scrub the emptied tub, tent or room with detergent and water before disinfecting, per our Martha cleaning steps.

To keep the next batch clean, see our prevention guide.

My grow kit has spots. Is it contaminated?

White fuzz on a grow kit is usually mycelium: "chances are it's not mold — it's mycelium" (North Spore — affiliate link: we may earn a commission at no extra cost to you). Black, green or orange patches, or a foul smell, deserve a photo sent to the seller. No spots and no mushrooms? See what to do when a kit does nothing.

How do you track suspect blocks in Hypha?

Hypha doesn't identify contaminants, but its records help you tell one bad block from a bad culture.

1. Tag the block suspect in Hypha.
2. Photograph it now and in 24 hours, and compare them side by side in its photo journal. Set a daily repeating task from the Schedule tab.
3. Clear the tag if it stays clean; if not, end the block as Failed and add the contaminated flag.

The strain view then shows each strain's success and contamination rates, with spawn and bulk kept apart. The family tree links each block created as a transfer to its parent, so you can check whether failed blocks share one culture. All of this is on every plan, including Free.

Hypha's contaminated flag doesn't record the contaminant's type, and its bulk rate counts colonizing and fruiting losses together (grow log guide). A photo check for possible contamination is in development, and growers don't see its results yet. Download Hypha to start logging.

FAQ

Is cobweb mold dangerous to humans?

Cobweb mold's effect on people isn't covered by the Penn State and AMI pages we checked. Its spores spread easily (AMI), so wear the EPA's "minimum PPE" for mold: an N-95 respirator, gloves and eye protection (EPA).

Does cobweb mold smell?

Cobweb mold has "a noticeable, damp, musty smell", according to GroCycle. The extension and AMI pages describe no smell.

What is the yellow liquid in my grow bag?

Yellow liquid in a grow bag is usually exudate, which North Spore calls natural, though it can mean the mycelium is fighting a rival. Wet slime or a sour smell nearby means bacteria, whatever its color.

Can you eat mushrooms from a block with mold on it?

We don't eat mushrooms from a moldy block. GroCycle says "eating mushrooms grown near black bread mold can cause illnesses in some people" (GroCycle). USDA says to discard moldy soft produce, which "can be contaminated below the surface" (USDA FSIS); mushrooms are soft, so we treat every mold that way.