Mushroom Contamination Identification — The Non-Green, Non-Cobweb Catalog

Contents

• What non-green mushroom contamination looks like
• Identifying contamination by smell
• Bacterial wet rot and bacterial blotch
• Pin mold: Mucor and Rhizopus
• Black mold: Aspergillus niger
• Yeast and sour rot on grain
• Scrap or save each contaminant
• How to prevent contamination
• Related problems

Something has gone wrong on your grow, and it isn’t the usual suspect. It’s not the bright forest-green of Trichoderma, not the grey wispy strands of cobweb, and there are no bugs crawling around. Instead you’re looking at something brown, black, cream-colored, or slimy, or you’ve caught a smell that stops you before you even open the bag.

This page is the catalog for exactly that. If your problem is green, start with our green mold guide. If it’s grey and cobweb-like, go to cobweb mold vs. mycelium. If it’s a pest, see mushroom mites. What’s left, bacterial wet rot, pin mold, black mold, yeast, and sour rot, is what we cover here.

What non-green mushroom contamination looks like#

These contaminants sort out fastest by two questions: where in the grow cycle did it show up, and what does it actually look and feel like. A slimy grain jar during the spawn run is a different problem from brown spots on caps at harvest, even when both are bacterial.

Contaminant	When it usually appears	What sets it apart	Commonly confused with
Bacterial wet rot	Grain spawn run, bulk colonization	Slimy, wet grain that’s breaking down structurally	Yeast in its early stages
Bacterial blotch	Fruiting	Superficial brown or yellow lesions on caps	Bruising, natural cap darkening
Pin mold	Bulk colonization	Tiny black “pin-head” sporangia on tall stalks	Cobweb mold, which has no pin-heads
Aspergillus niger	Colonization, fruiting	Jet-black, powdery spore heads	Other dark molds like Cladosporium
Yeast	Grain spawn run	Cream or pink slick, wet-looking, no fuzzy growth	Bacterial wet rot

The single most useful habit for this whole category is to smell before you look closely. Several of these announce themselves by odor well before the visual signs are definitive, which is why the smell test comes first.

Identifying contamination by smell#

A healthy grow smells faintly of fresh mushroom or damp earth. Anything sharp, sour, sweet-rotten, or boozy is a warning, and the specific smell narrows down the cause before you confirm it with what you see.

• Sour or vinegar points to lactic-acid bacteria and sour rot. The fermentation that lowers the substrate’s pH gives off that sharp acidic note.
• Sweet-rotten or putrid is the signature of severe bacterial wet rot, usually an advanced Bacillus infection or anaerobic conditions in substrate that’s far too wet.
• Fermented or yeasty, like rising bread dough or brewing beer, is a reliable tell for yeast contamination on grain.
• Ammonia usually means the substrate was over-supplemented or is breaking down high-nitrogen material, rather than a specific organism.
• Musty is the smell growers most often associate with Aspergillus and a composting or sterilization problem.

If you get a clear read from smell alone, you can often make the scrap-or-save call before the contaminant has fully shown itself.

Bacterial wet rot and bacterial blotch#

Bacteria cause two distinct problems that look nothing alike, because they strike at different stages.

Bacterial blotch shows up on fruiting bodies. The culprit is Pseudomonas tolaasii (named P. fluorescens biotype G in older literature), the most significant bacterial pathogen in mushroom growing. It starts as pale yellow lesions on the caps that darken to a golden yellow and then a rich chocolate brown. The discoloration is superficial, no more than 2 to 3 millimeters deep, and the caps turn slimy or sticky to the touch.

Blotch needs water to get going. The pathogen requires a film of water sitting on the cap for roughly four to six hours to establish an infection, so it follows prolonged wetness, high humidity without airflow, and condensation from temperature swings. That’s also how you tell it from bruising: blotch is yellow-to-brown and sticky and appears wherever water lingers, while bruising is bluish or dark and shows up only where the mushroom was physically handled.

Bacterial wet rot is the substrate-level version, and it hits grain or bulk during the spawn run. This is the “wet spot” or “sour rot” caused by Bacillus subtilis and its relatives: the grain turns slimy and wet-looking, breaks down, and throws off a foul, sour, or putrid smell. What makes Bacillus stubborn is that it forms heat-resistant endospores that survive inadequate sterilization and germinate once the grain sits warm and wet. A 2025 study in the Journal of Mycopathological Research on spawn-contaminating microbes identifies it alongside other spoilers, including Enterobacteriaceae and Pseudomonas fluorescens, that inhibit Pleurotus (oyster) mycelium outright.

One more bacterial problem worth naming so you don’t misread it: Ewingella americana, a less common enteric pathogen, causes internal stipe necrosis, a browning and collapse of tissue inside the stipe that’s visible only when you cut a mushroom open at harvest. It isn’t the surface cap blotch of P. tolaasii, and the two shouldn’t be lumped together.

Pin mold: Mucor and Rhizopus#

Pin molds are the fastest-growing contaminants you’ll meet, and they belong to the genera Mucor and Rhizopus (order Mucorales, phylum Mucoromycota, the group formerly lumped into the now-abandoned Zygomycota). They start as a dense, white, cottony mycelium that races across the substrate, then develop their tell: tiny black or dark-grey sporangia perched like pin-heads on the tips of tall stalks.

Those pin-heads are the whole ballgame for telling pin mold from cobweb mold. Stamets’ The Mushroom Cultivator describes pin mold’s sporangial heads as the feature that separates it from the purely fibrous, wispy growth of cobweb, which never forms them. If you see distinct black dots on stalks, it’s pin mold; if the growth is just grey-white fuzz with no heads, work through cobweb mold vs. mycelium instead.

Under magnification the two pin-mold genera separate too: Rhizopus anchors itself with stolons (runners) and rhizoids (root-like structures), while Mucor lacks those and has simpler sporangiophores. For deciding what to do, the genus barely matters, because both are opportunists that thrive on the same thing: excess free moisture. They show up on over-hydrated bulk substrate, on high-sugar grain, and wherever water pools at the bottom of a bag or bin, especially in stagnant air.

Black mold: Aspergillus niger#

Aspergillus niger is the one that reads as alarming, jet-black and powdery, but it’s distinct from the green-presenting Aspergillus species. It begins as a white to yellowish mat, then gets rapidly buried under a dense layer of black conidial heads with a dry, powdery texture. If the mold on your grow is olive or yellow-green rather than true black, it’s more likely a green-presenting species, and our green mold guide covers those.

Black mold is best read as a symptom of a composting or sterilization failure. Affected substrate smells musty and stops supporting vigorous mushroom mycelium, and warm conditions plus dusty air help A. niger’s prolific spores spread. One safety note worth respecting: at high spore densities Aspergillus can pose a respiratory risk, so seal contaminated material before it leaves the room and ventilate rather than disturbing a dusty colony indoors.

Yeast and sour rot on grain#

Yeast contamination has a look all its own. Unlike the filamentous molds, yeasts produce no aerial fuzz at all. Instead they form cream-colored, pinkish, or off-white slick, slimy patches on the surface of grain spawn, and the common genera include Candida and Rhodotorula, the pink or red one you’ll occasionally see. That wet, glossy sheen can read as early bacterial wet rot, but yeast lacks the extreme sliminess and structural breakdown of a bad Bacillus infection, and the fermented, bread-dough smell settles it.

Yeast exploits the sugars in grain. It follows over-supplemented grain, incomplete sterilization, or incubating spawn too warm and too wet. Because it coats the grain and competes directly for the mushroom’s primary food source, it’s a straightforward sterility-chain failure when it appears.

Sour rot is the bacterial cousin of the same wet, anaerobic conditions. Lactic-acid bacteria ferment available carbohydrates into lactic acid, dropping the substrate’s pH into a range mushroom mycelium can’t tolerate and giving off that sharp sour or vinegar smell. It sets in when substrate is too wet, packed too densely, or sealed too tightly before the mycelium can establish, and when substrate wasn’t cooled adequately after pasteurizing or sterilizing.

Scrap or save each contaminant#

The honest answer for most of this category is scrap, but the reasoning differs, and a couple of cases genuinely leave room to recover.

• Bacterial wet rot (Bacillus) — scrap. Bacteria spread through the moisture film in the substrate, and the structural breakdown and pH shift they cause make the material unrecoverable. Don’t try to nurse it back.
• Sour rot (lactic-acid bacteria) — always scrap. Anaerobic, acidified substrate cannot be brought back into a range mushroom mycelium will colonize.
• Bacterial blotch — often recoverable. Localized blotch on a few caps doesn’t mean the whole substrate is lost. Harvest and discard the affected mushrooms, then immediately correct humidity and airflow so the remaining crop dries and the spread stops.
• Pin mold — depends on extent and timing. Extensive pin mold usually means disposal. But a small, surface-only patch appearing very early in the spawn run can sometimes be recovered if you fix the moisture problem, increase airflow to dry the surface, and the mushroom mycelium is already vigorous.
• Aspergillus niger — scrap, and manage the dust. Treat it as a loss and prioritize not aerosolizing the spores while you clean up.
• Yeast — scrap the grain. Once yeast coats the grain, it’s competing for the same nutrients your mycelium needs, and the jar or bag should go.

Whatever the contaminant, dispose of it the same way. Seal the material in a bag before it leaves the grow area, so you’re not spreading spores or bacteria on the way to the bin. Then sanitize surfaces and any tools you used with a 10% bleach solution or a suitable commercial disinfectant, and give the space some downtime before the next run.

How to prevent contamination#

Prevention is specific to each contaminant, but almost all of it comes back to two disciplines: keep the sterility chain intact, and get the substrate’s moisture right.

• Bacterial wet rot: hold your sterilization times and temperatures so Bacillus endospores are actually destroyed, and don’t over-hydrate grain. Grain is pressure-sterilized at 250°F (121°C) with time scaled to jar size; our pressure cooker guide covers the sizing, and working inside a still air box closes the hygiene gaps at inoculation where bacteria get in.
• Bacterial blotch: manage the fruiting environment so caps dry quickly after watering. Keep air moving, control humidity so condensation doesn’t settle, and avoid leaving a water film on the mushrooms.
• Pin mold: hydration discipline is the single most important intervention. Prepare substrate to field capacity with no pooling water. Our substrate calculator and the substrate guide help you keep the water ratios in range.
• Aspergillus niger: sound composting and sterilization, plus dust exclusion in the grow area to keep spore loads down.
• Yeast: proper grain prep, restraint with supplementation, and complete sterilization. Yeast is less heat-resistant than bacterial endospores, so a sterilization cycle that reliably kills Bacillus will handle yeast too. Clean spawn from suppliers like North Spore or Out-Grow closes another common entry point.
• Sour rot: cool pasteurized or sterilized substrate fully before you inoculate, and don’t compact or seal it airtight before the mycelium has a chance to establish, since that’s what creates the anaerobic pockets lactic-acid bacteria exploit.

The same playbook holds whether you’re growing oyster mushrooms, which are especially prone to bacterial spawn problems, blue oyster, or anything else: clean work, clean spawn, and substrate mixed to the right moisture.

Related problems#

• Green mold on mushrooms — Trichoderma, Penicillium, and the green-presenting Aspergillus species this page routes around.
• Cobweb mold vs. mycelium — the grey, fibrous contaminant that pin mold is most often mistaken for.
• Mushroom mites identification — when the problem is a pest rather than a microbe.
• Common mushroom growing problems — the troubleshooting hub for contamination, pinning, and environment issues.