First rule: act on it early
By the time most contaminants are obvious, they're already releasing spores or bacteria. The single most expensive mistake is leaving a contaminated block "to see what happens" next to clean ones — one green-mold bag can seed an entire room. Inspect daily. When in doubt, seal the suspect block in a bag before moving it, take it well away from your clean area, and only then open it (if at all). Speed of removal matters more than a perfect ID.
What is growing on my block?
Answer what you can see from where you are standing. You do not need all four, but the more you give it the narrower the answer gets. Do not open a suspect bag to look closer — bag it first, carry it away from your clean blocks, and inspect it there.
This reads appearance, which is a strong hint and never a certainty — two organisms can look alike on a bad day and lighting lies. Where it is unsure it will say the more cautious of the two, because binning a block you could have saved costs one block and keeping one that is sporulating can cost the room.
Once you know what it was, the number that matters is how often it happens. Log the batch as contaminated in the free Grow Log and it becomes your real contamination rate by substrate, strain and method — and if the culture it came from is in the free Culture Library, flagging it shows every other jar and block that came off the same parent. Both are free, no account needed.
The usual suspects, at a glance
| What you see | Likely culprit | What it's telling you |
|---|---|---|
| Forest-green powder (was white & fuzzy first) | Trichoderma (green mold) | Spores got in + a foothold; often weak/old culture or slow colonization |
| Wispy grey "spider silk," spreads overnight | Cobweb (Dactylium) | Stagnant, too-humid air; poor fresh-air exchange |
| Black/blue-green fuzzy spots | Black / pin mold (Aspergillus, Rhizopus) | Airborne spores; under-sterilization or dirty air |
| Slimy grey patches, sour/rotten smell | Wet spot / sour rot (Bacillus) | Under-sterilized or substrate too wet |
| Sticky brown lesions on caps | Bacterial blotch (Pseudomonas) | Water sitting on mushrooms + poor air movement |
| Orange/pink slime on grain | Bacterial / yeast | Sterilization failure or contaminated culture |
Is it mold or just mycelium? Healthy mushroom mycelium is white, dense, and "rooted" — it grips the substrate and advances at a steady, even pace. Contaminants tend to be a different color (green, grey, black, pink), sit loosely on the surface, smell off, or spread visibly faster than the colony. A small fuzzy white tuft with tiny droplets can be normal aerial mycelium; a fuzzy patch that turns any color is not.
Why blocks get contaminated
Contamination is rarely bad luck — it's a process leak. Almost every case traces back to one of five things:
- Sterilization/pasteurization fell short — not hot enough, not long enough, or too big a load for the vessel. Supplemented substrate is especially demanding because the same nutrition that boosts yield also feeds mold.
- Dirty technique at inoculation — the open moment when spawn meets substrate is where most contaminants enter. Moving air, unwiped surfaces, and long exposure all raise the odds.
- Slow colonization — a thin spawn rate or a weak culture gives contaminants time to establish before your mycelium claims the substrate.
- Too much moisture — waterlogged substrate goes anaerobic and breeds bacteria (see the field-capacity guide).
- Stagnant, dirty fruiting air — high humidity with no fresh-air exchange invites cobweb and bacterial blotch once you're fruiting.
The prevention checklist
You can't sterilize the world, but you can stack the odds heavily in your favor. The growers who barely lose a block do these consistently:
- Sterilize/pasteurize correctly for the substrate — full temperature, full time, loads sized so heat actually penetrates the center.
- Work clean in still air — wipe down surfaces, 70% isopropyl on hands and tools, minimize the time substrate is open, and keep a still-air box or flow hood between you and moving air.
- Use a healthy, vigorous culture and a generous spawn rate. A 20–30% spawn rate colonizes fast and crowds out invaders — speed is a contamination defense, not just a yield lever.
- Hit field capacity, not soaking wet — squeeze test every batch.
- Give fruiting blocks fresh air and avoid water pooling on caps; match conditions to the species with the fruiting-conditions cheat-sheet.
- Separate clean from suspect. Colonize and fruit away from where you open and handle contaminated material; never let the two airflows mix.
- Inspect daily and cull early. The cheapest contaminated block is the one you remove before it sporulates.
Track it, and the pattern shows up
One contaminated block is noise; a contamination rate is signal. Growers who log every batch — substrate, spawn rate, who sterilized it and how, where it colonized — quickly see the real cause: the load size that's too big for the pot, the strain that comes in weak, the room that always grows cobweb. You can't fix a contamination problem you're only guessing at.
Mycro keeps score for you
Log each batch and its outcome and Mycro turns it into your real contamination rate by substrate, strain, and process — so you fix the actual leak instead of guessing. Stop losing blocks to the same mistake twice.
Try the free Grow Log → Become a Founding Grower — $99 →$99 once · opens a sign-in account today · lifetime founder pricing · a direct say in what we build first · fully refundable any time.
Frequently asked questions
What is the green mold on my mushroom substrate?
Bright forest-green powder on grain or substrate is almost always Trichoderma. It starts as a white fuzzy patch that is easy to mistake for mycelium, then turns green as it sporulates. By the time it is green it has already released spores, so bag and remove it immediately and do not open it near clean blocks.
What does cobweb mold look like?
Cobweb mold (Dactylium / Hypomyces) is a fast, wispy, grey-to-translucent growth that looks like spider silk and spreads across the surface visibly faster than mushroom mycelium — often overnight. Unlike the dense, rooted look of healthy mycelium, it sits loosely on top. It usually signals stagnant, overly humid air with poor fresh-air exchange.
What causes wet spot or bacterial blotch?
Wet spot (sour rot, usually Bacillus) shows as slimy, grey, foul-smelling patches in grain or substrate and comes from under-sterilization or substrate that is too wet. Bacterial blotch on caps is sticky brown lesions from water sitting on the mushrooms with poor air movement. Both are bacterial, not fungal, and the fix is cleaner process and less standing moisture.
How do I tell Trichoderma from healthy mycelium?
Judge it on texture rather than colour, because Trichoderma starts out white and is very easy to mistake for vigorous mycelium before it turns green. Trichoderma is powdery and stands up off the surface; healthy mycelium is ropey and lies tight against the substrate. The confirming test is a dry cotton bud: green spore dust lifts off onto the bud, while blue-grey bruising does not rub off and is harmless. By the time it is clearly green it has already released spores.
Can a block with cobweb mold be saved?
Sometimes, and it is the one common contaminant that is often recoverable. Caught early, while it is still grey and wispy and has not coloured up, cobweb can be halted by moving the block away from clean ones and sharply increasing fresh-air exchange. Once it has taken on colour it is sporulating and the block goes out with the rest. Cobweb is telling you about stagnant humid air rather than about your sterile technique.
How do I prevent mushroom contamination?
Prevention beats treatment: sterilize or pasteurize properly for the substrate, work clean (still air, wiped surfaces, alcohol, minimal exposure time), use a healthy high spawn rate so your culture colonizes before contaminants establish, hit field capacity instead of waterlogged moisture, and give fruiting blocks fresh air. Inspect daily and remove any contaminated block early before it spreads spores.
General guidance from common cultivation practice, not guarantees — identification can be uncertain and your results vary with process, substrate, and conditions. Mycro is for legal culinary & medicinal mushrooms only.