Guide
Oyster mushroom contamination : causes, identification, fixes
A green patch, a gray fuzz, a suspicious smell — contamination never gives advance notice. Here's how to identify it quickly, its most common causes, and why logging the cause every single time changes the game over the long run.
| Appearance | Likely contaminant | Common cause |
|---|---|---|
| Green, powdery patches, musty smell | Trichoderma ("green mold") | Insufficient pasteurization, or a torn bag letting in unfiltered air |
| Fine gray-white fuzz that spreads fast, engulfs the mycelium | Cobweb mold (Cladobotryum) | Humidity too high combined with poor room ventilation |
| Black or dark gray patches, sometimes spotty | Various black molds | Handling hygiene — unsanitized hands, tools or surfaces |
| Slimy substrate, sour or ammonia-like smell, no white mycelium | Bacterial contamination ("wet spot") | Substrate too wet from the start, excess soak water not drained properly |
| Several blocks affected at once, in the same area | Cross-contamination | An already-contaminated block left too long in contact with others |
General visual identification pointers, not a lab diagnosis — if in serious doubt (very strong smell, health concern), it's safer to discard the block than to try confirming the exact species.
A contaminated block: what to do
If the spot is small and recent, some growers cut it out (substrate plus a safety margin around it) and let the block continue its cycle. If contamination is widespread, the smell clearly foul, or neighboring blocks are starting to show signs too, the priority is pulling the block from the room quickly — preventing spread to healthy blocks matters more than trying to save the one already lost.
Logging the date and likely cause the moment you pull the block takes ten seconds and saves a lot of guessing three weeks later.
Reducing the most common causes
- Insufficient pasteurization — check the temperature actually reached at the core of the substrate, not just the water bath, and the hold time.
- Substrate too wet — drain thoroughly after soaking; substrate that still releases water under hand pressure is still too saturated.
- Handling hygiene — sanitize hands, surfaces and tools before inoculation, use a dedicated work area if possible.
- Cross-contamination — isolate and remove an affected block right away, never leave it "to see" in contact with others.
Why tracking history beats reacting case by case
One isolated contamination doesn't teach you much. What matters is the pattern over time: if "substrate too wet" comes up three times in a month, that's not bad luck, that's your soaking method. With few batches per month, that recurrence is hard to feel without a dated log — gut feel alone rarely catches "this is the third time this month."
That's exactly what MycoFlow does: every contamination is dated and tagged with a cause (same categories as this guide), contamination rate is tracked over time, and the period's climate readings are shown alongside — so you can see whether a drop in contaminations actually lines up with a change in practice, not just hope it does.
Note: MycoFlow's interface is French-only today. This page and the waitlist below are how we're gauging interest in an English version — see the FAQ on the main page for details.
Production tracking by flush, substrate needs calculation, stock and traceability — one tool, built with a real farm.
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