Contamination: identify it, then prevent it

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.

The usual suspects, at a glance

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:

1. 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.
2. 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.
3. Slow colonization — a thin spawn rate or a weak culture gives contaminants time to establish before your mycelium claims the substrate.
4. Too much moisture — waterlogged substrate goes anaerobic and breeds bacteria (see the field-capacity guide).
5. 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.

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.