Every contaminated block is a message, not just a loss. Trichoderma is telling you something different than bacterial wet spot, which is telling you something different than cobweb — and each one points straight back at a specific upstream cause. Read the contaminant instead of just seeing "something's wrong," and prevention stops being a mystery.
The usual suspects
Four categories account for nearly everything that takes down a grow. Here's the fast-reference version, then the detail on each.
| Contaminant | What it looks like | What caused it | What to do |
|---|---|---|---|
| Trichoderma (green mold) | White at first, then flashes bright forest-green | Incomplete sterilization; contaminated spawn or air | Bag and remove immediately |
| Bacterial (wet spot / sour rot) | Slimy, wet, gray or amber; sour/rotten smell | Under-sterilized or over-hydrated grain | Toss; fix hydration & sterilization |
| Cobweb (Dactylium) | Wispy gray fuzz, faster-growing than mycelium | High humidity + stagnant air (low FAE) | Increase FAE; spot-treat w/ peroxide |
| Airborne molds (Aspergillus, Penicillium, Neurospora) | Black/yellow, blue-green, or aggressive orange | Dirty air; poor sterile technique | Remove; clean air & sanitation |
| Bacterial blotch (on fruit) | Brown blotches/pitting on caps, esp. oyster | Water sitting on caps + poor FAE | Stop misting caps directly; raise FAE |
Trichoderma — the #1 killer
This is the contaminant that takes down more grows than everything else combined. Trichoderma starts out white and indistinguishable from your own mycelium — that's what makes it dangerous. By the time it flashes bright forest-green as it sporulates, it's already spread through the block, and it spreads fast from there.
Cause: incomplete sterilization on a rich, supplemented substrate, or contaminated spawn or air brought in from an outside source. Action: bag it and remove it immediately — do not open it indoors, spores go everywhere the moment that bag is cracked. Then go fix the upstream problem: sterilization cycle and technique, not the block in front of you.
Bacterial (wet spot / sour rot)
Shows up in grain or substrate as something slimy, wet, and gray or amber, with a smell that's unmistakably sour or rotten once you've smelled it once. Grain that's gone bacterial won't colonize, or looks like it's "sweating" inside the bag.
Cause: under-sterilized grain, or grain that was hydrated too wet — excess free water sitting in the jar or bag instead of being absorbed. Action: toss it. Then hydrate your next batch of grain to field capacity, not soggy, and make sure the sterilization cycle is actually reaching a full kill, not a knock-back.
Cobweb (Dactylium / Cladobotryum)
Shows up on the fruiting surface as a wispy, fast-growing gray fuzz — it genuinely looks like a fine spiderweb, and the tell is that it's grayer and grows visibly faster than your mushroom mycelium.
Cause: high humidity paired with stagnant air — low fresh-air exchange (FAE) is what lets it establish and spread. Action: increase FAE, drop humidity slightly, and spot-treat the affected area with 3% hydrogen peroxide. Caught early, this is one of the few contaminants worth treating in place rather than tossing the whole thing.
Airborne molds
A grab bag of opportunists that land from dirty air: Aspergillus (black or yellow), Penicillium (blue-green), and Neurospora — an aggressively fast-growing orange mold that should be treated as a farm emergency the moment you see it, not a routine cleanup.
Cause: dirty air and poor sterile technique — too much open-air time, an unfiltered workspace, or tools and surfaces that weren't actually clean. Action: remove the contaminated item, then address the air itself — a flow hood or still-air box (SAB) and real sanitation discipline, not just a one-time wipe-down.
Bacterial blotch (on fruit)
Different from the contaminants above — this one shows up directly on developing mushrooms, especially oyster, as brown blotches or pitting on the caps.
Cause: water sitting on the caps combined with poor FAE — wet caps in stagnant air are exactly the conditions bacterial blotch needs. Action: stop misting directly onto developing fruit, and increase fresh-air exchange so caps dry between waterings instead of staying wet.
Prevention: the habits that matter
Every contaminant above traces back to one of five habits. Get these five right and you'll rarely need the ID chart at all.
Sterilize, don't pasteurize supplemented substrate
Pasteurization knocks back competitors on low-nutrition substrate like straw. Supplemented sawdust and grain can't take that shortcut — the same nitrogen-rich supplementation that makes a block productive is exactly what gives a surviving contaminant, especially Trichoderma, a windfall if the kill isn't complete. See the full build in the Bubba Barrel Sterilizer guide →.
Sterile technique — flow hood/SAB, 70% iso, minimize open time
Work in a flow hood or still-air box, wipe tools and surfaces down with 70% isopropyl alcohol, and treat every minute something sits open to room air as a chance for a contaminant to land. This is the single habit that shows up as the cause behind most airborne contamination. Full walkthrough in Agar Plate Preparation →.
Fresh-air exchange + right humidity in the fruiting room
Cobweb and bacterial blotch both trace back to the same root cause: stagnant air holding too much moisture against a surface. Enough FAE moving through the room, paired with humidity that's high but not saturating, closes off the conditions both contaminants need to establish.
Grain hydration to field capacity
Grain hydrated past field capacity holds free water the bacteria need to take over a jar or bag before your spawn gets a foothold. Hydrate to field capacity — moist throughout, no standing water — every batch, not just when a batch goes bad.
Sanitation and airflow discipline
Clean surfaces, filters that actually get changed, and air that's controlled rather than incidental. This is the habit that turns the first four from "things I did once" into "things that keep working" — discipline, not a one-time deep clean, is what prevents a repeat outbreak.
Toss or treat?
Not every contaminated block is a total loss — but most are, and knowing which is which saves you from either wasting good substrate or risking a spore bomb going off in your grow room.
When it's a lost cause
Trichoderma and bacterial contamination are both tosses, full stop. There's no salvaging a block once green mold has sporulated or bacteria have taken over grain — cutting around the visible spot doesn't remove what's already spread through the substrate, and the risk of releasing spores into a clean room outweighs whatever you'd recover.
When spot-treatment is worth it
Cobweb, caught early, is the exception. While it's still a light, wispy gray fuzz and hasn't overrun the fruiting surface, increasing FAE, dropping humidity slightly, and spot-treating with 3% hydrogen peroxide will often stop it in place. The window for this is narrow — the same fast growth that identifies cobweb also means it can go from "treatable" to "lost cause" in a couple of days, so act the day you spot it.
When the standard advice is wrong
A lot of contamination advice treats the symptom instead of the cause. Here's where that goes wrong.
Chasing symptoms, not causes
Spraying down a room after every outbreak, or buying a stronger cleaner, treats the symptom without touching the cause. If the same contaminant keeps coming back, the fix is almost never a new spray — it's the sterilization cycle, the technique, or the air handling that let it in the first time. Find that, and the outbreaks stop; skip it, and you'll be back here next month.
"Just cut out the green" spreads Trichoderma spores
This is the single most common mistake I see. Cutting the visibly green section out of a block and keeping the rest doesn't work — Trichoderma has already colonized substrate you can't see is contaminated yet, and the act of cutting into a sporulating patch releases spores into the air of your grow room, seeding your next batch. The whole block goes, sealed, and it leaves the room before it's opened.
Over-supplementation without matching sterility
Higher supplementation rates mean more productive blocks — they also mean a substrate that's a better meal for anything that gets past your sterilization. If you're pushing supplementation rates up, your sterilization cycle and sterile technique need to be tightening at the same time, not staying where they were for a leaner substrate.
The lesson that actually changed how I run contamination control wasn't about a spray or a new cleaning routine — it was learning to read the contaminant to find the cause. Every green flush of Trichoderma was pointing at the same sterilization cycle problem; every bout of cobweb was pointing at the same corner of the fruiting room with weak FAE. Once I stopped treating each outbreak as its own mystery and started asking "which contaminant is this, and what does it always mean," the repeat losses stopped being repeats. The contaminant is diagnostic information, not just a mess to clean up.