Contamination Identification Field Guide
Visual guide to identifying common contaminants. Learn to recognize molds, bacteria, and other problems, trace their sources, and take appropriate action.
Contamination Identification Field Guide
Contamination is inevitable in mushroom cultivation. The key is recognizing it early, understanding what you're dealing with, and taking appropriate action. This visual guide helps you identify common contaminants and their likely sources.
How to Use This Guide
When you spot something suspicious:
- Compare to the descriptions below
- Identify the likely contaminant
- Trace the probable source
- Decide whether to isolate, discard, or attempt rescue
If you can't identify a contaminant or aren't sure if something is contamination, treat it as contaminated. The cost of one lost jar is far less than spreading contamination to your entire operation.
Mold Contaminants
Trichoderma (Green Mold)
The most common and problematic contaminant in mushroom cultivation.
| Characteristic | Description |
|---|---|
| Color | White at first, then bright green |
| Texture | Powdery, dusty when sporulating |
| Speed | Extremely fast-spreading |
| Smell | None to slightly earthy |
Where it appears:
- Grain spawn
- Bulk substrates
- Agar plates
Common sources:
- Under-sterilized substrate
- Environmental contamination
- Contaminated source cultures
Once Trichoderma sporulates (turns green), the battle is lost. Immediately remove from your growing area to prevent spore spread. Do not open the container indoors.
Penicillium / Aspergillus (Blue-Green Molds)
| Characteristic | Description |
|---|---|
| Color | Blue, blue-green, or gray-green |
| Texture | Powdery, often in circular colonies |
| Speed | Moderate |
| Smell | Musty, earthy |
Common sources:
- Environmental spores (very common in homes)
- Poor sterile technique
- Contaminated air during transfers
Cobweb Mold (Hypomyces)
| Characteristic | Description |
|---|---|
| Color | Light gray to white |
| Texture | Wispy, web-like, very fluffy |
| Speed | Fast—can cover substrate overnight |
| Smell | None |
Distinguishing from mycelium: Cobweb mold is much fluffier and more diffuse than mushroom mycelium. It grows faster and doesn't have the organized, rhizomorphic patterns of healthy mycelium.
Treatment option: Hydrogen peroxide (3%) sprayed directly on cobweb can kill it without harming mushroom mycelium. Only works if caught very early.
Black Mold (Aspergillus niger)
| Characteristic | Description |
|---|---|
| Color | Black |
| Texture | Powdery when mature |
| Speed | Moderate |
| Smell | Earthy, musty |
Serious health concern: Black mold spores can cause respiratory issues. Handle contaminated containers outside and wear a mask.
Pink Mold (Neurospora)
| Characteristic | Description |
|---|---|
| Color | Salmon pink to orange |
| Texture | Fluffy, spreading rapidly |
| Speed | Extremely fast |
| Smell | Sweet, bread-like |
Also called "bread mold," Neurospora is notorious for spreading throughout grow spaces. If you see it, clean your entire area thoroughly.
Bacterial Contamination
Wet Spot / Sour Rot
| Characteristic | Description |
|---|---|
| Color | Darker, wet-looking grain |
| Texture | Slimy, clumped together |
| Speed | Spreads from point of origin |
| Smell | Sour, fermented, foul |
Common causes:
- Grain too wet before sterilization
- Insufficient sterilization time
- Bacterial contamination during inoculation
Bacterial contamination is almost always a moisture or sterilization issue. If you're seeing it regularly, review your grain preparation and PC times.
Bacterial Blotch (on Fruiting Bodies)
| Characteristic | Description |
|---|---|
| Color | Brown, dark, or slimy spots |
| Texture | Wet, sunken areas on caps |
| Cause | Excess moisture on developing mushrooms |
Prevention:
- Improve air circulation
- Reduce direct misting on mushrooms
- Maintain airflow in fruiting chamber
Contamination by Stage
Agar Plates
| Sign | Likely Cause |
|---|---|
| Colored growth from edge | Airborne spores during pour or transfer |
| Colored growth from center | Contaminated culture |
| Bacterial spread (slimy) | Unsterile technique or media |
Grain Spawn
| Sign | Likely Cause |
|---|---|
| Wet, sour smell | Grain too wet, under-sterilized |
| Green patches | Trichoderma from environment or culture |
| No growth + smell | Bacterial contamination |
Bulk Substrate
| Sign | Likely Cause |
|---|---|
| Green patches | Trichoderma—often from contaminated spawn |
| Overlay with no pins | Not contamination—adjust fruiting conditions |
| Bacterial smell | Substrate too wet, poor pasteurization |
Decision Framework
Isolate Immediately
Move suspicious containers away from healthy ones. Contamination spreads through airborne spores.
Assess Stage
Early (pre-sporulation) contamination may be treatable. Green, blue, or black colors usually mean it's too late.
Consider Value
A contaminated agar plate costs little. A contaminated grain jar costs more time. A contaminated bulk substrate costs the most.
Dispose Safely
Take contaminated materials outside before opening. Bag and dispose. Don't compost near your grow space.
Tracing the Source
When contamination strikes, investigate:
| Question | What It Tells You |
|---|---|
| Is it one item or many? | Isolated incident vs systemic issue |
| What stage did it appear? | Points to specific process step |
| Did these items share materials? | Common spawn lot, grain batch, etc. |
| When was your last transfer session? | Technique-related vs material issue |
Log contamination events with as much detail as possible. Patterns become visible over time and point to root causes.
For prevention strategies, see Mycology 102: Sterile Technique. For broader troubleshooting, see Mycology 105.
Quick Identification Reference
| Color | Likely Contaminant | Action |
|---|---|---|
| Bright green | Trichoderma | Discard immediately |
| Blue-green | Penicillium/Aspergillus | Discard |
| Light gray (fluffy) | Cobweb mold | H2O2 if early, else discard |
| Black | Aspergillus niger | Discard safely (mask) |
| Pink/orange | Neurospora | Discard, deep clean area |
| Yellow/brown (wet) | Bacterial | Discard |