One of the biggest challenges in mushroom cultivation is mushroom contamination. No matter how careful you are, if you grow long enough, you’ll eventually encounter green mold, bacterial blotch, or other stubborn contaminants. Understanding how these molds and bacteria develop—and more importantly, how to prevent them—is crucial for successful cultivation.

This guide will cover the most common contaminants in mushroom growing, how to identify them, and practical ways to avoid and eliminate contamination before it spreads.

Trichoderma (Green Mold): The #1 Threat to Mushroom Growers

If you’ve ever seen green mold on your substrate or grain, you’ve likely encountered Trichoderma, also known as “trich.” This fast-spreading mold is one of the most common and devastating contaminants in mushroom cultivation.

How Does Trichoderma Develop?

• Improper sterilization or pasteurization – If your grain spawn or substrate isn’t heated long enough, trich spores can survive and thrive.
• High humidity with stagnant air – A lack of fresh air exchange (FAE) and standing water create an ideal breeding ground for mold.
• Environmental exposure – Trich spores are everywhere, especially in carpeted rooms or poorly ventilated spaces.

How to Prevent Green Mold

• Sterilize grain bags for 3–3.5 hours at 15 PSI to ensure the core temperature reaches sterilization levels.
• Pasteurize manure-based substrates for at least 2–3 hours at 150–180°F. If you’re using just coir and vermiculite, pasteurization isn’t as critical.
• Use grow bags with proper filtration – The Bag Tek method reduces contamination risks compared to open-air monotubs, which are prone to airborne spores.
• Increase fresh air exchange (FAE) and remove excess water to prevent mold growth.

What to Do if You Get Trichoderma

• Immediately remove and discard any contaminated substrate. Cutting out green mold does NOT work—it will return stronger.
• Disinfect the growing area with isopropyl alcohol or bleach to kill lingering spores.
• If using monotubs, avoid leaks between the lid and filter patches, as these can allow contaminants in.

Even experienced cultivators deal with Trichoderma—but by following strict sterilization and air exchange protocols, you can significantly reduce contamination risks.

Bacterial Contaminants: Identifying and Controlling “Wet Spot” and Bacterial Blotch

Bacillus spp. (“Wet Spot” or “Sour Rot”)

Bacillus spp., commonly referred to as “wet spot” or “sour rot,” is a milky, gray-colored bacteria that gives off a sweet, apple-like smell.

Signs of Bacillus Contamination

• A slimy, milky residue forms on the grain.
• The liquid on agar plates does not adhere to mycelium—it moves freely around the plate.
• A sour smell similar to fermenting fruit.

How to Prevent Bacillus Contamination

• Sterilize your agar for at least 45 minutes to kill bacteria before working with spores.
• Use a still-air box (SAB) or laminar flow hood to reduce airborne contaminants.
• Be cautious when handling syringes—accidentally touching the needle tip can introduce bacteria.

Pseudomonas (“Bacterial Blotch”)

Pseudomonas, known as bacterial blotch, is a common contaminant in button mushroom farms but can also affect Psilocybe cubensis.

Causes of Bacterial Blotch

• Overwatering – Water sitting on the surface of mushrooms causes soggy, discolored caps.
• Lack of airflow – High humidity and stagnant air encourage bacterial growth.

How to Prevent Bacterial Blotch

• NEVER spray water directly onto fruiting mushrooms, especially during the pinning stage.
• Mist the walls of the grow bag or tent instead of spraying directly on the mushrooms.
• Maintain proper air circulation to prevent water from sitting on mushroom caps.

Is Bacterial Blotch Safe to Consume?

Yes—if caught early, mushrooms affected by bacterial blotch can still be consumed. However, they should be harvested promptly, handled with gloves, and disinfected with alcohol spray before consumption.

Yellow Mold: Another Common Contaminant

Yellow mold can develop on petri dishes, grain bags, and grow substrates, often due to contaminated spore syringes.

How to Avoid Yellow Mold Contamination

• Test multi-spore syringes on agar plates first to confirm they are clean before inoculating grain.
• Store syringes properly—poor storage conditions can lead to contamination.
• Use isolated spore syringes instead of multi-spore syringes for faster colonization and reduced contamination risks.

Testing spores before inoculation eliminates guessing games—you’ll know whether contamination came from your syringe or your grain bag.

Best Practices to Prevent Contamination

Preventing contamination isn’t about being lucky—it’s about controlling your environment and refining your technique.

Key Takeaways for a Contaminant-Free Grow

• Sterilize agar plates for at least 45 minutes and liquid culture for the same duration.
• Always disinfect your injection ports with alcohol before inoculation.
• Avoid growing in windy or dusty environments—a still-air box is the best budget-friendly option.
• Use extra-large filtered grow bags instead of monotubs for better air control and contamination prevention.
• Harvest mushrooms correctly—twist and pull the entire fruiting body to avoid leaving behind aborts, which can rot and invite mold.
• Never leave standing water in your grow bags or monotubs—excess moisture fuels mold and bacteria growth.
• Consider adding calcium carbonate or oyster shell to your substrate to buffer pH and slow down acidic metabolite buildup.

Final Thoughts: Mastering Contamination Control

Contamination is inevitable, but understanding how and why it happens is the key to successful mushroom cultivation.

Whether you’re battling Trichoderma, bacterial blotch, or yellow mold, following proper sterilization, pasteurization, and air exchange protocols will significantly improve your success rate.

Every grower—from beginners to experts—faces contamination at some point. The difference between failure and success is how well you prevent it and how quickly you respond when it appears.

By refining your technique and controlling your environment, you’ll turn contamination from a frustrating setback into a rare occurrence.

Author: The Spore Guy