Cobweb Mold growing on mushrooms close up

Cobweb Mold on Mushrooms: Identification and Prevention Techniques

Cobweb Mold (Hypomyces rosellus) is one of the most common fungal pathogens that poses a significant threat to mushroom cultivation. It thrives in still air which makes it a greater threat to grain jars, substrate bags, and monotubs. It grows quickly. And when I say quickly, I mean it can go from the size of a dime to complete coverage in 48 hours. It’s important to take immediate action with early signs of contamination of cobweb mold on mushrooms.

Learning how to identify and even more importantly, preventing cobweb mold is key to a successful mushroom harvest. Here’s a high-level overview of Cobweb mold on mushrooms.

Cobweb Mold on Mushrooms Identification:

Cobweb mold typically manifests as a dense, cottony white or grayish mat, resembling spider webs ( hence the name).


It rapidly spreads across mushrooms if left unattended. Keep a close watch on your crops for any signs of mold growth and be prepared to take action immediately.


It’s easy to mistake your mycelium for cobweb mold. Healthy mycelium will be bright white and grow slower whereas the cobweb mold will be greyish-dull within a day and grows very quickly. 

All varieties of fungi in the Hypomyces group are parasites to other fungi. When left untreated the cobweb mold will cause your mushrooms to cease any healthy growth. And if cobweb mold appears on mature mushrooms they can quickly rot destroying an entire crop.


Cobweb Mold thrives in warm, stagnant, and humid conditions, competing with the desired mushroom mycelium for nutrients and space. Cobweb mold is known far too well in the mushroom community for its fast growth, aggressive nature, and ability to produce millions of spores. Fun stuff, right?


Cobweb Mold Prevention Techniques for Mushroom Cultivators

Environmental Conditions:


Cobweb mold thrives in warm temperatures as mentioned. Ensure your growing area remains within the optimal range for the target mushroom species you’re cultivating. Temperature requirements vary depending on the species. Keep a thermostat to watch your temperature closely and adjust as needed to stay in the target range for your mushrooms.

Humidity + Water:

Proper humidity control is crucial to prevent excessive moisture accumulation in which cobweb mold thrives. This is a delicate dance, and one all of us mycologists obsess over. If you notice signs of mold, consider lowering your humidity levels ASAP and reassessing your misting schedule.


Fresh Air Exchange (FAE):

Provide adequate airflow and ventilation to prevent cobweb mold on mushrooms. Stagnant air flow is the number one contributor to cobweb mold. Be sure your growing environment has good air circulation with proper filtration in place to prevent any introduction of contaminants and mold spores. When working on grain-to-grain transfers or inspecting your mushroom bags, using a laminar flow hood keeps your workspace contam free so you can focus on your research. Check out our shop featuring our lab-quality FloCube laminar flow hoods and clean booths with HEPA filtration.  

Sterilization and Sanitization to Prevent Cobweb Mold:

Substrate Preparation:

Always follow proper sterilization techniques for mushroom substrates. Pasteurization or autoclaving is a critical step to eliminate potential contaminants, including cobweb mold spores. Read our tips on sterilization here. 


Equipment and Work Area:

Regularly clean and disinfect all cultivation tools, trays, containers, and surfaces to minimize the risk of introducing cobweb mold or other contaminants to the environment. 

Personal Hygiene:

Maintain good personal hygiene standards when handling anything in your mushroom grow. Wash your hands thoroughly, wear disposable gloves, and avoid cross-contamination between different cultivation areas. Change your clothes and shoes before entering your grow room so you don’t carry in any pathogens from your quick trip to the store.

Tips on what to do if cobweb mold is growing on your crop

Regularly inspecting mushroom substrates, and fruiting bodies for the first signs of cobweb mold is your best bet. At the very first indication, immediately remove infected materials, increase airflow, adjust environmental conditions, and consider applying appropriate fungicides if approved for mushroom cultivation. 

Unfortunately, if the cobweb mold has already covered your substrate, the chances of saving your mushrooms are slim. Early prevention is crucial.


Isolation and Quarantine:

Separate and isolate any contaminated mushroom substrate or fruiting bodies immediately! This is essential to prevent the spread of cobweb mold. Isolate infected materials far away from healthy cultures. You may need to go ahead and dispose of them properly so you don’t risk mold jumping throughout your crop. 

Environmental Adjustment:

Adjust the environmental conditions to make your environment less conducive to cobweb mold growth. This may involve lowering the humidity levels and adjusting the room temperature. Cobweb mold tends to flourish in warm and humid conditions with poor air circulation. The first step – check the thermostat and hygrometer.

Fungicidal Treatment:

ONLY if necessary

Depending on the severity of the cobweb mold infestation and the specific mushroom cultivation practices, you can use approved fungicides. Be careful – not all fungicides are suitable for mushroom cultivation! Be sure to choose an appropriate fungicide that is safe for use on mushrooms and follow the instructions carefully.


Some growers use a Hydrogen Peroxide mixture. Recommended use of 97% distilled water to 3% hydrogen peroxide mixture. Example: 100mL bottle, containing 97mL water and 3mL peroxide. You then soak a sterile cloth and pat the infected areas if caught in its early stages. 

Increased Airflow:

Improve ventilation and air circulation in the growing area ASAP. Cobweb mold thrives in stagnant air as mentioned. Increasing airflow helps create an environment less favorable for its growth. You can manually fan your grow chamber or monotub several times a day if already in the fruiting stage.

Cobweb mold is a common problem that severely impacts mushroom cultivation and diminishes crop quality in a short amount of time. As with all mushroom cultivation, cleanliness, and precise growing conditions are best to combat cobweb mold. In order to maintain healthy and productive mushroom crops, regularly monitor your growing environment, and follow strict sterilization and hygiene practices. 

Cheers to a happy healthy harvest and keeping your mushrooms contam-free!


Cobweb Mold growing on mushrooms close up
Propane burner and canner for mushroom cultivation

We have affordable lab-quality mushroom cultivation equipment including; sterilizers, canners, and outdoor propane combos for sterilizing all your tools, grains, and spawns. Shop for mushroom cultivation equipment including flow hoods, HEPA filters, stainless steal tables and more.


FloCube products let you focus on your research.

Fast Safe shipping

USA & Canada

2 Year Warranty

Low Maintenance

International Warranty

Offered in USA, Canada

100% Secure Checkout

Visa / Mastercard / American Express / Google Pay / Apple Pay / Affirm / Crypto