Identifying, Preventing, and managing Penicillium contamination
What is Penicillium?
Penicillium is a common mold that produces airborne spores found indoors and outdoors. The genus includes over 300 species, with beneficial and harmful effects.
In nature this mold plays an important role — it helps decompose organic matter and contributes to food spoilage.
Some species can damage crops, produce toxins, or trigger allergies, asthma, and skin infections. On the bright side, some species are harmless and incredibly useful. They're used to make antibiotics (like penicillin) and immunosuppressants (like mycophenolic acid). Others are key to cheese-making, giving flavor and texture to gourmet cheeses like Brie, Camembert, Gorgonzola and more.
But what happens when Penicillium shows up in a mushroom grow?
Even tiny amounts of spores can take over and mess with your mycelium. Thus, preventive measures and catching the first signs early are the best ways to save your grow and stop it from spreading.
How to identify Penicillium on mushroom mycelium
Many molds look similar, making them hard to tell apart. Penicillium, Trichoderma, and Aspergillus can all appear GREEN!
That's why just visual identification isn’t enough — a microscope is needed for accurate identification. However, by learning and observing specific growth patterns, you can get very close to an accurate diagnosis. Even a rough mold classification or distinguish mold from healthy mycelium is enough for your practical needs in mushroom cultivation.
Key Identification Features:
Microscopy. The name Penicillium comes from the Latin word for “painter’s brush,” referring to its unique microscopic structure. Its conidia (asexual spores) form brush-like chains. Penicillium sp. under 1000x phase-contrast microscopy. Photo By: bacteriagram
Color. It starts off bright white, similar to Trichoderma, but gradually turns green, blue-green, green-grayish or yellow.
Growth Pattern. Typically forms a circular or oval patch that spreads outward in a uniform ring.
Growth Speed. Penicillium takes about 2–3 days from white to green color.
In contrast, Trichoderma can turn green in as little as 8–12 hours, maximum within 24 hours.
Texture. Its surface looks lumpy or bumpy, often compared to curdled milk on early stages.
Mature mold has powdery texture.
Spreading. Penicillium prefers growing ON the surface of the mushroom cake or block where it gets better oxygen. It may grow slightly deeper, but usually stays near the top instead of spreading deep inside the substrate.
Compared to Trichoderma – Trichoderma tends to grow deeper and can even emerge from the bottom of the cake or block, unlike Penicillium, which stays mostly on the surface.
✅ Smell – The odor can resemble a locker room, old cheese, dirty socks, or damp earth. That said, small patches might not produce any noticeable smell.
How to distinguish mycelium Bruising from Penicillium Contamination
Don’t confuse mold contamination with mycelium bruising.
Mold has a powdery texture.
While bluish mycelium and bruising is a normal reaction in psilocybin mushroom species. It doesn’t have a powdery surface—the mycelium tissue itself turns blue.
Bruising can be caused by hard fanning, misting, touching the mycelium, or breaking and shaking the spawn.
Check out more examples here: Mycelium Bruises or Mold Contamination
Q-Tip Test for Mold Detection
The Q-tip test isn’t scientific, but it’s a popular trick among beginner mushroom growers. While it won’t tell you the exact mold species, it can help you tell the difference between mycelium bruising and mold contamination (especially during the mold’s sporulation phase).
The idea is simple: mold spores are powdery and sticky, they will transfer to anything that touches them.
How to do the Q-Tip Test:
Take a clean white cotton swab or sterile Q-tip.
Gently swipe it across the suspicious area on your mycelium.
Check the swab. If it picks up powder, it’s likely mold. If it stays clean, it’s probably just bruising.
This test is handy for a quick check—but for true mold ID, you’ll need a microscope.
Can I save mushroom Spawn contaminated by Penicillium mold?
No. Once the mold turns green, blue-green, or yellow it has entered the sporulating phase.
At this stage, you can’t save any healthy part of the spawn—unlike with mild bacterial contamination. Discard the entire jar or bag!
In rare cases, if the mycelium is strong enough, it may fight off small patches of Penicillium — create a border around the contamination and literally “eat” it. As an example, here mushroom mycelium (red ganoderma aka reishi) created a border around contaminated spot and day-by-day this circle becomes smaller and smaller. A few similar spots have already disappeared.
However, even if it looks like the contamination disappeared completely, some spores still hiding in there.
Can I save a mushroom Cake Contaminated by Penicillium Mold?
Once Penicillium turns green, blue-green, or yellow, it’s in the sporulating phase, meaning it’s actively releasing spores into the air. These spores can contaminate anything they land on—textiles, carpets, furniture, your ventilation system. Air currents only make it worse, helping spores spread throughout the space.
Remember: if you see color, it’s already sporulating and contaminating your environment. At this point, there’s no way to save the cake. Stay calm, toss it out, protect your future grows, and treat it as a learning experience.
The Good News: treatment is possible in the early stage — BEFORE the mold turns green (or blue, or yellow).
How to Treat Early-Stage Mold Contamination
If you catch a small, round, white curdled patch, you can try treating it with salt, baking soda, or hydrogen peroxide solution (H₂O₂ 3%).
Make sure the treated area covers a bit more than just the visible contamination.
Salt or Baking Soda Treatment
Pour salt or baking soda directly onto the affected spot.
Make sure to seal the affected area completely, especially around the edges. Treated area should be slightly bigger than the visible contaminated spot! Here is the example, where more salt needed.
Or: soak a paper towel in salt or soda solution, place it over the patch, and pour some dry salt/soda around the edges. Seal the area completely to limit spore exposure.
Here is the real case with such example on the forum: Is it green mold? How to save the cake?
Hydrogen Peroxide Treatment
Soak a paper towel in 3% (or slightly stronger) hydrogen peroxide until thoroughly wet.
Place it gently on the contaminated area.
You can spray a little extra H₂O₂ on the paper towel afterward for extra effect.
Don’t spray H₂O₂ directly onto the contamination! If some spores are already mature — even if not yet visible — spraying can send them flying all over the cake.
Note that any treatments aren’t a guarantee that contamination won’t return! Here is an example — mold appeared again after salt application, which means spores spread on the entire top layer of the cake. Toss it immediately!
Can I notice Penicillium mold in Liquid Culture?
Yes, sometimes, blue-green mold patches may appear floating on the surface of the liquid culture (LC), but most of the time, contamination isn't visible until after inoculation.
Can I USE Liquid Culture contaminated by Penicillium Mold?
No! Once the LC is contaminated, spores have already spread throughout the liquid.
Can I save a Mushroom Genetics from Liquid Culture contaminated by mold?
It's tricky, but possible! If you really need to rescue the mushroom genetics, you can try cleaning the culture through Agar-to-Agar transfers. This involves isolating healthy mycelium from the contaminated sample on agar plates and transferring it until you get a clean culture.
Can I save Mushroom Mycelium on Agar contaminated by mold?
Yes! By cleaning through agar-to-agar transfers. This cleanup method works for any type of mold:
Cut a healthy piece of mycelium from the contaminated agar plate.
Transfer it to a fresh, sterile agar plate.
It may take 1 to 10 (or more) transfers to get clean, contaminant-free mycelium.
Pro Tip: Take a sample as far away from the contamination as possible. Avoid using a laminar flow hood while working with contaminated plates — it can spread the spores. Use Still Air Box (SAB) instead.
Main causes and how to prevent Penicillium Mold Contamination
Household Routine to Reduce Airborne Mold Spores:
Discard moldy food, textiles, and furniture.
Clean any area where moldy items were stored or touched.
Regularly clean your refrigerator and surrounding areas.
Clean the ventilation system and A/C every 3–6 months.
Replace or wash air filters every 3–6 months.
Inspect and treat mold in walls, corners, bathroom, and kitchen promptly.
Mycology Routine for Penicillium prevention
Clean Genetics. Get cultures from trusted vendors or prep your own under sterile conditions. If unsure, test spores, syringes, or LC on agar or with 2–3 small grain jars before scaling up.
Here is an example of Penicillium contamination from a spore swab:
Sterilization & Pasteurization. Penicillium spores die at 122°F (50°C) after 25–40 minutes. So that, it's easy to kill all spores by proper sterilization or pausterization.
Casing Layers. pH adjusted (alkaline) casing layers slow Trichoderma, but don’t reliably prevent Penicillium. It grows in acidic, neutral, and even alkaline environments. The only way - pausterization.
Sanitizing Tools & Surfaces. Wipe scalpels, needles, syringes, and your workspace with alcohol. Use sterile, disposable items whenever possible (petri dishes, syringes, containers).
Equipment for Sterile Work. Use a Still Air Box (SAB) or a Laminar Flow Hood. Turn off fans/ventilation 30 minutes before your work to reduce air currents.
Personal Hygiene & Grow Room Cleanliness. You are (grower!) also a contamination vector: remember to wear gloves, a face mask, clean clothes, or a lab gown. Remove carpets, curtains, and textiles that trap dust. Regularly wipe surfaces.
Even with perfect routines, contamination can still happen, but careful prevention greatly lowers the risk.
Bottom Line
There are hundreds of Penicillium species—some helpful, some harmful. Their spores are everywhere and can easily ruin a mushroom grow. Spore levels vary by season and from room to room, so the risk of contamination is different for every grower.
But don’t worry! With proper sterility routine, sterilization, pausterization, contam-free genetics, and selective transfers, you can keep Penicillium in check. Agar-to-agar transfers are a great way to clean up your culture and get back on track.
Stay alert through every stage of cultivation. If you catch contamination on mushroom cake early, you may still save your harvest.
Contamination might feel like a setback, but it’s really a learning opportunity. Every grow teaches you something. Keep going, keep growing and keep getting better!
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Check out more examples of Penicillium contamination in the photo gallery below.
Have a happy growing and healthy shrooms!
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