Green mold (Trichoderma) is one of the most common problems that causes spoiled mycelium, especially in oyster mushroom cultivation.
How do you know whether it's mycelium or mold in your block?
In the early stages of colonization, green mold is usually invisible. During the first days, Trichoderma can grow along with the oyster mushroom mycelium. Its young hyphae are pale white and almost indistinguishable from oyster mushroom mycelium.
When Trichoderma matures, it releases spores in large quantities — they are green and vary in shade. That green color is what makes the mold clearly visible.
Some growers try to treat green spots with various solutions — but this rarely works.
Online articles mention a range of substances: table salt, hydrogen peroxide, sodium hypochlorite, lime — or combinations of them, such as salt with lime or bleach with lime.
You can try treating a small mold spot under the plastic film with one of these solutions.
But if the spot is small, the oyster mushroom mycelium often outgrows it on its own.
However, if the contamination is widespread (see photo), chemical treatment will not help.
Can mycelium fight green mold?
As mentioned earlier, if the Trichoderma patch is small — about the size of a fist — the oyster mushroom mycelium can sometimes take over the area once Trichoderma has released its spores. Oyster mushroom hyphae secrete substances that inhibit mold hyphae.
But if the contamination is heavy, the mushroom mycelium won’t be able to stop it.
Fun fact: mold struggles to grow in substrate with alkaline pH. More on that below.
Trichoderma is a common soil mold. It plays a key role in enriching soil, so its spores are everywhere — especially in mushroom farms where conditions are favorable for its growth.
To avoid contamination, follow these best practices:
✅ Disinfect growing areas regularly. (See link to disinfection article below.)
✅ Properly dispose of spent substrate.
✅ Follow the correct substrate preparation process for your method (pasteurization, sterilization, hot water treatment). This includes observing the right temperature, timing, and clean sowing procedures.
This is the #1 reason. Problems arise when:
▶️ The substrate was underprocessed (temperature and/or time too low);
▶️ The substrate is overly wet;
▶️ The pH is below 7.
Often, more than one of these issues is present.
In such cases, green mold colonizes the entire substrate. Sometimes mycelium doesn’t grow at all, and spawn grains become covered in gray-green fuzz.
💡The problem usually isn’t with the quality of the spawn. Mold attacks the boiled grain first, since it’s rich in nutrients, and then spreads to the straw or husk.
How to fix it:
✅ Adjust your substrate preparation protocol.
📌 Read more here
✅ Measure the substrate’s pH and keep it between 7.8 and 8.5.
✅ Maintain strict cleanliness in the inoculation room. Use positive pressure to keep contaminants out.
Mold growth slows above pH 8 — and oyster mushroom can tolerate this.
⚠️ Just be cautious: at pH above 9, oyster mushroom growth is also inhibited.
If mold is found only on the surface, under the plastic film, it often means that temperature fluctuations caused condensation — creating the perfect conditions for mold to grow.
Water flows from the center of the substrate to the cooler film layer.
That free moisture creates ideal conditions for mold or bacterial growth.
💡Even though a cross-section shows the substrate is fully and evenly colonized (as in the right photo↓), with no visible Trichoderma except for a thin layer just under the plastic film — such batches rarely produce fruit.
If there is a bacterial issue, you’ll also notice a bad smell in the bags or the incubation room.
How to fix it:
✅ Keep the temperature in the incubation room stable and consistent throughout the entire incubation period. Fluctuations — especially local hot spots — create favorable conditions for mold and other problems.
✅ Let the blocks cool naturally through properly arranged airflow from ducts. The best option is to maintain a uniform room temperature using air conditioning or a controlled supply of fresh air. Avoid cold air streams blowing directly onto the blocks.
If overheating has already started, do not rush to cool the blocks.
Sudden cooling with cold air leads to condensation — moisture collects between the plastic and the substrate, creating ideal conditions for Trichoderma and bacteria.
⚠️ Cooling must be gradual and controlled.
Focus on keeping the air temperature in the room consistent, allowing the blocks to cool slowly through even airflow and a balanced environment — not through sudden cold drafts.
In this case, mold is found only in the perforation holes of the bags. The substrate inside is fully colonized and white.
This means the contamination likely came from spores in the room’s air, which settled into the bag holes.
It’s often Trichoderma, but sometimes you’ll see gray or black mold as well.
The treatment is the same regardless of mold type.
How to fix it:
✅ For isolated spots: remove infected straw by hand, wipe with 5% hydrogen peroxide or bleach solution (wear gloves), and tape over the hole.
✅ If most holes are infected: the room is likely heavily contaminated. Air may look clean, but Petri dishes will grow colorful colonies within days.
📌 In that case, remove all blocks and treat the space with sporicidal disinfectants or fungicides.
📌 Read about other types of mold that can appear during mushroom cultivation here.
Emoji guide: 📌 link | ✅ advice | ▶️ fact | 💡 important | ⚠️ warning
