Are your oyster mushroom grow bags fully colonized but still not producing mushrooms? In this guide, we'll cover the most common reasons — from temperature imbalances to hidden fungicides — and how to fix them.
Table of Contents:
Here are the main reasons why the mycelium might not be developing properly:
✅ The spawn is old, weak, or was stored incorrectly. It might have enough energy to colonize the substrate, but not to initiate fruiting.
✅ The substrate lacks sufficient nitrogen and/or other essential nutrients (especially phosphorus).
✅ The humidity during incubation was too low.
✅ The final stage of incubation wasn't done correctly. 📌 See details here
✅ Temperature is too high and/or humidity too low in the fruiting room.
If several of these factors are present at once, oyster mushrooms may appear only after some of them are corrected.
💡 Always double-check whether your growing conditions match the needs of the mushroom strain you're using.
⚠️ The most common cause of poor fruiting is incorrect temperature and humidity.
If humidity in the grow room is too low, the cuts in the bags may dry out, and the mycelium at those spots dies due to dehydration — making primordia formation impossible.
To clarify, we are talking about well-colonized blocks. Below is a photo of an oyster mushroom block with stroma.
You’ll notice that the mycelium does not look healthy.
📷For comparison, a healthy block is shown on the right.
The white substance you see inside a mushroom bag might be stroma — not mold. Stroma is a dense layer of hyphae that forms between the substrate and the plastic.
This structure acts as a protective barrier for the block during high temperatures and low humidity.
Many people assume that a bag filled with dense white mycelium is a sign of good colonization. But that’s not always true.
⚠️ If the white layer is gelatinous and slimy (left), or forms crusty patches between the plastic and substrate (right), it’s likely stroma↓.
In some cases, mushroom pins may still appear. But most often, stroma forms when the substrate overheats during incubation. That's why it's critical to monitor internal temperatures — don’t let the center of the block exceed 32–33 degrees Celsius (about 90–91 degrees Fahrenheit).
Why mushroom grow bag not fruiting and there are no mushrooms on a well-grown block?
Especially in warm weather, a block might fail to fruit if the switch from incubation to fruiting conditions wasn’t done properly.
Here’s how to check:
We measure
the temperature of the center of the block - T3 and under under the plastic (between plastic and substrate) - T2.
T1 - the air temperature 10–15 cm away from the block.
If the air temperature (T1) has been stable for some time, and the under-film temperature (T2) is higher than the average of T3 and T1, primordia (pins) will form slowly — or not at all.
Example:
T1 = 23
T2 = 26
T3 = 28
(23 + 28) / 2 = 25.5 → Since T2 is higher, fruiting is delayed.
✅ In this case, gradually lowering the grow room temperature will help initiate fruiting.
Many growers prefer not to bother with formulas and just lower the room temperature via air conditioning, or even refrigerate individual blocks. But that’s not necessary — modern strains do not require a strong “temperature shock.”
⚠️ Without checking the temperature balance, you may misdiagnose the cause and waste electricity trying to fix the wrong issue.
Once mushrooms begin to form, continue monitoring your growing conditions — they directly affect mushroom quality.
Another reason for poor fruiting is the presence of chemical residues from fungicides or herbicides in the substrate — especially if you’re using straw or sunflower husk.
These substances inhibit mycelium growth and reduce or prevent fruiting.
There’s no easy way to tell if straw or husk contains fungicides — there are no visual signs.
⚠️ In wet harvest seasons, fungicide use increases dramatically. Residual chemicals break down only after 4–6 months, so substrates made from freshly harvested straw or husk may cause problems in the fall and early winter. Later, as the chemicals degrade, yields may improve on the same material.
📌Read: Why primordia turn yellow and dry.
If you see a thin-stemmed mushroom with a bell-shaped cap, it’s probably a coprinus — not oyster mushrooms. Such a block won’t produce oyster mushrooms.
There are several coprinus species.
Some have white caps that darken over time, others are gray from the beginning. The caps spread, turn black, and in “ink” species, even smear like ink when touched.
Some coprinus mushrooms are technically edible — but unless you're an expert, it’s safer not to eat them.
⚠️ The appearance of foreign mushrooms indicates that your substrate wasn’t properly pasteurized or sterilized.
Coprinus often show up in nitrogen-rich substrates combined with poor heat treatment.
This is especially common when growers make substrate at home using hot water poured over straw, without pre-soaking it.
In most cases, a lack of fruiting comes down to growing room conditions or substrate quality.
Start by checking humidity, temperature balance, and substrate history. And if you suspect fungicides — try switching suppliers for a few batches. Consistent monitoring and small adjustments usually lead to a good harvest.
Emoji guide: 📌 link | ✅ advice | ▶️ fact | 💡 important | ⚠️ warning
