Oyster mushroom diseases and defects usually show up as changes in the shape, color, or surface of the caps and stems — making the mushrooms look less appealing and harder to sell.
In most cases, these issues are caused not by infection, but by imbalances in the growing environment.
The most common reasons include:
✅Incorrectly set up or miscalculated ventilation, humidification, or heating/cooling systems
✅Sudden fluctuations in temperature or humidity
✅Inconsistent control settings — for example, using timers to regulate humidity can easily cause spikes and drops
📌To maintain stable conditions, humidification should ideally be controlled automatically.
If you don’t have climate controllers in place, it’s easy to miss how much your room’s conditions are fluctuating — even small shifts can affect mushroom development.
The good news is: oyster mushrooms usually “tell” you what’s wrong. Within a few days, signs begin to show on the caps and stems.
Once you learn to recognize these patterns, you’ll often be able to tell exactly what environmental condition caused each defect.
Table of Contents
- Moldy oyster mushrooms
- White fuzzy mold on oyster mushrooms
3. Oyster mushrooms lack moisture
6. Growths and pimples on oyster mushroom
7. Curved and wavy oyster mushroom caps with pimples on the edges
9. Why is oyster mushroom blue?
10. The effect of furnace gas on oyster mushroom
11. Black spots on oyster mushrooms
12. How to solve my problems with oyster mushroom diseases
Many growers refer to any infection on dead mushrooms as a "bacterial infection," but that’s not always accurate.
True bacterial infection in oyster mushrooms is caused by Pseudomonas bacteria and is known as Brown Blotch Disease (BBD).
📌Read more about bacterial infections.
Let’s take a look at the photo.
Why did mold appear on these mushrooms?
Initially, the air was too dry, which caused the central mushroom to dehydrate. Its cap turned lifeless and brown.
Later, the two mushrooms seen higher up and to the left began to grow when the humidity level was suddenly increased. But the change was too sharp or too extreme.
In response, the mushrooms turned their caps upward to help release excess moisture from their gills — a defense mechanism.
However, this abrupt fluctuation in humidity stressed the mushrooms and eventually killed them.
Once the mushrooms died, condensation began to collect on them. This moisture created ideal conditions for mold to grow.
White fuzz doesn’t always mean mold.
If oyster mushrooms are harvested when the caps are fully mature, they tend to store poorly. White spots often appear after a few days in the fridge.
If you see a light white coating in the center of the cap — and it wipes off easily with a finger — this is most likely aerial mycelium, not mold.
These mushrooms are still safe to eat.
But if the white fuzz covers most of the cap and doesn’t wipe off easily, it’s a sign of spoilage.
In that case, it’s best to discard them.
In the photos below, you can see oyster mushrooms with a delicate white coating on the stems — it closely resembles the mycelium that naturally grows on the caps.
This is typically aerial mycelium, which can form when mushrooms are stored in a bag under high humidity and elevated CO₂. It’s a harmless reaction to storage conditions.
However, if the fuzz is thicker, discolored, or has an unpleasant smell, it’s more likely to be mold — and the mushrooms should be discarded.
✅Mold usually appears on dead mushrooms when condensation forms due to sharp humidity changes.
✅White fuzz isn't always mold — it’s often aerial mycelium, especially on refrigerated mushrooms.
✅To tell the difference: mycelium wipes off easily and has no odor, while mold is persistent and may smell unpleasant.
When the airflow in the growing chamber is too strong, it can cause mushroom caps to dry out, crack, and turn yellow — even when the humidity seems fine.
You may notice:
▶️Brown edges that feel dry or even warm to the touch
▶️Cracks or splits forming around the edges of the cap
▶️Yellowing of the surface
▶️Caps curling downward to protect the gills from drying out
This damage often appears on mushrooms still growing in the bags but can also happen after harvesting.
Sometimes caps may crack even under normal airflow if humidity dips temporarily — for example, when the floor is watered and humidity spikes, only to drop again soon after. These sharp fluctuations stress the mushrooms.
Strong airflow can also create glossy, lacquer-like caps, as seen in the photo.
This usually happens when the air is not just dry, but also too cold or too hot for oyster mushrooms.
In such conditions, the cap surface becomes shiny, almost polished.
Keep humidity stable in the chamber — ideally between 83% and 90%, depending on the room temperature and mushroom strain.
Make sure fans aren’t blowing directly onto the mushrooms, especially if the air is too dry or if heating/cooling systems create hot or cold drafts.
If you see cap edges starting to curl or develop a leathery texture, check both airflow direction and air temperature — not just humidity levels.
Some oyster mushroom strains — especially many Pulmonarius types — naturally have brownish caps.
That’s normal and healthy.
A stressed mushroom, however, will look dull, unevenly colored, or cracked, unlike the rich, even tone of a healthy brown cap.
📌Learn more about natural cap color
✅Strong airflow can dry out caps, causing cracks, curling, or yellowing.
✅If airflow is both strong and the air is too cold or hot, the cap may develop a shiny, lacquered surface.
✅Stable humidity and even air distribution are key to preventing these issues.
When oyster mushrooms don’t get enough moisture, their appearance changes. You may notice:
▶️Wavy or uneven caps
▶️Small growths or bumps around the edges (see photo)
▶️Caps curling downward — this usually means humidity has dropped slightly to around 80–83%
If the humidity falls even lower, mushrooms start to look dry and feel light.
The gills thin out, turning white or yellowish, and the cap curls up, exposing the gills (see photo below).
In these conditions, the mushrooms age faster to release spores. They grow without proper flesh inside, making them feel hollow and fragile — just skin and gills.
As a result, the caps curl up, exposing the hymenophore. See the image below.
Don’t rely on just one hygrometer — they can be inaccurate. Try one of these:
▶️Use a second humidity sensor to confirm your current readings
▶️Or recalibrate your existing device in a metrology lab
👉 If your mushrooms look dry or yellow, but your sensor shows 89%, it’s a clear sign that something’s wrong with the reading.
Also:
▶️Place humidity sensors directly near the growing clusters, not just at the top of the chamber
▶️Use several sensors at different points in the room for better accuracy.
If your system uses air ducts, take these steps:
▶️Measure airflow speed using an anemometer — place it directly at the nozzle cup in the duct (you can see what we mean by “nozzle cup” in this photo.)
→ The airflow should be between 26.4–32.8 ft/s (8–10 m/s)
▶️Check airflow direction — if your nozzle cups are pointing sideways or have slipped out of place, adjust them
▶️Inspect the recirculation filter — if it’s dirty, it can reduce humidity in the chamber
✅Low humidity leads to light, dry mushrooms with thin gills and curled caps
✅The appearance of the mushrooms is often more accurate than your hygrometer
✅Always confirm humidity readings with multiple, properly placed devices
✅Airflow issues or dirty filters can also cause low humidity levels
If your oyster mushrooms are growing with:
▶️Long, thin, or curved stems
▶️Small, underdeveloped caps that don’t fully open
▶️Deformed or twisted shapes
...then high levels of carbon dioxide (CO₂) are the most likely cause.
These symptoms show up when fresh air exchange is insufficient and CO₂ builds up in the growing chamber.
📌You can read more about CO₂ levels, how to measure them, and how to adjust your airflow in our dedicated article: Full guide to CO₂ and oyster mushroom growth
A common cause of abnormal shapes and yellowing in mushrooms is an waterlogged substrate. For instance, twisted oyster mushrooms may have their entire fruiting body turn inside out (bend) and become wavy, with the cap turning yellow.
The hymenophore (gills) may grow abnormally, remaining white or also turning yellow.
If mushrooms bend solely due to increased carbon dioxide, they will have normal flesh.
However, if there is excess water in the substrate, the oyster mushroom will feel non-elastic and watery to the touch.
If you squeeze it, water may flow out, and the flesh may break apart between your fingers. Often, the mushroom emits an unpleasant odor. Such oyster mushrooms cannot be eaten or pickled.
For more information about the problem of oversaturated substrates, click here.
White growths or a white mesh on the caps of oyster mushrooms, resembling semolina or netting, form under low temperatures combined with high levels of CO2 in the growing chamber.
This white substance is known as secondary mycelium, where hyphae develop on the cap in response to adverse environmental conditions. Small primordia may even emerge on the mushroom cap from these hyphae.
These mushrooms are safe to eat, provided you can assure buyers that they are perfectly fine.
A white powdery residue on mushrooms can also be from spores. It has a different appearance, resembling dust, and can settle not only on the mushroom cap but also on surrounding surfaces.
For more information on the risks associated with fungal spores, click here.
White fuzz can also appear on small mushrooms, resembling tiny scales that can be easily wiped away with a finger.
Growths or warts along the edge of the cap that look like pimples occur with a significant increase in humidity (by 3-5%) alongside an increase in CO2 concentration.
More details about such mushrooms in a separate article.
In extreme cold (below 10°F / -12°C), outdoor air holds very little moisture. If mostly cold outside air is used with a low rate of recirculation, a paradox occurs: humidifying nozzles operate at maximum, yet humidity remains low.
This happens because the airflow doesn’t have enough time to absorb moisture, as the flow speed is high and air interaction with the nozzles is brief.
To address this, many growers hang additional nozzles directly in the grow chamber, near the ceiling. When these nozzles operate periodically—controlled by a timer or humidity sensor—it can result in a wavy edge on the mushroom caps or even make the entire cap wavy.
This effect is especially noticeable if the air in the ducts isn’t humidified after passing through the heat exchanger. As outside air is warmed up, it becomes much drier. This dry, warm air then flows into the grow chamber, reducing the humidity even further.
In such conditions, it’s common to see scales on the mushroom cap, as shown in the photo—these are dead, exfoliated skin cells. These dandruff-like scales are especially noticeable on the left side of the cluster.
1. Adjust Ventilation: Aim to use the minimum amount of outside air to help maintain humidity. Monitor cap shape to ensure it remains acceptable for sale.
2. Manage Temperature Gradually: If it’s difficult to heat the grow chamber during extreme cold, lower the temperature gradually by 1–2°C (1.8–3.6°F) every 4–5 hours.
3. Adjust Humidity Accordingly: Reduce humidity according to the specific requirements of your mushroom strain.
When temperature, moisture, and CO₂ levels are balanced, mushrooms grow slowly but develop a beautiful, marketable shape.
Coral-like formations on oyster mushrooms are often due to insufficient light and poor conditions during the early primordia stage in the incubator.
In the first two photos, the tiny pins have developed without air access.
The later photos (3-4) show how these shapes changed once the plastic was opened and a small draft introduced.
Many growers note that these formations resemble a hedgehog or, as some describe, a head of cauliflower (see photos below).
It’s worth noting that there is indeed a mushroom that naturally forms similar shapes—Hericium erinaceus, also known as "lion's mane" or "monkey head," sold by labs.
However, if you planted oyster mushroom grain spawn and ended up with coral-like or spiky white growths, this likely results from an imbalanced microclimate.
High CO₂ levels, lack of ventilation, and absence of light at the primordia stage can all impact oyster mushroom shape. Even if such clusters are later moved to a growing chamber, they won’t develop properly.
To fix this, remove these abnormal formations. If the microclimate is corrected, typical mushrooms should appear within 3-4 days.
If you notice that your typically gray oyster mushrooms have developed a bright blue hue, it often results from a sudden drop in temperature in the grow room. This temperature drop needs to be sharp to trigger the blue coloration.
The younger the mushrooms are when the temperature decreases, the greater the likelihood of them changing color.
What happens to the primordia in this situation?
There are two possible outcomes:
1. If the primordia turn blue, they may have a chance to survive and will grow into a gray-blue color. It's crucial to gradually raise the temperature in the growing chamber after the drop; a sudden increase can cause the mushrooms to drown in excess moisture.
2. If the primordia turn brown or yellow, it indicates that they are dead.
Typically, blueing results in mushrooms that have an unattractive appearance. You may observe clusters where many caps do not open, and some mushrooms appear dry and shriveled.
If the growing chamber is heated by a stove or wood-fired boiler, oyster mushrooms can be poisoned by flue gases. The mushrooms may take on an unusual light brown color that is not characteristic of the strain.
Continued exposure to smoke can lead to:
Flue gases can enter the chamber in two ways:
Mushrooms affected in this way should not be eaten. The discoloration of the cap is likely caused by the partial dissolution of sulfur oxides in the humid air of the growing chamber. This condensate settles on the caps and contains trace amounts of acids formed from sulfur oxides, which disrupt the development of the mushroom gills, leading to the death of the fungus.
Black spots on oyster mushrooms can be in the form of streak.
Dark, radially spreading stripes can appear on the caps of oyster mushrooms due to fluctuations in moisture levels in the growing room.
These stripes form as a result of ruptures in cell layers that occur during periods of lowest humidity.
On some mushrooms, the stripes turn black, which can deter buyers who may view them as a defect.
In the photo, you can see a cluster that developed under conditions where humidity fluctuated between 80 and 86% (relative humidity). The disk humidifier was only activated twice a day, with additional moisture provided by wetting the floors.
Also note the downward-curved, wavy edges of the oyster mushroom caps.
If you encounter this issue, it’s essential to adjust the humidification system to prevent humidity fluctuations.
Additionally, temperature fluctuations may also contribute to these dark streaks, as rapid shifts can disrupt the normal development of cell layers in the mushroom cap, further increasing the chances of pigmentation changes.
In the article, I’ve provided a brief overview of the most common types of oyster mushroom deformities.
If you encounter any issues with the appearance of your oyster mushrooms, feel free to reach out to me for a consultation!
You can find the consultation fees here.
