It’s not high room temperature that kills your mycelium — it’s what happens inside the block. In this article, we’ll go over the temperature thresholds that are actually dangerous for oyster mushroom mycelium and how to manage them during incubation.
Most new growers worry about keeping their incubation room warm enough. But fewer realize that overheating is actually a much bigger risk — especially during the first week after inoculation.
In this article, we’ll talk about how high the temperature can get before the mycelium inside the substrate starts to die, what signs to look for, and how to prevent block overheating.
Oyster mushroom mycelium can usually tolerate short spikes in heat, but if the core temperature inside the substrate block stays above 36–37°C (96.8–98.6°F) for more than 24 hours, the mycelium begins to die.
This doesn’t mean the temperature in your incubation room needs to be that high — the inside of the block can heat up all on its own, especially if:
▶️The room is too warm (above 24–25°C / 75–77°F)
▶️Blocks are packed too tightly and can’t release heat
▶️The substrate wasn't properly pasteurized or sterilized, and contaminating microbes are active.
💡This is important — please don’t skip it❗️
There’s a common myth among mushroom growers that the temperature in the center of the block should never go above 30 °C (86 °F). So when the blocks are developing well, they start actively cooling them down to stay below that line.
But in reality, for strong and healthy oyster mushroom mycelium, a core temperature of 31–32 °C (88–89.5 °F) is not dangerous — even if it stays that high for 3–4 days!
In fact, many strains actually give better first flush yields when the core temperature is held in that range.
So even if the blocks keep self-heating and you have to cool them down, do it gradually, and don’t drop the temperature below 30–31 °C (86–88 °F).
The first sign of trouble is slowed or stalled mycelium growth. Then, a brown or dark spot may appear in the center of the block — this is dead mycelium. On a cross-section, it often looks burned or cooked.
Even if the surface continues to colonize, the inner core may remain sterile or contaminated. This usually leads to poor fruiting or complete failure later on.
Here’s a cross-section of a colonized mushroom block.
You can clearly see the damage: the dark patches are areas where the mycelium overheated and died.
But look at that bright white spot in the middle — that’s fresh mycelium starting to regrow after things cooled down. Along the edge, there’s a dark wet line near the plastic — that’s where the substrate got too soggy from a blast of cold air. This kind of temperature shock can really mess with the block’s recovery and performance.
Sometimes, yes. If the temperature drops slowly (not abruptly!), the surviving mycelium may start regrowing into the dead zone. But this process takes time — and even then, the final yield will most likely be lower.
Fast cooling, especially with cold outside air, can make things worse by causing condensation under the plastic. That creates a perfect environment for mold and bacteria.
✅Monitor the temperature inside the block, not just in the room. Use a long probe thermometer to measure the center temperature during days 3–7 of incubation.
✅Provide good airflow around the bags, so they can release heat evenly. Recirculation ventilation is ideal.
✅Avoid packing blocks too tightly — leave space between them for air movement.
✅Keep incubation room temperature between 20–23°C (68–73.5°F) for most oyster strains. If the blocks start heating up, lower the room temperature by 1–2°C gradually.
Oyster mushroom mycelium is sensitive to both cold and heat — but heat stress is harder to fix. That’s why it's essential to track not just the air temperature, but also what's going on inside the blocks. A little prevention early on can save you a lot of frustration (and yield) later.
