Shiitake Mushroom Spawn Run: 6 Common Problems, Causes and Treatment Measures

1. Mycelium Recession in Spawn Bags

Symptoms: During the spawn run stage, the white mycelium turns yellow, and the areas once colonized by mycelium fade back to the natural color of the sawdust substrate.

Causes: The temperature during spawn run exceeds 35°C, and the bags are not turned frequently enough.

Prevention & Treatment:

• Avoid making bags for cultivation during hot seasons.
• For logs requiring a long spawn run, puncture aeration holes to increase oxygen supply.

2. Weak or Failed Browning of Mushroom Logs

Symptoms: The logs show a light yellow-brown color (insufficient formation of the dark brown protective bark).

Causes:

• Insufficient mycelial age — logs are de-bagged too early, before the mycelium is physiologically mature.
• Poor moisture retention on the mushroom bed — humidity too low to meet the browning requirements.
• Temperature too high at de-bagging with water spraying started too late, or temperature below 12°C at de-bagging.

Treatment:

• Spray water for moisture retention combined with ventilation — once a day for 2–3 consecutive days.
• Inspect the bed-covering film, repair any holes, and tighten the film to improve moisture retention.
• Lay the logs flat on the ground to use ground temperature and moisture to promote browning on one side, then turn them over to brown the other side.
• If caused by low temperature: remove some shading to let in light and raise the temperature, and ventilate at noon.
• If caused by high temperature: increase ventilation frequency and spray cool spring water to lower the temperature; open the film at both ends of the bed at noon, and ventilate in the morning and evening for 30 minutes each time.

3. Mycelial Dehydration

Symptoms: The log surface becomes rough and prickly to the touch, and the weight drops noticeably.

Causes:

• Strong sun exposure during the spawn run, or the tape over inoculation holes falling off, causing water to evaporate from the bag.
• Dry northwest wind or hot dry wind at de-bagging, causing mycelial cells to rupture, nutrients to leak out, and the surface to dry up.
• Large moisture difference between the cultivation site and the mycelium — common in dry-land mushroom farms where the ground absorbs moisture from the logs.
• Poor moisture retention of the bed film (old film with holes, loosely covered film, or excessive airflow in the bed).
• Improper ventilation — the film is left open too long and the logs are exposed to hot dry wind.

Treatment:

• Increase water spraying — use a watering can to spray plenty of water onto the logs for 2 consecutive days until the surface is soft rather than prickly.
• Cover the film tightly and shorten ventilation time, maintaining a relative humidity of about 90%.
• Irrigate the furrows on both sides of the mushroom bed with running water to increase ground humidity.

4. Thickened Bark (Rind) Layer

Symptoms: The bark layer becomes hard and dark brown, making fruiting difficult.

Causes:

• Delayed de-bagging — mycelial age is too long, nutrients are continuously transported to the surface, and the mycelium knots and thickens layer by layer.
• Improper substrate formula — excessive nitrogen source, imbalanced carbon-to-nitrogen ratio, mycelial overgrowth, delayed collapse, and thickened bark after browning.
• Improper ventilation — not following the browning-stage ventilation schedule, or too few and too short ventilation sessions.
• Overly shaded mushroom site with insufficient light.

Treatment:

• Strengthen ventilation — at least 2 times per day, 1 hour each time.
• Adjust lighting — maintain a "three parts sun, seven parts shade" environment.
• Increase the dry-wet difference and temperature difference to force the mycelium to shift from vegetative growth to reproductive growth.
• Use the "log-tapping fruiting method" — brush the log surface back and forth with a palm brush dipped in water, or gently squeeze the log by hand to vibrate and break the mycelium, exposing white mycelium through cracks to promote knotting and fruiting.

5. Yellow Water Secretion from Bags

Symptoms: As the bags gradually mature, yellow-brown water droplets (commonly called "yellow water") keep oozing from the bag surface. Yellow water contains nutrients; if it accumulates excessively over time, it can easily attract Trichoderma and other contaminants, eventually causing the bags to rot. It must be dealt with promptly.

Treatment:

• Strengthen ventilation to prevent bacteria from breeding in an oxygen-deficient environment; ventilation also helps evaporate the yellow water.
• Use a sharp, sterilized knife or needle to puncture the bag at the yellow water accumulation points (pierce only the plastic film, not the mycelium) to let the yellow water drain out. Avoid letting the yellow water run into the inoculation holes. When spraying water to cool the bags, rinse away the yellow water, but avoid letting water get inside the bag.

6. Mold and Rot of Mushroom Logs

Symptoms: During de-bagging and browning, black spots appear on the logs; pressing the affected area makes black water seep out, with a foul smell.

Causes:

• High temperature after de-bagging, especially on rainy days with poor ventilation — high temperature and high humidity promote contamination.
• Mold not detected in time during the spawn run, remaining latent in the substrate and multiplying rapidly after de-bagging when temperature and humidity become suitable.
• De-bagging too late — yellow water penetrates into the log and causes contamination.
• Overly shaded site, poor surrounding environment, and poor airflow.

Treatment:

• Isolate the affected logs — gather moldy logs on one bed and sprinkle lime powder on the ground.
• Manual removal combined with chemicals — first cut away the rotten parts with a knife to expose healthy tissue (leave no rotten material behind), then wipe or apply 50% carbendazim diluted 200–300 times onto the affected areas to effectively inhibit further rot.
• Control water spraying to avoid excessive humidity.
• Increase ventilation — open the film once a day to keep the bed air fresh.
• If the shading around the shed is too dense, open the north–south ventilation windows to allow air circulation.