Mycelium overlay is a common issue for mushroom cultivators, where the dense, white fungal network forms a thick, matted layer across the substrate surface. This condition, sometimes called “stroma,” seals the growing medium and prevents the formation of primordia, the tiny structures that develop into mushrooms. Overlay is a response to an imbalance in the fruiting environment, causing the mycelium to remain in a vegetative growth phase instead of switching to reproductive growth. This issue is generally fixable through manual surface treatment and environmental adjustments. The goal is to stress the mycelium slightly to signal that conditions are appropriate for reproduction, triggering the desired pinning response.
Recognizing True Mycelium Overlay
Identifying true mycelium overlay is the first step before attempting any fix. Overlay appears as a dense, leathery, thick white layer completely obscuring the substrate underneath. This mat is notably different from the delicate, wispy strands of aerial mycelium, which often form when air exchange is insufficient.
The overlay forms a rubbery “skin” that is difficult to penetrate, unlike the fluffy, cotton-like appearance of healthy mycelial knots that precede pinning. It is important to distinguish overlay from cobweb mold, a common contaminant that is typically gray, wispy, and grows explosively fast. True mycelium overlay is bright white and spreads slowly, representing healthy but non-fruiting fungal mass.
Manual Surface Treatments
Once a dense overlay is confirmed, a physical intervention called “scarification” is necessary to break the tough mycelial mat and expose the substrate to air. The most common method, often called “Fork Tek,” involves using a sterilized fork to gently disrupt the surface. Sterilize the tines of the fork by wiping them thoroughly with a 70% isopropyl alcohol solution to minimize contamination risk.
The scraping action should be light and shallow, aiming only to score the top one or two millimeters of the overlay. Avoid deep gouging that could damage the substrate or create pooling water. This mechanical disturbance mimics a natural stress cue, signaling the mycelium to shift from vegetative growth to fruiting growth.
Following scarification, some cultivators apply a thin, fresh layer of pasteurized casing material, such as peat moss or vermiculite. This re-casing step provides a favorable microclimate that retains moisture and encourages surface evaporation, a known pinning trigger. Breaking the barrier and offering a fresh surface can force the mycelium to initiate hyphal knot formation, the earliest stage of pin development.
Optimizing the Fruiting Environment
Manual treatment alone is rarely sufficient, as mycelium overlay is primarily an environmental problem, meaning climate correction must follow the physical fix. The underlying cause of the dense, non-fruiting mat is almost always insufficient Fresh Air Exchange (FAE), which leads to a buildup of carbon dioxide (CO2). Mycelium thrives in high CO2 levels, often above 5,000 parts per million (ppm), but to form a mushroom, the CO2 level must drop significantly, ideally below 1,000 ppm.
To correct this, drastically increase the FAE by fanning the container several times a day or modifying the grow chamber for constant air flow. This flushes out the dense CO2 layer near the substrate surface. This change from high to low CO2 is one of the most powerful signals for the mycelium to transition to reproductive growth.
Alongside increased FAE, ensure the substrate surface experiences gentle evaporation, another major pinning trigger. While high humidity (typically 85% to 95%) is necessary for pin formation, a slightly lower ambient air humidity encourages surface evaporation without drying out the substrate. Furthermore, a slight drop in temperature, sometimes 5 to 10 degrees Fahrenheit lower than the colonization temperature, serves as an additional environmental shock to stimulate pinning.