Mycelial Overlay (Overgrowth)

Definition: Overlay or Overgrowth is a cultivation disorder in which mushroom mycelium remains in a vegetative state and overgrows the casing layer, forming a dense surface mat. This “sealed” layer reduces pin formation and interferes with watering and fresh air because it can’t penetrate the casing effectively. It is most often remedied in home mushroom growing with the fork technique to break up this mat of mycelium.

Where it shows up

Overlay is primarily a risk in species and systems that use a non-nutritive casing layer (e.g., commercial Agaricus bisporus), but can be present in any mushroom grow system. Proper fruiting requires a shift from vegetative, or colonization growth to primordia initiation most often by slightly lowering temperature and CO2, so anything that delays or blunts that shift can predispose a crop to overlay.

How to Identify Overlay

• Dense, continuous white mat across the casing surface.

• Water beading and run-off: the surface “skins over,” so irrigation sits on top rather than soaking in.

• Poor or uneven pinset, including side pinning and delayed cropping.

What Causes Overlay

Overlay reflects continued colonial growth in and over the top layer of the substrate due to delayed or inadequate initiation (“pinning”) conditions. Common contributing factors are related to delayed fruiting conditions, which include:

• CO2 left too high for too long after colonization is complete.

• Temperature not lowered soon enough (prolonged vegetative-range temps). Some mushroom species are more sensitive to temperature reductions than others.

• Overly wet, stagnant air and insufficient fresh air exchange.

Mechanistically, successful initiation of fruiting conditions by reducing both air temperature and CO2, as well as the introduction of moisture through misting, failure to do so favors vegetative overgrowth at the surface, which will make fruiting difficult.

Prevention

• Time the initiation of fruiting conditions correctly: After the substrate is colonized to the desired degree, initiate fruiting conditions instead of holding vegetative or colonization conditions.

• Maintain a breathable casing: Avoid compaction and manage moisture so the casing stays porous; follow good casing prep and watering practices. In the case of mushroom grow kits like RyzaPods, this means a timely introduction of misting and fresh air exchange when colonization is done.

• Provide fresh air exchange (FAE): Prevent stagnant, humid, CO₂-rich boundary layers at the casing surface. In the case of RyzaPods, this means unsealing the lid and placing the humidity dome, along with consistent misting and indirect light exposure.