Cobweb mould and Trichoderma are the two most common causes of misdiagnosis in mushroom cultivation. They can look similar in early stages — both white, both filamentous — but they are fundamentally different organisms with different implications for the grow.
Misidentifying Trichoderma as cobweb mould leads to the most costly mistake in cultivation: doing nothing while a batch becomes a total loss.
What Trichoderma looks like
Trichoderma is a fast-growing mould that starts white and turns green as it sporulates. In the very early stages — before spore production — it can appear as a white, slightly fuzzy patch that is easy to dismiss or misidentify.
The green colouration typically develops within 24–48 hours of the white stage. Once visible, it spreads rapidly and produces spores that disperse through the growing environment. This is the point at which a contaminated container becomes a risk to other containers in the same space.
Key characteristics:
- Starts white, turns green or blue-green as it matures
- Dense, compact texture — not loose or web-like
- Does not collapse when misted
- Grows in a defined patch that expands outward
- Strong, musty smell
What cobweb mould looks like
Cobweb mould (Cladobotryum and related species) is a naturally occurring fungus that colonises mushroom grows, particularly in high-humidity environments. It is not a contaminant in the same sense as Trichoderma — it does not destroy a batch on its own.
It appears as a fine, white, web-like growth — loose and irregular, resembling a thin layer of spider’s web across the substrate surface. It spreads across the top of the substrate rather than into it.
Key characteristics:
- Fine, loose, web-like texture
- Collapses visibly when misted — this is the definitive test
- Spreads across the surface rather than growing in a defined patch
- Does not produce green colouration
- Associated with low FAE and high humidity
The definitive test: misting
When in doubt, mist the growth lightly and observe. Cobweb mould collapses immediately under moisture. The fine filaments mat down and become nearly invisible. Healthy mycelium and Trichoderma do not collapse.
This is the single most reliable field test and should be the first response when unidentified white growth appears.
What each one means for your grow
If it is cobweb mould:
Cobweb mould is manageable. It indicates that fresh air exchange is insufficient or humidity is too high. Increase FAE, reduce surface humidity slightly, and mist the affected area to collapse the growth. It typically resolves without further intervention once environmental conditions are corrected.
It does not require discarding the substrate.
If it is Trichoderma:
The batch is a loss. Trichoderma competes aggressively with mushroom mycelium and, once established, cannot be removed. Do not attempt to cut out the contaminated area — the mycelium of the mould extends beyond the visible surface growth.
Remove the container from the growing environment without opening it. Trichoderma spores spread easily and will colonise other substrates in the same space. Bag the container before removing it if possible.
After removal, audit the batch against the timing of appearance. [See: How to Identify Mushroom Contamination — using timing and location to find the entry point.]
Why misidentification is common
The window during which Trichoderma and cobweb mould look similar is short — typically 12–24 hours — but it is exactly the period at which growers are most likely to decide to monitor rather than act.
The logic seems reasonable: white growth that resembles mycelium could be healthy. But the misting test resolves the question immediately, and the cost of waiting — a full batch loss plus potential spread to other containers — is high.
Default to the misting test on any unidentified white growth. The test takes ten seconds.
Other white growths that cause confusion
Overlay: Dense, thick white mycelium that forms a continuous layer over the substrate surface, blocking pins. Not contamination — a cultivation variable related to CO₂ levels and humidity. Texture is much denser than cobweb mould and does not have the web-like appearance.
Aerial mycelium: Healthy mycelium growing upward off the substrate surface. Common in certain conditions. White, filamentous, but grows in a pattern consistent with the mycelium colony — not patchy or irregular.
Bacterial blotch: Can appear as white or off-white wet patches. Slimy texture distinguishes it from mould growth.
Summary
| Trichoderma | Cobweb Mould | |
|---|---|---|
| Colour | White → green | White only |
| Texture | Dense, compact | Fine, web-like |
| Misting response | No change | Collapses |
| Spread pattern | Expanding patch | Surface layer |
| Action | Remove batch | Adjust FAE/RH |
| Outcome | Batch loss | Manageable |
The misting test is definitive. Use it immediately on any unidentified white growth.
The full Contamination Pattern Recognition matrix — including entry point identification by timing and location — is included in the Environmental Calibration Sheet. Get the sheet →
Shroom Crafter 
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