Mould or mycelium? How to tell them apart

Mycelium is the main body of a fungus. It's a root-like structure made of a mass of branching, thread-like strands called hyphae¹. A mushroom is only the part that comes up later to release spores. Mould is also fungus, so the question isn't really "fungus or not". It's "the fungus you wanted, or one that's taking over".

What mycelium is

Colonies made of mycelium live in and on soil and many other substrates, which is the word for whatever a fungus feeds on¹. In a grow bag or tub, the substrate might be straw, sawdust or a similar material. The mushroom you're growing spreads through it as mycelium first.

Fruiting bodies, the mushrooms themselves, can form once two compatible mycelia have joined¹. That's the stage growers wait for.

So white, thread-like growth on a substrate is expected. Wikipedia describes mycelium as slender, entangled, branching threads¹. The trouble is that several contaminants start out white too.

Where white fuzz misleads

Green mould, caused by Trichoderma, is the best-known example. Penn State Extension says it starts out grey, later turns white along the edges, and turns green as it sporulates, meaning as it makes spores². In its white stage it can look much like healthy growth. A related Penn State page describes it as a dense layer on the casing surface that changes from white to green³.

Cobweb is another. It's a weft of cotton-like mycelium that's usually white but sometimes grey, pink or yellow, and it can cover casing, mushrooms or pins, which are the tiny first mushrooms². Colour alone won't settle it, then.

Mushroom growers also meet contaminants that don't fuzz at all. Some fungi simply outcompete the mushroom mycelium for food, and some attack it directly².

Signs the sources list

Nothing here replaces an expert eye, and these sources describe commercial farms, not kitchen bags. Still, they agree on a few warning signs.

• Colour change. Green spores, or patches that start grey and turn green, point to Trichoderma².
• A distinct smell. Some Trichoderma species give off a sweet or "coconut" odour⁴. A UK industry guide also lists an unusual smell among the signs to watch for⁵.
• Strappy white growth that appears alongside green sporulation. The same guide tells growers to look for both⁵.
• Spots on the mushroom itself, not the substrate. Penn State lists flesh-coloured spots under cobweb².

Not every species has the odour. Wikipedia says only that some do⁴.

Why it matters which one it is

Trichoderma species are responsible for green mould disease in mushroom cultivation⁴. Wikipedia reports that infected substrate leads to lower yields through competition, and that mushrooms can come out malformed or discoloured⁴.

The usual suspects are described in common contaminants.

Exceptions

The advice above describes commercial button, oyster and shiitake crops. Home grows differ in scale, substrate and species, and a grower's own supplier may give species-specific notes. Follow those first.