Green Mould

Meet the number-one culprit

If you grow mushrooms long enough, you will meet Trichoderma — green mould. It is the single most common and most destructive contaminant in mushroom cultivation, and it has cost commercial growers dearly. In the early 1990s an aggressive strain known as Trichoderma harzianum biotype 4 (Th4) triggered a green mould epidemic in Pennsylvania that caused tens of millions of dollars in crop losses.

Trichoderma is a fast, opportunistic mould that lives almost everywhere — in soil, on wood, in dust. It is not looking for your mushrooms specifically. It simply wants the same food your mycelium wants, and it is very good at grabbing it first.

What it looks like

Here is the tricky part: Trichoderma does not start out green. It begins as a patch of white, cottony mycelium that can look almost identical to healthy mushroom growth. This is why it fools so many growers.

The give-away comes a day or two later, when it produces spores. The patch turns a distinct forest or emerald green, often with a bright white advancing edge where new growth pushes outward. Once you see that green, the mould is already sporulating — releasing millions of tiny spores into the air.

Watch for these signs:

• A white patch that grows faster and flatter than your mushroom mycelium

• Green colouring appearing from the centre outward

• Browning or a wet, breaking-down look in the substrate at the leading edge, where Trichoderma degrades the block

• A patch on grain spawn that stays suspiciously smooth and pale before turning green

Why it spreads so fast

Trichoderma is dangerous because of its spores. A single green patch can throw off enormous numbers of spores that drift on air currents and settle on every surface in the room. Those spores are hardy and hard to remove. This is why one ignored green block can seed contamination across an entire crop.

The mould enters a grow space mainly on people, tools, and clothing, in poorly pasteurised or under-sterilised substrate, and through contaminated spawn. It thrives where mushroom mycelium is weak or slow — cool, stalled, or damaged colonisations give it the opening it needs. It also tends to favour slightly acidic conditions, which is one reason casing and substrate pH matter.

First response

When you spot green mould, treat it as a containment problem, not a cleaning problem. Do not carry the block through the room or open it near other blocks — disturbing it launches spores everywhere. We will cover the full cull decision in a later lesson, but the instinct to protect the room starts here.

A note on safety: this is about protecting your crop, not about eating anything. Never eat a mushroom growing on a contaminated block, and never taste-test to judge contamination. Kits and cultivated crops give you known, safe species grown under clean conditions; a mouldy block is compost, not food. As always, cook cultivated mushrooms fully and never eat them raw, and young growers should always work alongside an adult.

Green mould is beatable — but only if you read it early and respect how fast it moves.