Why Does My Mushroom Substrate Have Green Mould on It?

Green mould on a mushroom block or bag is one of the most disheartening sights for a home grower, and it is extremely common, especially among beginners. The green colour almost always indicates Trichoderma, a genus of soil mould that is phenomenally good at competing with mushroom mycelium. It is fast, aggressive, and produces spores by the millions. Understanding why it appears and how to respond is essential knowledge for any mushroom grower.

What Is Trichoderma and Why Is It So Aggressive?

Trichoderma is a fast-growing mould found virtually everywhere in soil, on wood, and in the air. It has several features that make it a serious competitor in mushroom growing: it germinates and spreads faster than most mushroom species at typical colonisation temperatures, it produces antifungal compounds that actively kill neighbouring fungi including mushroom mycelium, and it can reach sporulation within days once established. The characteristic green colour comes from its spores, which turn green as the mould reaches maturity. Early-stage Trichoderma may appear white and be mistaken for mycelium before turning green.

Why It Appears: The Main Entry Points

Trichoderma enters your substrate through three main routes. The first is inadequate sterilisation or pasteurisation: surviving Trichoderma spores germinate as soon as conditions are suitable. The second is contamination during inoculation, introduced from the air, your hands, tools, or contaminated spawn. The third is air contact after the bag is opened for fruiting, which is essentially unavoidable but manageable if the block is already well colonised before opening. A fast-colonising block has a significant competitive advantage over any Trichoderma that enters at this stage.

Can a Green-Moulded Block Be Saved?

The honest answer is rarely. If Trichoderma is visible on the surface, its mycelium has already penetrated deeper than what you can see or remove. In a few cases where contamination is at a very small inoculation point and caught extremely early, removing the affected section and sealing the remaining block with wax has saved grows. However, the risk of the contamination spreading and releasing spores into your growing space usually outweighs the potential yield from a compromised block. Isolate, bag, and remove from your growing area immediately.

Prevention: Sterilisation and Spawn Rate

The two most effective prevention strategies are proper sterilisation and a high spawn rate. Sterilise supplemented sawdust substrate at 15 psi for 2.5 to 3 hours minimum. For straw, use hot water pasteurisation at 75 to 82 °C held for one hour. Increasing the amount of spawn relative to substrate, using 15 to 20 percent spawn by dry weight rather than the minimum 5 to 10 percent, helps the mycelium colonise faster and take hold before Trichoderma can establish. Inoculate in the cleanest possible environment and always wipe surfaces and tools with isopropyl alcohol first.