Contamination is the greatest challenge in mushroom cultivation – and yet much of it is preventable. The most common culprits are few: learn to recognize and prevent them.
Identifying the most common contaminations
Trichoderma is the classic mushroom pest. Under a microscope, you will see characteristic pyramidal or Y-shaped branched conidiophores with spherical, green spores in sticky heads – not "spore chains." You will see it immediately with the naked eye anyway: a fast-growing, intense green that later turns blackish-green and spreads rapidly. Trichoderma spores are everywhere in the air and the environment – therefore, consistent prevention is crucial.
Cobweb is caused by Cladobotryum spp. (Dactylium dendroides; the main fruiting form of the genus Hypomyces). It looks like a fine spiderweb, silvery-gray and delicate. It spreads faster than mycelium and often smells unpleasant.
Bacillus bacteria are less visually distinct, but cause slimy or foul-smelling substrates – often a sign of poor sterilization or excessive moisture.
Mycelium or mold?
Healthy mycelium is uniformly white, thread-like, and smells pleasantly mushroomy. Green, gray, black, pink, or yellow discoloration, slimy spots, "dusty" areas, or a musty-sour smell, on the other hand, are clear signs of contamination. When in doubt: it is better to discard one culture too many than to let a contamination spread.
Prevention is your best strategy
Prevention is your best strategy. Work under aseptic conditions: wear gloves, disinfect your work surface, and let everything dry. For surface disinfection, 70% isopropanol (alcohol) or diluted bleach (sodium hypochlorite, about 1:10) is suitable. Important: Never mix bleach with other cleaners (especially not with acid or ammonia-containing agents – toxic chlorine gas can form). Ensure good ventilation and protect your skin.
Good sterilization and pasteurization practices are fundamental – no compromises. Store your substrates and cultures in cool, dry places, as many contaminants love damp environments. Ventilation is important, but unfiltered outside air can bring in spores – use HEPA filters for fruiting chambers.
If contamination occurs: act quickly
Small spots of Trichoderma can sometimes be cut out with a sterile scalpel and the affected area disinfected with sterile alcohol – but honestly, complete removal is usually safer. In the case of widespread contamination, it is better to discard the entire substrate so as not to jeopardize your whole project.
Do not open, consume, or shake contaminated cultures. Keep contaminated bags closed, re-sterilize or re-pasteurize them before disposal if possible, and then dispose of them closed in the residual waste. Open composting of contaminated substrates is discouraged: an open compost heap is an ideal spore spreader (especially for Trichoderma) and can re-infect neighboring cultures and the surrounding area. Do not leave contaminated material lying open in your home – the spores can spread.
⚠️ Safety information – Mushroom spores (respiratory tract)
Mature mushrooms release spores. Inhaling large amounts of spores can irritate the respiratory tract and lungs and, with repeated exposure, trigger allergic reactions up to extrinsic allergic alveolitis ("Mushroom Worker's Lung").
- Allow mushrooms to fruit in well-ventilated rooms; do not place them in bedrooms or living areas.
- Harvest mushrooms just before they release their spores to reduce the spore load.
- Sensitive or allergic individuals, those with pre-existing conditions, or those with continuous exposure: wear respiratory protection (FFP2/FFP3).
- Never consume contaminated or moldy cultures; dispose of the bag closed and do not stir up or inhale spores while doing so.
Note on protective equipment: An FFP2/FFP3 mask serves to protect humans from spores, not to maintain the hygiene of the culture. It is particularly useful when handling sporulating or contaminated material.
Learning from contamination
You learn from contamination. Document what went wrong: was the sterilization sufficient? Did you work under sterile conditions? Was the storage too damp or too warm? Every "failure" is an opportunity to refine your craft.