Mushroom Contamination Guide: Green Mould, Cobweb and Bacterial Blotch

Read this first

Nothing in this guide identifies an organism. Visual symptoms overlap between very different problems, several distinct fungi produce similar-looking growth at similar stages, and bacterial problems in particular usually require laboratory work to identify with any confidence. What a grower can realistically do is recognise a pattern, act conservatively, and change something in the process next time.

If you want to work through what you are looking at question by question, the mushroom contamination checker walks the same reasoning as this guide and ends with concrete next steps.

Everything here refers to legal gourmet and culinary cultivation. Do not eat mushrooms from a grow you are unsure about.

1. What contamination actually means in cultivation

Cultivation is a competition you have rigged. You sterilise or pasteurise a substrate to remove the existing microbial community, then introduce one organism with a head start and hope it claims the material before anything else arrives.

Contamination is simply that rigging failing — something other than your culture establishing itself. It usually arrives from one of four places: material that was never fully treated, air reaching the substrate while it was open and vulnerable, hands, tools or clothing carrying spores in, or a grow space that has been quietly accumulating spores from previous failures.

That last route matters more than beginners expect. Penn State Extension's guidance on green mould specifically identifies contaminated personnel, equipment and poorly cleaned rooms as ways contamination moves through a growing operation. One failed bag left open on a shelf can seed the next three.

2. Green mould (Trichoderma-type) patterns

This is the contamination most growers meet first, and the one most often misread early.

What the pattern looks like. Penn State Extension describes green mould as growth that can begin as white mycelium before producing green spores. That is the key point: at the start it does not look green at all. A patch that looked like healthy white growth two days ago and is now grey-green, olive or bright green fits the pattern. The greening is sporulation.

Because of this, "it was white so it was fine" is not a safe conclusion. What tells you something is wrong is the change, not the initial colour.

Why it spreads. Once a patch has sporulated, the spores are on your shelf, your tub, your hands and your sleeves. Penn State's guidance is explicit that contaminated personnel and equipment, and rooms that are not cleaned properly, are how it travels. Cutting the green patch out of a block on your kitchen worktop is one of the most effective ways to distribute it around a room.

What to do. Isolate the container first. Leave it sealed. Take it outdoors before opening or disposing of anything, and dispose of or compost it according to your local waste practice. Then clean the shelf, tub, tools and surfaces near it before you touch anything clean. Do not eat mushrooms from an affected block.

3. Cobweb-type fungal growth

What the pattern looks like. Penn State Extension describes cobweb mould as loose, cotton-like or weft-like white to grey growth spreading across a surface. Compared with the mycelium you were expecting, it tends to look wispier, flatter and faster — a haze creeping over the casing or substrate rather than a dense mat rooting into it. Some cobweb-type growth later develops a greyish cast.

Why handling matters. The same guidance notes that its spores disperse readily with air movement and watering. This is the practical reason cobweb-type growth deserves different handling from green mould: fanning it, misting across it or wiping it is exactly the action that spreads it.

What to do. Move the container out of your grow space before doing anything with it, and do not mist, fan or wipe across the growth. If it is widespread, seal it, take it outdoors and dispose of it. Clean the tub and shelf afterwards. Reducing standing surface wetness and improving air movement in the chamber both remove conditions this kind of growth enjoys.

4. Bacterial blotch on fruit bodies

This one appears on the mushrooms themselves rather than in the substrate, and it is a moisture problem before it is anything else.

What the pattern looks like. Penn State Extension describes lesions that can progress from pale yellow toward brown, and notes that prolonged wet cap surfaces favour the disease. AHDB associates blotch with persistent moisture on caps, high humidity, and temperature fluctuation. The marks are superficial discolouration of the cap surface — not a fuzzy growth sitting on top of it, which points elsewhere.

Why it happens. A cap that stays wet for hours is a different environment from a cap that is damp and then dries. Free water sitting on the surface is the condition being described in both sources. Fluctuating temperature makes this worse, because a cool cap surface in warm humid air collects condensation without you misting at all.

What to do. AHDB's practical levers are airflow and keeping cap surfaces from staying wet, and that maps directly onto home setups: stop misting developing caps directly, mist the chamber walls instead, and raise fresh-air exchange so surfaces dry between waterings. Remove and discard badly affected fruit bodies, and clean handling surfaces and knives between grows.

If you are unsure whether your chamber is actually moving enough air, the air exchange calculator sizes it from the room dimensions, and the air exchange guide explains the trade-off between ventilation and holding humidity.

5. Wet, slimy substrate and sour odours

These are warning signs, not an identification. A patch of uncolonised material that stays wet, turns slimy, or discolours, and a sour or rotten smell you can notice in the room, are non-specific. They can indicate bacterial contamination, they can indicate other organisms, and they can indicate substrate that was simply too wet to colonise.

Naming a pathogen from a smell is not something this guide will do, and neither should anyone else. What the signs justify is caution: isolate the container, do not use the material, and do not try to rescue it into a bulk substrate.

Do not lean in and deliberately sniff a suspect bag. You gain nothing reliable and you risk inhaling a lungful of spores. If a smell is noticeable at normal distance, that is your answer.

The usual process causes are worth reviewing honestly: hydration level, sterilisation or pasteurisation time and temperature relative to load size, whether bags had fully cooled before inoculation, and how clean the inoculation step really was. The substrate hydration calculator makes the first of those repeatable rather than a matter of feel.

6. Normal mycelium versus contamination

More grows are binned unnecessarily than are eaten dangerously, and the reason is almost always this section.

Healthy mushroom mycelium is white, and it does not look uniform. It produces dense ropey rhizomorphs in some places and soft, fluffy, tomentose patches in others, sometimes side by side on the same block. Fluffy white growth on its own is close to meaningless as a contamination signal, and teaching people that every cotton-like patch is cobweb produces a lot of wasted substrate.

Other perfectly normal things that alarm beginners: clear or amber droplets on the mycelium surface, which is metabolic guttation; patchy or slower colonisation across a grain bag; and browning or a light crust on some species as they mature.

What genuinely deserves attention is a change: colour appearing where there was none, growth of a visibly different texture spreading across a surface, an odour noticeable in the room, or wet slimy patches in material that should be dry and colonised. Photograph the area and compare it in two or three days rather than deciding in the first thirty seconds.

Unusual fruit body shapes — long stems, small caps, aborted pins — are much more often an environment problem than a disease. Penn State's guidance associates high CO2 with elongated stems and small caps in oysters, but there is no universal morphology chart, and it is a reason to check fresh air rather than a diagnosis.

7. What to do when you are not sure

The safe default is the same in almost every case, which is convenient, because the safe default does not require you to know what the organism is.

1. Isolate. Move the container away from your other grows and away from your clean work area.
2. Do not disturb it. Keep it sealed. Do not cut into it, shake it, wipe it or open it indoors to have a closer look. Disturbing sporulating material indoors is how one problem becomes several.
3. Document it. Photograph it with the date. A comparison photo in three days tells you more than any amount of staring.
4. Dispose outdoors if it progresses. Seal it, take it outside, and dispose of or compost it according to your local waste practice.
5. Clean afterwards. Surfaces, tubs, tools, shelves — anything the container touched or sat near.
6. Do not eat it. No suspect grow is worth it.

8. Prevention themes that actually move the numbers

None of these are exotic, and none of them require a laminar flow hood.

• Clean the space, not just the moment. Rooms and shelves accumulate spores between grows. Wiping down between batches is more effective than any single heroic sterile technique.
• Hands, tools and surfaces immediately before contact. Not ten minutes before. Wipe the injection port, sterilise the needle, work on a cleaned surface.
• Treat the substrate properly for its load size. A larger batch needs longer to reach temperature throughout, and a partially treated core is a reliable source of failure.
• Get hydration right. Free water pooling in the base of a bag is a standing invitation. Weigh rather than judge by feel.
• Reduce cross-contamination between grows. Do not handle a suspect container and then a clean one. Keep isolated grows genuinely isolated.
• Control persistent surface moisture and airflow during fruiting. Caps that dry between mistings are the single most useful lever against blotch.
• Keep records. Contamination is a pattern problem. One bad bag is bad luck; three from the same shelf, the same grain batch or the same lineage is a cause you can actually fix.

If you are still putting a setup together, the equipment checklist separates what is genuinely essential from what is optional for your workflow, and the mushroom growing equipment guide covers the buying decisions behind it. For clean-work kit specifically, see the sterile workflow gear; for airflow and humidity control during fruiting, the fruiting environment gear.

Tracking contamination rather than just reacting to it

Hyphae Fidelity lets you mark a batch as contaminated, record which type it appeared to be and your own notes, and then look across your grows to see how often it happens, which stage it tends to appear at, and where in a lineage it first showed up. Your grow records are private to your account.

That last part is the difference between losing bags and fixing a process. A single contaminated block tells you almost nothing. Ten logged events with dates, stages and types tell you exactly which step of your workflow to change.