Start here: it is probably not cobweb
Almost every week somebody photographs a white fuzzy patch on a block and calls it cobweb, and almost every week it is the block behaving normally. Three things get mistaken for it constantly, and all three are signs of a healthy colony in the wrong conditions:
- Aerial mycelium. A block sitting in its own carbon dioxide throws fluffy white growth up off the surface instead of fruiting. It is white, not grey, it grows over days rather than hours, and it is the same organism you inoculated. The fix is fresh air, not the bin. Our pinning guide covers the same symptom from the other side.
- Hyphal knots and early pins. Just before primordia form, mycelium gathers into dense fuzzy lumps. They look alarming for about a day and then turn into mushrooms.
- Mycelium healing a wound. Anywhere you have cut, scratched or pinned the bag, the colony throws a fresh white bloom over the damage. It is bright white and tight to the surface.
None of those is grey, and none of them moves while you watch. That is what the test below is really for: not confirming cobweb, but ruling it out cheaply before you destroy a block that was fine.
The three-hour test
Cobweb is the fastest-growing thing that will ever appear in your fruiting room. That speed is the only signal that is hard to fake, so make it the one you decide on.
- Mark the edge of the patch. A pen line on the bag, or a photograph with something fixed in frame, is enough.
- Leave it three hours in the same conditions. Do not mist it and do not poke it.
- Look again. A visibly larger patch is cobweb. An unchanged patch is your own mycelium, whatever it looks like.
If you cannot wait three hours, the second-best check is a dry cotton bud drawn lightly across the growth. Cobweb lifts away in a loose sheet because it is barely attached. Mycelium resists, because it is gripping the substrate it is eating.
The four tells, ranked by how much they are worth
| Signal | Cobweb | Healthy mycelium |
|---|---|---|
| Speed (the decider) | Advances in hours, can cover a surface overnight | Advances over days, nothing changes in an afternoon |
| How it sits | Loose on top, lifts off in a sheet | Rooted into the substrate, resists a wipe |
| Colour | Grey and translucent, sometimes almost colourless | Bright white, opaque, sometimes with clear droplets |
| Smell (weakest) | Musty and damp, or nothing at all early on | Faintly mushroomy, or nothing |
Colour is where most misdiagnosis happens, because early cobweb can look white and vigorous mycelium can look grey in poor light. It is third on the list for a reason. If you want to work through colour, texture, where it is growing and smell together, the free contaminant identifier asks for all four and names the likeliest culprit plus the test that confirms it.
What cobweb actually is, and why it turns up in the fruiting room
Cobweb is Dactylium (also catalogued as Hypomyces or Cladobotryum), and the important thing about it is that it is a mycoparasite: it eats mushroom mycelium and mushroom fruit bodies. It is not competing with your culture for sugar the way green mould is. Your crop is its food.
That single fact explains its whole behaviour on a farm:
- It appears after the block is colonised and out on a fruiting shelf, not during incubation, because that is when its food is available and exposed.
- It moves faster than anything else in the room, because it is not building a colony from scratch, it is consuming one that is already built.
- It will jump to neighbouring blocks on the same shelf. A spot of cobweb is a room problem, not a block problem.
- It spoils mushrooms directly, leaving soft brown wet patches on caps rather than just spoiling the substrate.
Cobweb is a message about your air, not your sterile technique
This is the part worth internalising, because it changes what you fix. Green mould tells you a sterilisation cycle fell short or an inoculation was open too long. Cobweb tells you the fruiting air is stagnant and too wet. You can run a flawless lab and still get cobweb, and tightening your lab will not stop it.
The conditions it wants are: humidity held very high with no fresh-air exchange to move it, standing water on surfaces and bag tops, warm still corners, and blocks packed tightly enough that air does not pass between them. Every one of those is a fruiting-room setting rather than a bench habit.
Caught early, the block can usually be saved
Cobweb is the one common contaminant that is often recoverable, which is exactly why the three-hour test is worth doing before anything drastic. Working order:
- Get it out of the room first, gently. Bag the block where it stands rather than carrying an open patch past twenty clean blocks. Disturbing it is how one block becomes a shelf.
- Stop misting that block. Free water on the surface is what it is living on.
- Raise fresh-air exchange sharply for the whole room and let humidity drop for a day. Drier moving air is the actual treatment; everything else is tidying.
- Check the neighbours daily for three days. Cobweb that has been in the room a while is usually on more than one block by the time you see it on one.
- Once it has taken on colour, it goes out. A patch turning yellow, pink or rusty is sporulating and no longer worth saving. That is the line.
On spot treatments, be honest with yourself about what they do. A light spray of 3% hydrogen peroxide or a dusting of salt on an isolated patch kills what it touches and buys time on a surface, and plenty of growers use both. Neither reaches mycelium that has already run below the skin of a sawdust block, and neither changes the conditions that invited it in. If you treat a patch and leave the airflow alone, expect it back on the same shelf within the week.
What to change so it does not come back
- Fresh-air exchange before humidity. Most small rooms are run to hold a humidity number and the air never moves. Aim for genuine air changes, not a humidifier fighting a sealed box.
- Stop water pooling. Sloped shelves, no puddles on bag tops, no misting directly onto colonised surfaces late in the day.
- Space the blocks. Air has to reach the back of the rack, not just the front of it.
- Clear spent blocks out promptly. A finished block on a shelf is exposed mycelium with nothing left to defend itself, which is where cobweb establishes first.
- Keep incubation and fruiting apart. Cobweb's food is out on the fruiting shelves; do not give it a path back to blocks that have weeks of colonising left.
- Write down which room and which shelf. Cobweb recurs in a place. You will not remember in six weeks that the last two outbreaks were both on the bottom tier by the door.
Our contamination guide covers the other four culprits and the habits that prevent each of them, and the fruiting conditions cheat-sheet gives the humidity and fresh-air targets for fourteen species so you know what "too humid" actually means for what you are growing.
Find out what is really costing you blocks
Mycro's free Grow Log records what you lost each run and what took it, then ranks the causes by blocks lost rather than by how recent they feel. The Culture Library traces a flagged plate or jar down to every block that came off it. Free, no account, no cap on how many batches you track.
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Frequently asked questions
How do you tell cobweb mold from mycelium?
By speed first, then by how it sits. Mark the edge of the patch, leave it three hours and look again: cobweb mould visibly advances in that time and mushroom mycelium does not. Cobweb is grey and translucent, fine as spider silk, and lies loosely on the surface so a dry cotton bud lifts it away in a sheet. Healthy mycelium is white, ropey and rooted, and resists the bud rather than peeling off.
Is cobweb mold dangerous?
It is dangerous to the crop rather than to you. Dactylium is a mycoparasite, meaning it feeds on mushroom mycelium and the fruit bodies themselves, which is why it can take a whole shelf in a couple of days. It is not regarded as a human pathogen the way Aspergillus is, but do not eat mushrooms off an affected block, do not disturb a coloured patch indoors, and be careful not to mistake black or blue-green mould for it, because that one genuinely is a respiratory hazard.
What does cobweb mold smell like?
Musty and damp, like a cloth left wet in a bucket, and often nothing at all in the first day. Smell is the weakest of the signals and should never be the one you act on: early cobweb is frequently odourless, and a sour or rotten smell points at a bacterial problem such as wet spot instead. Judge cobweb on how fast it moves and how loosely it sits, and treat the smell as confirmation rather than evidence.
Can you get cobweb mold on agar or in grain spawn?
It is possible but uncommon, and if you think you have it on a plate it is worth a second look. Cobweb lives on mushroom mycelium and normally arrives on moist air in the fruiting room, not through a sterilised jar, so grain and agar much more often carry Trichoderma, black mould or a bacterium. On agar the giveaway is the same as on a block: a fine grey growth that crosses the plate in a day while your own culture takes several. Discard the plate, never try to cut it out, and check whether your still-air work or your incubation space is the real source.
Identification from appearance is always a judgement — confirm with the test given above before you destroy a block, and when a patch is black or blue-green do not put your face near it. Mycro is for legal culinary & medicinal mushrooms only.