Yellow liquid appeared on the mycelium, mycelium turned a yellowish color, strange yellow-brown line appeared on the mycelium. What could it be? Contamination or not?

Yellow mycelium or Yellow liquid on mushroom mycelium: contamination or not?

Examples of strange yellow liquid on mycelium

Sometimes you can observe healthy mycelium with small amounts of clear yellow, yellowish, orange or brown liquid forming on the surface of the mycelium. Examples in short video:

It can be noticed in spawn jars during colonisation period

Secondary metabolites in spawn jar
Yellow liquid (metabolites) in spawn jar


It can be noticed on mushroom cake during top layer colonisation or fruiting period

Mushroom piss
Mushroom secondary metabolites or exudate


What is it? Is there something to worry about?

What is mushroom metabolites?

Secondary metabolites (aka mushroom exudate, mushroom urea, mushroom pee) β€” it is a waste products of fungal metabolism or mushroom exudate.

πŸ”¬ Biochemical analyses of the exudates showed that acid phosphatase, Ξ²-glucosidase, acid and alkaline protease, RNase polygalacturonase and cellulase enzymes as well as oxalic acid and ammonia were present

In short and simple metabolites consist of acids, exopolysacharides, waste products and antibiotics, which are naturally secreted by mycelium cells.They can be found on different species of mushrooms: wild mushrooms, magic mushrooms, gourmet and edible mushrooms.

Metabolites is a natural immune response of the mycelium to stress or pathogens

How to determine mushroom pee?

βœ… Color of liquid: yellow, orange or brown

βœ… Liquid is transparent

βœ… NO slime

βœ… NO smell, except smell of soil and mushrooms. Fermented, rotted, cider, sweet and sour, stale, yeasty, smell of dirty socks β€” those are clear signs of contamination.

Secondary metabolites on mycelium
Orange and yellow metabolites on mycelium
Mycelium waste products and overlay
Mycelium metabolites and overlay
Psilocybin mushrooms mycelium products
Brown mycelium and yellow liquid as mushroom metabolites example

Main causes of mycelium metabolites

Metabolites is a immunity reaction to:

πŸ†˜ lack of fresh air exchange

πŸ†˜ direct sunlight

πŸ†˜ high temperature

πŸ†˜ lack or excess of nutrients in the substrate

πŸ†˜ mycelium tissue damage

πŸ†˜ a symptom that mycelium is fighting with pathogenic microorganisms (bacteria, mold). In this case you can see yellow or brown lines like borders between them

πŸ†˜ this also happens if you mix several strains of mushrooms. Their colonies struggle with each other and create yellow/brown borders between them

πŸ†˜ stroma and overlay are accompanied by mushroom pee

πŸ”΄ Recommended: Fruiting period. Pinhead initiation. Growing parameters for Psilocybe Cubensis

If mushroom mycelium struggles with contaminants, such mushroom cake requires close attention. Very often mycelium copes with mild contamination successfully. However, if slime and/or smell appears, such mushroom cake or spawn jar should be thrown away immediately

What is the strange yellow-brown line on the mycelium?

Let's figure out one case from Shroomok Discord Community. This is a great example to show you metabolites sectoring.

Question: Strange yellow-brown line on the mycelium. Anyone know what this could be?

Contamination or metabolites?
Contamination or metabolites?

Answer from Shroomok:

Here are mushroom metabolites and sectoring process. Why did this happen?

In this way different colonies of mushroom mycelium or bacteria/mold can temporarily share territory and useful nutrients in the substrate. Since we are dealing with two different microorganisms or different mushroom colonies that compete with each other, usually clear boundaries form between them where they cross aka sectors.

I don't see bacterial or mold contamination so far. Just a struggle between mushroom relatives. As a result we see such immunity reaction of mycelium β€” metabolites.

On the photo below more intense metabolites. It is a sign of struggling with pathogens (i.e. mild bacterial contamination). Spawn can be used for the next steps of cultivation, but you should check it thoroughly (smell, colour) and remove weird areas

Mushroom metabolites and sectoring as a sign of possible bacterial contamination
Mushroom metabolites and sectoring as a sign of struggling with contamination

Yellow mycelium as a type of mushroom metabolites


Occasionally, snow-white mycelium can turn beige, yellow or even brown during colonisation period in the jar or during fruiting period in the fruiting chamber. It is also a kind of fungal exudate β€” waste products of mycelium.

Yellow spots on mushroom mycelium
Mycelium pigmentation


This case can be as a reaction of fungal immunity to a minor contamination. It means mycelium struggles successfully! Observe and be attentive.

Yellowish mycelium is not contam
Yellowish mycelium is not contamination

Minor fungal pigmentation is very common in mushroom growing and nothing to worry about!

Yellowish mycelium
Yellow mycelium is NOT a contamination here

Mycelium metabolites or Wet Bubble disease?

If mycelium ingests excess of anything that may become toxic enough to eventually harm, disable or kill it, mycelium produces waste exopolysaccharides as antimicrobial agents, acids, excrete enzymes and tries rid itself off that excess.

Metabolites is a reaction to stress or response to mild contamination (especially bacteria). Overlay or Stroma is often accompanied by mushroom urea.

Mushroom metabolites is a natural mushroom antibiotic and immunity reaction. This means your mycelium is fighting off pathogens or stress. It’s not contamination itself!

Mycelium urea. Mycelium pee. Mycelium piss. Metabolites or exudate
Metabolites or exudate as immunity reaction of mycelium


πŸ”΄ Newbie cultivators often confuse secondary Metabolites with Mycogon aka Wet Bubble contamination.

It seems similar to metabolites, but Mycogon has specific features. An amber liquid with a rotten smell oozes from pins, primordia and clots of mycelium.

Mycogone or Wet Bubble contamination
Mycogone or Wet Bubble contamination

Metabolites or yellow mold?

πŸ”΄ Newbie cultivators confuse pigmentation with Aspergillus yellow spp. (Aspergillus flavus). This mold has yellow powder without any liquid on mycelium.

Yellow Aspergillus on mushroom mycelium
Aspergillus flavus on mushroom mycelium


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Have a happy growing and healthy mushrooms!

  1. Growing Troubleshooting
  2. Mushroom metabolites