Contamination guide
Mould or normal? Contamination in a grow kit
Not every colour you see on a grow kit is mould, but some colours are a sure sign. Below are the harmful contaminants and the normal things that look like mould, side by side with photos.
Never eat mushrooms from a block with coloured (green, blue, black, orange) mould. Do not open, sniff or shake a mouldy block indoors; bag it twice, tie it and put it in household waste. People with asthma or a weakened immune system should not handle a mouldy block.
Harmful: types of contamination
Vicenny · CC BY-SA 4.0
Green mould (Trichoderma)
- What it looks like
- A dense, bright white mat first; within days it turns into dark green powdery patches.
- Cause
- A wet, stuffy or tired block; dust in the room.
- What to do
- Bag the block twice, tie it and put it in household waste. Do not try to cut the patch out.
Photo: Trichoderma on a petri dish; it shows the same green rings on a block.
Luis Fernández García · CC BY-SA 4.0
Blue-green mould (Penicillium)
- What it looks like
- Blue-green powdery mould spreading in rings.
- Cause
- Airborne spores; opening the bag more than needed.
- What to do
- Close up and discard the affected block. Do not eat its mushrooms.
Photo: Penicillium on fruit; the colour and texture are the same.
Nikolai Bulykin · CC BY-SA 4.0
Black mould (Rhizopus, Aspergillus)
- What it looks like
- Grey fluffy threads topped with black (sometimes yellow-green) spore heads; a musty smell.
- Cause
- Warm, humid spells, flies, household dust.
- What to do
- Close it up and discard at once; do not open or sniff it indoors.
Photo: Rhizopus (black bread mould) on cooked grain.
Orange mould (Neurospora)
- What it looks like
- Bright orange clumps of spores that spread within hours.
- Cause
- Warm, humid conditions; substrate that was not sterile enough.
- What to do
- Bag it without touching, tie and discard. Moving it releases a spore cloud.
An et al. (2022) · CC BY 4.0
Cobweb mould (Cladobotryum)
- What it looks like
- Sparse, wispy grey-white threads creeping over mushrooms and pins.
- Cause
- Poor airflow, high humidity.
- What to do
- If very early, cover the spot with a damp paper towel and salt it; if it has spread, discard the block. Improve airflow.
Photo: affected mushrooms and culture (scientific figure).
Bacterial blotch (Pseudomonas)
- What it looks like
- Yellow-brown, sticky, wet spots on the caps; sometimes a bad smell.
- Cause
- Water sitting on the mushrooms, condensation.
- What to do
- Discard spotted mushrooms and never eat soft or smelly ones. Mist the air, not the caps.
John Tann from Sydney, Australia · CC BY 2.0
Fungus gnats
- What it looks like
- Tiny black flies; holes in the mushrooms, larvae trails on the block.
- Cause
- Damp organic matter, pot plants, an old block.
- What to do
- Hang a yellow sticky trap; finish and discard an old block.
Photo: an adult fungus gnat (Sciaridae).
Normal: things that look like mould
Rob Hille · CC BY-SA 3.0 Bright white, silky mycelium
The white web through the block is the mushroom itself. It smells of fresh mushrooms or soil.
Rickproser · CC BY-SA 3.0 Cotton-like fuzz at the stem base
White, fluffy aerial mycelium: not enough fresh air, so ventilate. If it turns green, it is mould.
Yellow or amber droplets
Mycelium metabolites; normal if odourless. If heavy, move the block somewhere a little cooler and airier.
Keeblur · CC BY-SA 3.0 White, pink or lilac powder
Oyster spores: a sign of a late harvest. Pick the next cluster a little earlier.
Pradejoniensis · CC BY-SA 3.0 A brown crust on a shiitake block
A normal ripening stage; shiitake fruits through this skin.
Quick decision table
| What you see | Smell | Most likely | Decision |
|---|---|---|---|
| Bright white, silky, spread through the block | Mushroomy, earthy | Healthy mycelium | Normal |
| White turning to green powder | Mouldy | Trichoderma | Discard |
| Blue-green rings | Mouldy | Penicillium | Discard |
| Grey fluff + black dots | Mouldy, sour | Rhizopus / Aspergillus | Discard |
| Bright orange clumps | Sour, yeasty | Neurospora | Discard |
| Wispy grey web over mushrooms | Mouldy | Cobweb mould | Early: salt; spread: discard |
| Yellow droplets | None | Metabolites | Normal |
| Lilac, pink, white powder | Mushroomy | Spore dust | Normal; harvest earlier |
| Wet yellow-brown spots on caps | Bad | Bacterial blotch | Discard spotted mushrooms |
Questions
There is mould on my kit. Can I eat the mushrooms?
No. Do not eat mushrooms from a block with green, blue, black or orange mould. Bag the block twice, tie it and put it in household waste.
Is white fuzz mould?
Usually not. Bright white, silky mycelium and cotton-like fuzz at the stem base are normal and point to too little fresh air. If a white mat turns green within days, it is Trichoderma.
There are yellow droplets on the block. Is that a problem?
Yellow or amber droplets are mycelium metabolites and normal if odourless. If there is a bad smell or coloured mould with them, discard the block.
How do I prevent mould?
Open the bag only at the cut, mist the cut and the air (do not let water pool), air the room a few times a day, keep the kit away from pot plants and fruit bowls, and do not keep an old block around.
Sources
- Penn State Extension: Green Mold of Mushrooms
- Penn State Extension: Fungal Diseases of Mushrooms (IPM)
- Penn State Extension: Sciarid Fly in Commercial Mushroom Production
- GroCycle: Mushroom Contamination, 11 Common Contaminants
- North Spore: Common Contamination in Mushroom Cultivation
- North Spore: Is my mushroom spawn moldy?
- Frontiers in Industrial Microbiology (2025): Neurospora sitophila orange mold
- AHDB: Symptoms of bacterial blotch on mushrooms
- Out-Grow: Shiitake bag cultivation
Sources checked on 5 November 2026. Photos from Wikimedia Commons; licence and author on each photo and on the image credits page.
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