Most contamination in grain spawn is easy to identify once you know what you are looking at. The hard part is the in-between: patches that look odd but turn out to be normal, and early contamination that still looks white. This guide covers both. One rule applies throughout: if you are unsure, keep the bag sealed and wait 24 to 48 hours. Mycelium gets whiter and more even. Contamination gets more obvious.
What healthy grain spawn looks like
- Colour: white to off-white, spreading outward from where the inoculant went in.
- Texture: cottony fuzz (called tomentose growth), ropey branching strands (rhizomorphic growth), or a mix of both.
- Progress: visible spread every day or two at the right temperature, around 21 to 24°C (70 to 75°F) for most species.
- Smell: fresh and earthy, like fresh mushrooms or a forest floor. Never sour, rotten or sharp.
- Kernels: plump with a dry-looking surface, no pooling water.
Some species look different, and new growers often mistake these for problems:
- Lion's mane grows thin, wispy, almost transparent mycelium at first. It looks weak next to oyster, but it is normal.
- Shiitake turns patchy brown as it matures, especially where light reaches it. Browning is an expected stage for shiitake.
- Reishi often takes on a yellowish tint and a tougher, leathery feel as it matures.
- Pink oyster may show pink patches in a mature bag. These are usually the first pins forming.
- Oysters in general grow dense, bright white and fast.
Normal things that look scary
Yellow or amber liquid (metabolites)
Droplets of yellow to amber liquid on the mycelium, or a little pooled at the bottom of the bag, are metabolites. The mycelium releases them, often when it is older, warm, or pushing against something. A little is normal and harmless. Heavy metabolites are a cue to look closely at the rest of the bag. If the liquid is cloudy, the nearby kernels look slimy, or the bag smells sour, suspect bacteria.
Ropey strands
Rhizomorphic growth, thick strands that branch like roots, is healthy and often a sign of vigorous mycelium. It is not mould.
Patchy, uneven early growth
Mycelium starts from wherever the inoculant landed, so early growth is often lopsided, with white islands and bare grain between them. That is what the break and shake is for. Our guide on how long grain spawn takes explains the timing.
Pins inside the bag
Small mushrooms forming inside a fully colonised bag, especially with oysters, mean the spawn is ready and wants to fruit. Use it soon.
Condensation
Water droplets on the inside of the plastic come from small temperature swings and are fine. A real pool of water sitting in the bottom of the bag is different: it means the grain is too wet, which raises the risk of bacteria.
Dense white knots
Mycelium sometimes thickens into dense white clumps, especially in spots with more air. That is normal. The difference from early green mould is time: mycelium stays white for days, while Trichoderma turns green within one to three days.
Mould contaminants
Green mould (Trichoderma)
Trichoderma is the most common contaminant of grain spawn. It usually starts as a bright white, dense, fast-growing patch that can look like extra-healthy mycelium. Within one to three days it turns green from the centre outward, pale mint at first, then deep forest green and powdery. You may see a clear boundary or a yellowish line where it meets your mycelium. A bag with green mould will not recover.
Blue-green mould (Penicillium)
Smaller, slower powdery spots in blue-green to grey-green, often starting on individual kernels.
Black, yellow-green or brown powdery mould (Aspergillus)
Powdery patches in black, yellow-green or brown. Some Aspergillus species can be a health concern if their powder is inhaled, so never open or sniff these bags.
Grey fluff with black dots (pin mould)
Very fast, grey, wispy growth with tiny black or grey dots on stalks, like pins in a pincushion. It often follows grain that was too wet or not fully sterilised.
Orange or salmon-pink mould
Fast, powdery orange to salmon-pink growth. It spreads aggressively and is notorious for getting into neighbouring bags, so remove the bag from your incubation area straight away.
Bacterial contamination
Bacteria do not make fuzz, so they are easier to miss. Look for:
- Wet, glossy or slimy kernels, often in a patch, that look darker or greasier than the rest
- A milky, grey or cloudy film on the grain or the plastic
- Mycelium that stops at the edge of certain kernels and refuses to grow over them
- A stalled bag that started well and then stopped spreading
- A sour smell near the filter patch: vinegar, rotten fruit, sour milk or dirty socks
The usual causes are grain that was too wet, burst kernels, incomplete sterilisation that let heat-tolerant bacteria survive, or a contaminated inoculant.
The signs table
| What you see or smell | Most likely | What to do |
|---|---|---|
| White fuzz or ropey strands, fresh smell | Healthy mycelium | Keep incubating |
| Yellow or amber droplets, no bad smell | Metabolites | Normal; use the spawn once colonised |
| Brown patches on shiitake | Normal browning | Nothing |
| Pink patches (pink oyster) or small mushrooms in the bag | Pins forming | Spawn is ready; use it soon |
| Droplets on the plastic | Condensation | Normal; keep the temperature steady |
| Water pooled in the bottom | Grain too wet | Watch closely for bacteria |
| Bright white patch turning green in 1 to 3 days | Trichoderma | Dispose, sealed |
| Small blue-green powdery spots | Penicillium | Dispose, sealed |
| Black, yellow-green or brown powder | Aspergillus | Dispose, sealed; do not open |
| Grey wispy fluff with black dots | Pin mould | Dispose, sealed |
| Orange or salmon powder | Orange mould | Remove at once, away from other bags |
| Wet, slimy, glossy kernels, sour smell | Bacteria | Dispose, sealed |
| No growth at all after 10 to 14 days | Weak or dead inoculant, or too cold | Check temperature; reinoculate only if the grain shows no other signs |
When it appears tells you why
- In the first few days, scattered through the bag: the grain or sterilisation was the problem, or the bag was damaged.
- Starting at the injection point: something came in with the inoculation, from the needle, the port, your hands or the inoculant itself.
- Every bag inoculated from the same source: the inoculant is contaminated.
- One bag out of several from the same source: technique, or a fault in that one bag.
- Late, near the filter patch or a seam: a wet or damaged filter, a weak seal, or a contaminant that was hiding until the mycelium slowed down.
What to do with a contaminated bag
- Do not open it indoors. Opening releases mould and bacteria into the room.
- Seal it in a second bag and take it out of your incubation area.
- Dispose of it with household waste. Some growers pressure cook contaminated bags before throwing them out.
- Wipe down the incubation shelf and anything the bag touched with 70% isopropyl alcohol.
- Work out the cause using the timing list above, so the next bag goes better.
If a bag from us arrives contaminated or damaged, send a photo within 7 days and we will replace or refund it. Details are on our returns page.
How to prevent contamination
- Clean technique at inoculation. Wipe the injection port with 70% isopropyl alcohol, flame the needle, work in still air, wash your hands and wear gloves. Our clean technique guide and inoculation tutorial cover it in full.
- Inoculate generously. Faster colonisation leaves contaminants less time to get established.
- Keep the temperature right. 21 to 24°C (70 to 75°F) suits most species. Above about 27°C (80°F), bacteria and Trichoderma speed up while most gourmet mycelium slows.
- Do not shake too early. Wait for clear, actively spreading growth before the break and shake, and never shake a bag that looks suspect.
- Keep the filter patch dry and uncovered. A wet or blocked filter is a weak point. Our guide to grain spawn bags explains why.
- Start with good grain. Our grain spawn arrives fully hydrated and pressure sterilised, so the most common causes of bacteria, wet grain and under-sterilisation, are dealt with before it reaches you.
Quick checklist
- White, fresh-smelling and spreading: healthy.
- Yellow droplets, ropey strands, patchy early growth, pins: usually normal.
- Any green, blue-green, black, grey or orange powder: mould. Dispose, sealed.
- Wet, slimy kernels and a sour smell: bacteria. Dispose, sealed.
- Unsure? Keep it closed and look again in 24 to 48 hours.
- Use the timing of the problem to find the cause.
Frequently asked questions
What does green mould on grain spawn mean?
What does bacterial contamination look like in grain?
Is yellow liquid on mycelium contamination?
Can you save contaminated grain spawn?
