Mycelium vs mold on a grow kit: how to tell
White, soft, fuzzy growth on a fresh block is usually mycelium, the mushroom itself. North Spore says black, green or orange patches, or a foul smell, are the signs of a real problem. Photograph it and contact the seller before you open the bag.
What healthy mycelium looks like
A grow kit is a block of sawdust with mushroom mycelium growing through it. North Spore says a healthy block has white or off-white mycelium weaving through brown substrate, and an earthy smell. White growth is usually mycelium, the mushroom itself, and it is normal.
How much you see depends on the species:
- Oyster: can form a thick, leathery mat. Blue oyster may show yellow-orange droplets, and pink oyster may look rosy.
- Golden oyster: thick and white.
- Lion's mane: faint, and often hard to see at all. North Spore says a lion's mane block often looks uncolonized when it is fine.
Amber beads on the surface are harmless mycelial exudate, per North Spore.
What points to a real problem
North Spore says black, green or orange patches may be transit damage, and a foul smell is the other warning sign. If you see green or black discoloration or smell something foul, the seller says to contact it with photos before you open the kit, and to keep the packaging and the batch sticker.
Shiitake blocks need extra attention. North Spore's shiitake pages suggest spraying Trichoderma, a green mold, with 3% hydrogen peroxide. Field & Forest, another seller, says to add 2 capfuls of bleach per gallon of soak water for green mold on its shiitake blocks. The two sellers give different fixes, so follow the instruction sheet that came with your kit.
What to do next
- Take a clear photo in good light, and note the batch sticker.
- If you have not opened the bag yet, do not. Contact the seller first.
- Read your seller's guarantee. North Spore says its kits are guaranteed to fruit within two weeks or it sends a replacement.
- If you are unsure what you are looking at, our contamination identifier asks a few questions about color, smell and location and points you to a next step. It is a guide, and the seller has the final say.
For the full method, see how to grow mushrooms at home. If the kit simply is not doing anything, try troubleshooting.
Keep reading
- Mushroom grow kit troubleshooting: kits that look fine but are not growing
- What is mycelium?: what the white growth is and what it does
- Contamination identifier: three questions on color, smell and species
Back to How to grow mushrooms at home
Common questions
Is white fuzz on my mushroom kit mold?
Most likely not. North Spore says white growth is usually mycelium, the mushroom itself, and it is normal. Oyster mycelium can form a thick, leathery mat, while lion's mane mycelium is faint.
What color is mold on a grow kit?
North Spore says black, green or orange patches may be transit damage or mold. Send the seller a photo with your batch sticker before you open the kit.
Are the amber droplets on my block a problem?
North Spore describes amber beads as harmless mycelial exudate.
My lion's mane block looks dead. Is it?
Not necessarily. North Spore says lion's mane often looks uncolonized, and kits are guaranteed to fruit within two weeks or a replacement is sent. Keep the block moist and wait.
Sources
- Is my kit growing mold?, North Spore help center. Checked November 11, 2026.
- Spray & Grow Kit FAQ, North Spore. Checked November 11, 2026.
- Using a Spray & Grow Kit, North Spore. Checked November 11, 2026.
- Fruiting Block Kit Instruction Booklet (PDF), North Spore. Checked November 11, 2026.
- 3 pc. Organic Shiitake Fruiting Block Kit Bundle, North Spore. Checked November 11, 2026.
- Shiitake Ready-To-Fruit Blocks, instruction sheet, Field & Forest. Checked November 11, 2026.
How this page is checked
Cedrick Reese wrote this page from North Spore's help-center and FAQ answers on mold versus mycelium, plus shiitake mold advice from North Spore and Field & Forest. Where the two sellers disagree, the page says so. We cannot see your kit, so nothing here is a diagnosis, and a photo sent to your seller beats any guide online. The date at the top shows when each source was last read.