Clean agar is where every clean grow starts — the plate is your quality gate before a contaminant ever reaches grain or a fruiting block. Get the pour right and every downstream step gets easier; get it wrong and you're chasing green mold through your whole line for weeks.
What agar is for
A poured plate does three jobs in a mushroom operation, and it's worth being clear on which one you're doing before you mix media:
- Isolate a clean culture — take a tissue sample, spore print, or vendor culture and get it growing on a surface where you can actually see what it's doing.
- Expand it — multiply a clean culture across more plates so you have enough mycelium to inoculate grain spawn or liquid culture.
- Clean up a contaminated one — chase a healthy leading edge of mycelium away from bacteria or mold by transferring wedges across successive plates (more on this below).
All three depend on getting the media right first. If you want exact weights scaled to the batch size you're pouring, run them through the Agar & Media calculator → rather than eyeballing it — agar percentage in particular is unforgiving in both directions.
The media recipes
Three recipes cover nearly everything a gourmet mushroom operation needs. All are given per litre of water; scale up or down from there.
| Recipe | Per litre | Notes |
|---|---|---|
| LMEA | Light malt extract 20 g + agar 20 g | Simplest, most forgiving — best for beginners |
| MEA | Malt extract 20 g + agar 20 g (+ peptone 2 g optional) | General-purpose; peptone gives a small nitrogen boost |
| PDA (DIY) | Potato 200 g infusion + dextrose 20 g + agar 18 g | Vigorous growth; more prep work (boil, strain the potato) |
| PDA (commercial powder) | 39 g powder | Same result as DIY PDA, far less hands-on time |
At roughly 20 mL per 90 mm plate, a litre of media pours 40–50 plates, depending on how generous you are with the pour and how much you lose to the sides of the flask.
Which to reach for: start with LMEA if you're new to plate work — it's the easiest recipe to get right and grows clean enough that you can actually learn to read healthy mycelium versus early contamination. Move to PDA when you want more vigorous, faster colonization for a species that's slow to establish. Use MEA as your default general-purpose media once you're comfortable — it sits in the middle and covers most species well.
Pouring plates, step by step
Mix the media
Weigh your agar and nutrient source per litre of water per the table above, and get it into solution — agar needs heat to fully hydrate, so bring it to a simmer while stirring before it goes in for sterilization, not after.
Sterilize 30–45 minutes at 15 psi
Run the media in a pressure cooker or autoclave at 15 psi (250°F) for 30–45 minutes. This is doing two jobs at once — killing anything that could contaminate the batch, and fully dissolving the agar so it sets evenly.
Cool to ~120–130°F
Let the media cool before you pour — straight off the sterilizer is far too hot. You want it liquid but no longer scalding, roughly 120–130°F. Too hot and you cook condensation into the lid; too cool and it starts gelling in the flask before you're done pouring.
Pour ~20 mL per plate
Pour in a still-air box or flow hood, working quickly. About 20 mL per 90 mm plate gives you a good working depth without wasting media. Keep the stack of open plates small — every second a lid is off is a second for something to land on it.
Let set, then rest 2–3 days
Let poured plates gel undisturbed on a flat surface, then don't use them right away. Rest them 2–3 days before inoculating — this is free contamination insurance. Anything that snuck in during the pour will usually show itself in that window, and it gives condensation on the lid time to clear instead of dripping onto the agar surface later.
Sterile technique that actually holds
The recipe gets you clean media; technique is what keeps it clean once the lid comes off. A still-air box — a clear plastic tub with arm holes — is enough for most home and small-farm work. It removes the moving air that carries spores onto an open plate. A laminar flow hood is faster and more forgiving at volume, and worth the upgrade once you're pouring and transferring at production scale, but it isn't a requirement to get clean plates.
Inside either one, the fundamentals don't change:
- 70% isopropyl on gloves, tools, and the work surface before you start — and again between contaminated and clean transfers.
- Flame the scalpel or transfer tool to red heat, then let it cool a few seconds before it touches agar. Touching hot metal to media cooks and cracks the surface right where you're trying to work.
- Minimize open time. Have everything staged and open the plate for seconds, not minutes. Every extra second with a lid off is exposure you didn't need to take.
- Work away from your own air. Don't talk, breathe, or lean directly over an open plate — you are, yourself, a contamination source.
Transfers and cleaning up a culture
Once you have a clean, colonized plate, three transfer moves cover most of what you'll do with it:
Agar to agar
Cut a small wedge from the leading edge of mycelial growth — the actively growing rim, not the older center — and place it mycelium-side-down on a fresh plate. This is how you multiply a clean culture or keep a strain going indefinitely without it aging out.
Sectoring away from contamination
If a plate shows a patch of mold or bacteria but the mushroom mycelium is still growing cleanly elsewhere, take your wedge from the point on the leading edge farthest from the contamination. Transfer that wedge to a new plate and watch the next generation closely — repeat over a few transfers if needed, chasing the clean, actively growing edge away from the problem each time. It doesn't always work, but it's usually worth one or two attempts before writing a culture off.
Agar to grain, agar to liquid culture
Once a plate is fully and cleanly colonized, wedges from it can inoculate grain spawn jars directly, or go into a jar of sterilized liquid culture (usually a light sugar-water solution) to multiply the mycelium in suspension before it's used to inoculate grain at volume.
When the standard advice is wrong
A few habits that "seem" careful actually cause the failures they're trying to prevent:
- Pouring too hot. Media straight off the sterilizer will stream condensation onto the lid as it cools sealed, and pouring at that temperature can shock-kill the very culture you transfer onto it later if you rush the next step. Wait for ~120–130°F.
- Too many plates open at once. Staging a big stack of empty plates so you can "pour faster" means most of them sit open and exposed while you work through the batch. Pour in smaller batches with lids on between.
- Skipping the 2–3 day rest. Using a plate the same day it's poured skips the window where contamination would have shown itself. It feels efficient and costs you a ruined transfer later.
- Leaning on antibiotics instead of technique. Gentamicin gets added to agar to suppress bacterial contamination, but reported doses in the wild span a huge range — 10 to 500 mg/L — and it only knocks back some bacteria, not all of them, and does nothing for mold. It's a patch for bad technique, not a substitute for it. Fix your sterile technique first; treat antibiotics as a narrow tool, not a safety net.
The lesson that took me longest to actually internalize wasn't a recipe — it was that contamination is almost always a technique problem wearing a "bad batch" costume. Early on I'd blame the agar, the strain, the humidity. Nearly every time, it traced back to how long a lid was off, or a tool that wasn't actually cool when it touched the plate. The plates that fail are the ones where I got impatient. Slow down at the pour and the transfer, and the failure rate drops more than any single ingredient change ever will.
FAQ
Do I need a flow hood or is a still-air box enough?
A still-air box is enough for most home and small-farm culture work. It removes the air currents that carry spores onto an open plate, and combined with good technique — flamed tools, minimal open time, working away from your own air — it holds up fine. A flow hood is faster and more forgiving at volume, worth the upgrade once you're pouring and transferring at production scale, but not a hard requirement to get clean plates.
What agar is best for beginners?
Light malt extract agar (LMEA). It's the simplest recipe to get right, forgiving of small measurement errors, and grows clean enough that you can actually learn what healthy mycelium looks like versus early contamination. Move to PDA for more vigorous growth and MEA as a general-purpose default once you're comfortable.
How long do poured plates keep?
Once rested 2–3 days to confirm they're clean, sealed plates stored in the fridge (around 35–40°F) typically hold for several weeks to a couple of months before they dry out or the media degrades. Check for shrinkage, cracking, or off color before using an older plate.
Why did my plates get contaminated?
Almost always one of a handful of causes: too many plates open at once, working in moving air instead of a still-air box or flow hood, skipping the 2–3 day rest so contamination wasn't caught before use, an under-sterilized batch of media, or a tool that wasn't actually cool after flaming when it touched the agar. Technique and open-air time explain far more contamination than a "bad batch" of media does.