Agar is where you prove a culture is clean. Grain is where you multiply it. The agar-to-grain transfer is the bridge between the two, and it is one of the most rewarding moments in a grow: a small wedge of mycelium goes into a jar, and a few weeks later that jar is a solid white block of spawn ready to feed a whole tub.
It is also the step where a lot of beginners first meet contamination. In this guide we explain why the transfer works the way it does, what tends to go wrong, and how to read your jars while they incubate, so you can move cultures from plate to grain with confidence.
Hands-on version: Prefer to learn by doing? Work through the free step-by-step module Agar-to-Grain Transfer and track your progress as you go.
Why go from agar to grain at all?
You can inoculate grain straight from a spore syringe or liquid culture, so why bother with agar? Because agar lets you see what you are working with. On a plate, mycelium and contaminants grow out as separate, visible colonies. A clean culture looks like even, white, spreading growth. Mold, yeast, and bacteria show up as off-colors, shiny wet spots, or fuzz that looks different from the rest.
Grain, by contrast, is a nutrient-rich, opaque environment. Once something unwanted gets into a jar, it usually has a head start you cannot see. Starting from a plate you have already inspected stacks the odds in your favor: you are only putting tissue you trust into the substrate that will carry your whole grow.
Why the leading edge?
The outer rim of a colony is the youngest, most actively growing mycelium. It recovers quickly after being cut and moved, so it tends to "take" faster on grain. It is also the part of the plate that has had the least time to pick up anything that landed on the older center. That is why the module has you cut from the edge rather than the middle.
The science of a clean transfer
Sterile grain is a blank check
Properly prepared grain spawn (covered in How to Prepare Grain Spawn) has been pressure-sterilized, so nothing is living in it. That is exactly what makes it valuable and vulnerable at the same time. Whatever gets into the jar first, and grows fastest, wins. Your job during the transfer is to make sure the first and only arrival is your mycelium.
Why the still air box and the settling time
Most contamination during transfers comes from airborne spores and bacteria drifting into an open jar or landing on a tool. A still air box cuts down on air movement so particles settle instead of swirling past your work. Letting the box sit for 10–15 minutes after wiping it down gives dust time to fall and gives the 70% alcohol time to evaporate. (If you want the reasoning behind 70% rather than 99%, see Why 70% Alcohol Beats 99%; for box technique, see the Still Air Box Guide.)
Why one blade per jar
If a single jar or wedge harbors something you could not see, a reused blade carries it into every jar that follows. Using one sterile blade per jar keeps any problem isolated to one jar instead of the whole batch. It is a small cost that protects your entire run.
Safety: Never bring an open flame into a still air box. Alcohol vapor collects inside the box and can ignite. If you prefer to flame-sterilize a blade, do it outside the box only after the alcohol has fully evaporated, and let the blade cool before it comes back in — a hot blade will also kill the very tissue you are trying to transfer.
Why room-temperature grain matters
Grain straight from the pressure cooker is hot enough to kill mycelium, and warm jars pull in outside air as they cool. Waiting until jars are at room temperature protects both your culture and your seal.
Incubation: what is actually happening in the jar
After the transfer, the mycelium needs a few days to recover and start growing into the grain. In the module we recommend a dark spot around 21–24 °C (70–75 °F). That range suits common gourmet species like oyster, lion's mane, and shiitake during colonization, and it is warm enough for steady growth without favoring the fast molds and bacteria that love heat.
Stable temperature matters as much as the number. Big swings cause condensation inside the jar, and pooled moisture on grain is an invitation for bacteria.
Why shake at 20–30%?
Mycelium spreads from each point of contact. Early on, there are only a few points of contact (your wedges). When a jar is roughly 20–30% colonized, shaking breaks those colonized kernels apart and scatters them through the jar, turning a single front into dozens of new starting points. That is why colonization usually finishes about 7–10 days after shaking, much faster than if you left the jar to grow out from one spot.
Shaking too early, before the mycelium has really established itself, can stall it. Shaking much later gives fewer benefits. The 20–30% mark is a practical middle ground.
Common mistakes
Transferring from a "mostly clean" plate. If any part of a plate looks doubtful, clean it up by moving healthy tissue to a fresh plate first. Grain will not fix a questionable culture; it amplifies whatever you give it.
Opening lids too wide or too long. Lift the lid only enough to drop the wedge in, and close it immediately.
Rushing the settle time. Working in a box that is still full of drifting particles and alcohol vapor undoes the point of using it.
One giant wedge. Two or three small wedges per jar give the mycelium more starting points and usually colonize faster than one large piece.
Incubating somewhere convenient rather than stable. A shelf near a window or heater can swing a lot over a day.
Not labeling. Without culture name and date on every jar, you cannot trace which plate a problem came from.
Troubleshooting: reading your jars
Check jars every day or two. Most issues show up as one of a few patterns. These are typical signs, not a lab diagnosis — but you do not need certainty to make the right call.
What you see or smell
Likely cause
What to do
Green or blue-green patches
Often a mold such as Trichoderma
Keep sealed and dispose of the jar
Black or dark spots
Often a mold such as Aspergillus
Keep sealed and dispose of the jar
Pink or orange fuzz
Other molds
Keep sealed and dispose of the jar
Wet, slimy, or cloudy grain; sour or rotten smell
Bacterial contamination
Keep sealed and dispose of the jar
No visible growth after several days
Weak wedge, too-hot grain at transfer, or cold incubation
Check temperature; give it a little more time before writing it off
Yellowish droplets on healthy white mycelium
Often metabolites from stressed mycelium
Usually not contamination on its own; watch closely
When in doubt, throw it out. Do not open contaminated jars indoors — mold spores spread easily and some can irritate the lungs. Dispose of them sealed, away from your grow area. If you have asthma, allergies, or a weakened immune system, wear a mask when handling contaminated material.
Logging each contaminated jar in WeMush, alongside the plate and date it came from, helps you spot patterns: a single bad plate, a batch of under-sterilized grain, or a technique slip on one particular day.
FAQ
How many jars can one plate inoculate?
Usually 4–8 quart jars, depending on how well the plate is colonized and how many wedges you use per jar.
How long until I see growth?
You should typically see white mycelium spreading from the wedge within a few days of incubation at the recommended temperature.
Can I use a plate that has a small spot of contamination far from where I cut?
We would not. Spores and bacteria are invisible long before colonies are. Move clean leading-edge tissue to a fresh plate, confirm it grows clean, and transfer from that.
Do I need light during incubation?
No. Keep jars somewhere dark. Light is a fruiting signal and is not needed while the mycelium colonizes grain.
