Your agar plate is contaminated if you see growth that is any color other than clean white, a shiny or wet looking patch, colonies popping up in random spots away from where you inoculated, or more than one clearly different kind of growth on a plate that was supposed to hold a single culture. Healthy mushroom mycelium is white, matte, and organized. Anything that breaks that pattern gets treated as contamination until proven otherwise.
Out-Grow Quick Answer
- 🍄 Clean mycelium is bright white, matte, and grows in an organized circle from the inoculation point.
- 🟢 Any green, yellow, black, pink, or grey growth is contamination. So is a shiny, wet, greasy patch, which is bacteria.
- ⏱️ Give a plate 24 to 72 hours for first growth at 68 to 77 degrees F, and 5 to 10 days for a full 90 millimeter plate.
- 🕸️ A plate covered in wispy growth in under 24 hours is cobweb mold, not a fast strain. Toss it.
- 🗑️ More than two clearly different colony types on one plate means discard, or cut out a clean sector only if the strain is worth saving.
How to Tell If Your Agar Plate Is Contaminated at a Glance
A customer emailed me last spring with a photo of a lion's mane plate, sure it was ruined because the center looked fuzzy and uneven. It was fine. That fuzz was normal aerial mycelium. Two days later he sent a second photo of a different plate with a dime sized green spot near the edge, and that one was Trichoderma, dead on arrival. The difference between those two plates is the whole skill of reading agar, and once you have it, most calls take about three seconds.
The fastest read is color. Mushroom mycelium is white. Not cream, not beige, not grey with a green tint, white. When I glance at a plate the first thing my eye does is hunt for any color that is not white, because color is the single most reliable contamination tell on agar. Green, blue-green, yellow, black, pink, and orange are all contaminants. After color I look at texture and pattern, but if I see color I usually do not need to look any further.
Here is roughly how often each problem shows up on the plates growers send me. Green mold is far and away the most common, which is why it is worth learning to spot first.
That gap between Trichoderma and everything else is why I always scan for green before I look at anything else.
Here is that contrast in person, clean white next to an obvious green colony.
What Contaminated Agar Looks Like: Color, Texture, and Pattern
The worst plate I ever tried to talk myself into keeping was an oyster culture years back that had one small milky, shiny patch riding along the edge of otherwise clean mycelium. I wanted it to be condensation. I told myself it was just wet. It was a bacterial colony, and when I streaked from that plate onto grain, the grain went sour and slimy inside a week. That plate cost me a jar of spawn and taught me to read texture as carefully as color.
Reading a plate comes down to four things you check in order. Color first, because it is the loudest signal. Then pattern, meaning where the growth is and how it is organized. Then texture, matte and fluffy versus shiny and wet. Then the count, how many clearly different kinds of growth are on the plate. Clean mycelium passes all four. Contamination fails at least one.
That ranking is also why the comparison below leads with color and ends with colony count.
| What you check | Healthy mycelium | Contaminated plate |
|---|---|---|
| Color | Bright white | Green, yellow, black, pink, grey, or creamy off-white |
| Pattern | Organized circle radiating from the inoculation point | Random splodges, or colonies starting away from the inoculation point |
| Texture | Matte, fluffy, or ropey | Shiny, wet, greasy, or jelly-like |
| Colony count | One uniform type | Two or more clearly different types |
| Growth speed | Covers a 90 millimeter plate in 5 to 10 days | Explosive, wispy coverage in about 24 hours, or scattered late arrivals |
That last row matters more than beginners expect. Speed is a tell. Nothing gourmet covers a full plate overnight, so growth that looks like it happened while you slept is almost always a mold, not a strain that got lucky.
Healthy Mycelium vs Contamination on an Agar Plate: the Numbers to Know
The question I get more than any other on agar is some version of "how white is white enough," and it usually comes from someone staring at a plate that is 90 percent clean with one thing nagging at them. So here are the reference numbers I keep in my head, because reading a plate is partly a color read and partly a timing read.
Two clearly different colony types on a plate meant for one culture is the line I do not cross. It is a simple, honest rule that saves more grows than any fancy technique.
Here is what that one clean, uniform type of growth actually looks like close up.
Common Contaminants on Agar Plates and How to Identify Each
I keep a mental rogues gallery from years of plates crossing the bench, and four culprits account for almost everything a beginner will see. Learn these four and you have covered the vast majority of contaminated agar plates you will ever read.
Cobweb mold is the one that fools people most, because at a glance grey wispy growth can read as mycelium. The tells are the color and the speed. Real mycelium is white and anchors into the agar. Cobweb is grey, lifts up off the surface like a loose web, and moves far too fast.
Side by side, the two are easy to tell apart once you know what to look for.
How Long to Incubate Agar Plates Before You Read Them
Early on I threw out a perfectly good reishi plate at day two because nothing much was happening and I assumed it had failed. Reishi is just slow. Had I waited, it would have colonized fine. The flip side happened a month later when I kept a plate that looked clean at day three, expanded it at day four, and watched a green colony surface on the grain that had been hiding on the agar the whole time. Both mistakes came from reading the plate on the wrong timeline.
The window is the fix. Most gourmet mycelium shows first growth in 24 to 72 hours at 68 to 77 degrees F, and takes 5 to 10 days to cover a standard plate, with oysters on the fast end and reishi and shiitake slower. But a plate that looks clean early can still surprise you, so I do not declare a plate clean until it has stayed uniformly white, matte, and organized for 3 to 5 days, and then I give it one more look 48 hours after that before I expand it.
How to Read Your Agar Plate: a Step by Step Check
When a new grower shadows me at the bench, I walk them through the same short routine every time, and by the third or fourth plate they are calling it before I do. It is not a feel thing. It is a fixed order of checks, and here it is as I actually run it.
When to Throw Away a Contaminated Agar Plate, and When to Rescue It
A grower called me once about a rare culture he had waited months to get, one small green fleck near the edge and clean white mycelium everywhere else. For a routine oyster plate I would have said toss it and start over, no hesitation. But that strain was worth the extra effort, so we talked through a sector transfer, cutting a clean wedge of mycelium well away from the contamination and moving it to a fresh plate. It worked. That is the whole decision in one story: what the plate is worth changes the answer.
Cutting a clean wedge of mycelium away from the contaminated part of a plate and moving it to fresh media, done only to save a rare or hard to replace strain rather than as a routine fix for an ordinary contaminated plate.
For everyday grows my rule is simple. Discard any plate showing clear colored mold, widespread bacterial slime, or multiple colony types. It is faster and cheaper to pour a new plate than to fight a contaminated one, and expanding a bad plate risks the grain and liquid culture downstream of it. Rescue is for a valuable or hard to replace strain, and even then only when the contamination is small and clearly separated from a clean sector you can cut out.
- Any clear green, black, or pink mold
- Widespread shiny or milky bacterial slime
- Wispy grey growth or full coverage in about a day
- A routine strain you can easily re-pour
- A rare or hard to replace strain
- Small contamination clearly separated from clean growth
- A clean sector you can cut well away from the problem
- You have a still air box and can work cleanly
Whichever side of that list your plate lands on, do not reach for chemicals as a quick fix.
How to Safely Dispose of Contaminated Agar Plates at Home
The first time I had a stack of green plates pile up, I made the mistake of carrying them open across the room to the trash, and I am fairly sure that is how I seeded a contamination streak that dogged me for a week. Now I never move a contaminated plate around the space unsealed. Treat a contaminated plate as biological waste, wear gloves, and inactivate it before it goes in the trash.
If you have a pressure cooker or autoclave, seal the plates in an autoclavable bag and run them at 121 degrees C, which is 250 degrees F, at 15 psi for 15 to 20 minutes. If you do not, cover the colonies with a 20 percent household bleach solution and give it 20 to 30 minutes of contact time before bagging and binning per your local rules. Either way, do not open a heavily contaminated plate indoors near food, living space, or your fruiting room, since spores from something like Trichoderma travel and will find your next grow.
Common Bad Advice About Reading a Contaminated Agar Plate
Two pieces of beginner advice cost people more clean cultures than almost anything else, and I have watched both play out on plates growers sent me hoping I would tell them it was fine. Here is what is actually true.
Cobweb mold and healthy mycelium look the same, so grey fuzzy growth is probably fine.
They do not look the same once you know the tells. Cobweb mold is grey, wispy, fast spreading, and grows up off the surface like a loose web. Healthy mycelium is white, anchors into the agar, and looks ropey or fuzzy but attached. Color and growth rate give it away.
Yellow or amber droplets on the plate always mean it is contaminated and dying.
Amber or yellow metabolite droplets are often normal exudate from healthy, actively colonizing mycelium, and are especially common on species like reishi. Read the whole picture, color, smell, and colony shape, before you toss a plate. Colored mold or a foul smell is the real warning, not the droplets alone.
The thread running through both is the same. A single feature in isolation rarely settles it. You read the plate as a whole, color and pattern and texture and count together, and the honest calls get easy fast.
"The plate that gets people in trouble is never the obvious one. It is the plate that is 95 percent clean with one thing they want to ignore. In more than 20 years of this, that one thing has almost never turned out to be nothing."
Mike Wiberg, Out-GrowOut-Grow Supplies for Cleaner Agar Work
Reading plates is a skill, but cleaner inputs mean fewer contaminated plates to read in the first place. These are the supplies that genuinely help, with an honest note on each.
If you are pouring your own plates, our culture media collection covers the agar and prepared plates for the job, and starting from clean, properly poured media removes one of the common ways contamination sneaks in. When your goal is to get a clean culture into liquid faster once a plate reads clean, our liquid culture collection and the broader mushroom culture collection give you tested starting cultures so you are not fighting a questionable inoculum from the first step. None of this replaces good sterile technique. It just stacks the odds in your favor.
The bottom line on reading agar plates
Contamination on agar is loud once you know what to look for. White, matte, and organized is clean. Colored, shiny, random, or explosively fast is not. Check color, pattern, texture, and colony count in that order, give the plate its full 3 to 5 days plus 48 hours before you trust it, and when in doubt, pour a fresh plate rather than gambling your grain and liquid culture on a questionable one. That single habit will save you more grows than any piece of gear.
About the author
Mike Wiberg is the founder of Out-Grow, a mushroom cultivation supply company he has run since 2009. He developed the original all in one mushroom grow bag in 2014, a format now used across the hobby, and over more than 20 years of hands-on mycology he has grown hundreds of species and researched substrate formulations from the lab bench. He maintains one of the largest commercial culture libraries online, including a working collection of more than 300 species under a USDA permit. His cultivation work has been cited in peer reviewed research published in Frontiers in Cellular and Infection Microbiology, and he is one of only a handful of growers with a documented successful clone of Matsutake, one of the most difficult species in cultivation. He has helped growers troubleshoot cultivation problems for nearly two decades, and he writes from the lab and the grow room, not from a keyword list.