A customer emailed me a photo last spring of a rye jar with a dark, glossy wet patch spreading across the bottom third, and the subject line just said "is this contamination or wet rot?" The short answer I gave him is the same one I will give you here. Wet rot is contamination. It is a specific kind of bacterial contamination, usually driven by too much water and under-sterilized grain, that turns kernels soft, slimy, and sour smelling. Contamination is the bigger umbrella word for any unwanted organism in your grow, from bacteria to green mold to yeast, so wet rot lives inside it rather than sitting beside it.
Once you see it that way, the practical question stops being "which one do I have" and becomes "is this the bacterial, wet, sour kind, or the mold kind," because the two look and behave differently and you fix them differently.
Out-Grow Quick Answer
- Wet rot is a bacterial contamination, most often Bacillus, so it is one type of contamination and not a separate category.
- Wet rot reads as soggy, slimy, sour or vinegar smelling grain with translucent wet pockets. Mold contamination reads as colored powdery or fuzzy patches, often green, blue-green, or black.
- The usual root causes are too much surface water on grain and under-sterilization that lets heat-resistant bacterial spores survive.
- Condensation on a bag by itself is not contamination. Judge by texture, smell, and growth pattern, not by water droplets on the plastic.
- Heavily wet, sour grain does not come back. For an indoor grow, discard it rather than trying to reheat or salvage it.
Wet Rot Is a Type of Contamination, Not a Separate Problem
The reason that customer was confused is the reason almost everyone is confused, and I get some version of his question most weeks. Beginner guides tend to list "wet rot" in one column and "contamination" in another, as if you are looking at two different failures. You are not. When I open a jar that has gone soft and sour, I am looking at bacteria that outcompeted the mushroom, and bacteria are contaminants. Wet rot is just the name for what bacterial contamination looks like when excess water is the thing that gave those bacteria the opening.
Here is the mechanism, plainly. Wet rot is usually a Bacillus infection of overly wet or poorly sterilized grain. Bacillus forms heat-resistant spores called endospores that can ride out a sterilization run that did not fully finish. When the grain is warm, wet, and full of leftover nutrients, those spores wake up, multiply faster than your mycelium can, and digest the kernels into a soft, slimy, sour mess. Mold contamination is a different niche. Airborne fungal spores like Trichoderma land on the surface and spread as colored patches. Bacteria travel through films of free water and stay more localized. Mold travels through the air and can hit several containers at once.
A bacterial contamination of grain or substrate, usually caused by species of Bacillus, in which excess moisture lets the bacteria decompose the grain into soft, wet, sour smelling material and outcompete the mushroom mycelium. Also called wet spot or sour rot.
How to Tell Wet Rot From Mold Contamination in Grain
The fastest read I ever learned came from a bad week early on when I had three jars go sideways at once and lined them up on the bench to compare. One was sour and wet, one had a spreading green patch, and one was fine. Set side by side, the difference is obvious, and that is the trick. You are not trying to name the exact organism. You are sorting what you have into wet and sour, or colored and fuzzy, or actually healthy. Wet rot contamination and mold contamination give you different signals in texture, color, and smell.
Texture tells you the most. Wet rot makes grain soft, swollen, and slimy, sometimes clumping into a gelatinous mass, with kernels that look almost boiled apart. Mold sits on the surface as a powder or a fuzz. Color sorts the molds: Trichoderma starts white and turns bright green, while Penicillium and Aspergillus come in blue-green, gray-green, or black. Smell is your confirmation. Wet rot smells sour, like vinegar, sour milk, or dirty socks. Healthy grain smells like clean mushroom or like nothing much at all.
| Signal | Wet rot (bacterial) | Mold contamination | Healthy grain |
|---|---|---|---|
| Texture | Soft, swollen, slimy, kernels clumped and wet | Powdery or fuzzy patch on the surface | Firm kernels bound by dry white mycelium |
| Color | Darkened, translucent, glossy wet pockets | Green, blue-green, gray, or black patches | Even bright white, ropey or cottony |
| Smell | Sour, vinegar, sour milk, rotten | Musty or faint, sometimes none | Clean mushroom or neutral |
| Spread | Stays local, moves through wet pockets | Airborne, can hit several jars fast | Even advance across the whole jar |
What Wet Rot Looks and Smells Like: Sour Grain, Slime, and Wet Spots
The jar that finally taught me to catch wet rot early was one I almost tossed too soon. It sat at day four looking a touch wetter than its neighbors on one side, nothing dramatic, and I marked it and left it. By the time I opened it a week later the wet side had gone soft and sour and the mycelium had crawled everywhere except that corner. That progression is the tell. Wet rot does not usually show up all at once. It builds, and if you know the stages you can pull a jar before it takes the whole batch of your attention.
What Causes Wet Rot in Mushroom Bags and Jars
Every time I trace a wet rot jar back to its start, it lands on one of two things, and usually both. There was too much free water in the grain going into the jar, and the sterilization did not fully finish. Bacillus spores survive a short or overloaded run, and then all that surface water hands them the wet, warm home they need. The other causes I see are real but they are the supporting cast, not the lead.
Surface water is the one beginners underrate. When you cook or soak grain, you want the water absorbed into the kernel and the outside surface dry to the touch before it goes in the jar. Grain that still has a film of water on it, or a puddle in the bottom of the jar, is a wet rot setup no matter how well you sterilize. After that comes dirty inoculum. If the liquid culture or grain-to-grain source you inoculated with already carried a bacterial load, you planted the problem yourself, which is why I confirm cultures on agar before I trust them. Tight seals with no gas exchange favor the bacteria over your aerobic mushroom too, and over-misting a cake or block during fruiting can drive the same wet, bacterial rot at the fruiting stage.
Normal Condensation vs Contamination: When Not to Panic
I probably talk more growers off the ledge over condensation than anything else in the contamination bucket. A grower will see beads of water on the inside of a colonizing bag, decide it is wet rot, and toss a perfectly good grow. Water on the plastic is not contamination. It is usually just a temperature difference between the substrate and the room, the same reason a cold glass sweats on a warm day. The diagnosis has to come from the grain, not the bag wall.
Some species also throw off a little yellow or amber liquid on the surface as they colonize, and that metabolite, what growers call mycelial exudate, is often normal on a healthy, actively growing block. The read is always the same three things together: texture, smell, and growth pattern. Firm grain, a clean or neutral smell, and mycelium advancing evenly means you are fine, condensation or not. Soft clumped grain, a sour smell, and stalled mycelium means wet rot, and the droplets on the plastic never made the call either way.
If you want a quick way to weigh what you are looking at, some signals matter far more than others. Here is how I rank them after years of fielding these emails.
How to Prevent Wet Rot and Bacterial Contamination
After enough years shipping grain and hearing where it goes wrong, my prevention routine has boiled down to a short sequence that heads off almost every wet rot jar before it starts. None of it is fancy. It is mostly about controlling water and finishing your sterilization, in that order.
Preventing Wet Rot, Step by Step
Surface-dry your grain
Hydrate grain so water is absorbed into the kernel, then let the outside dry to the touch. No puddle in the jar, no film on the grain.
Sterilize fully, do not overload
Run grain jars and small bags at 15 PSI for 90 to 120 minutes and leave room for steam to move. A packed cooker leaves cold spots where spores survive.
Inoculate clean cultures only
Use liquid culture or spawn you have confirmed clean on agar. Dirty inoculum plants the bacteria before the grow even begins.
Give the jar gas exchange
Use a filter patch or a filtered lid so the aerobic mushroom is favored over the bacteria that like still, sealed, wet conditions.
Mist the chamber, not the fruit
At fruiting, mist chamber walls to hold 85 to 95 percent humidity and let evaporation work. Soaking pins directly invites bacterial blotch and wet rot.
If you just want the short version to keep near your grow log, here is wet rot boiled down to the facts that matter.
When to Throw Away a Wet Rot or Contaminated Grow
The hardest email I answer is the one where a grower asks if they can save a jar that has clearly turned, because I understand not wanting to lose the work. But I have never once seen heavily wet, sour grain come back. Once bacteria have broken down the kernels, the structure and the nutrition are gone, and reheating cooks the damage in rather than reversing it. For an indoor grow, the honest call is to discard it.
Throw it out when you get a strong sour or putrid smell on opening together with slimy, obviously wet grain, or when a green, black, or otherwise strongly colored mold appears and is expanding. You will read about cutting away a small bacterial pocket and fruiting the rest outdoors as a triage move. It is genuinely contested, and for beginners and any indoor grow I do not recommend it. The risk to the rest of your space is not worth the partial jar.
To dispose of it safely, seal the contaminated material in a heavy bag for the trash under your local rules, or fully cook it and compost it well away from your grow area. Then clean any reusable jars and gear with hot water and detergent followed by a disinfectant wipe-down. High loads of bacterial or mold aerosols off rotten substrate can bother allergies and airways, so it is worth wearing a mask and working in a ventilated spot when you handle it.
Common Bad Advice About Wet Rot and Contamination
Two pieces of advice in this corner of cultivation cost people good grows and clean jars, and I want to correct them head on. Neither is malicious. They are just repeated so often that they sound true.
You can fix wet rot by re-pasteurizing or reheating the substrate.
Once bacteria have colonized and broken down the grain, heat does not restore the structure or nutrition. Most growers discard heavily affected grain rather than trying to salvage it.
If it is white, it is right. Any white growth is healthy mycelium.
Several contaminants, including Trichoderma in its early stage, look white before they change color. Texture, structure, and spread speed tell you more than color alone at the earliest stage.
Relevant Out-Grow Supplies for Clean Grain and Substrate
Most wet rot starts with grain prep and sterilization, so the supplies that actually help are the ones that take that variable off your plate or make your own prep more consistent.
- Sterilized grain bags: pre-sterilized grain spawn in filter-patch bags removes the two biggest wet rot causes at once, surface water and an unfinished sterilization run. A good fit if your losses trace back to the cooker.
- Mushroom substrate and bulk substrate: properly prepared substrate at field capacity, not dripping, so you are not fighting excess water at the fruiting stage.
- All in one grow bags: a self-contained sterilized grain and substrate bag, a forgiving option while you build up your sterile technique.
- Culture media and mushroom cultures: agar plates and clean cultures so you can confirm your inoculum is not carrying the bacteria that cause wet rot in the first place.
Related Out-Grow Guides
If you are working through contamination questions, these guides go deeper on the surrounding steps.
The Bottom Line on Wet Rot vs Contamination
Wet rot is not the opposite of contamination. It is a specific, bacterial, water-driven form of it, and once you sort a bad jar into wet and sour or colored and fuzzy, you know what you are dealing with and how to respond. Control your grain moisture, finish your sterilization, judge the grain and not the condensation, and discard the jars that have truly turned. Do that and wet rot stops being a mystery and becomes a rare, catchable event.
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.