Mold on mushrooms typically appears as fuzzy patches in white, green, blue, or black. The exact look depends on the type of mold, but the most common signs on store-bought mushrooms are soft spots covered in fine fuzz, discoloration that wasn’t there before, and a musty or sour smell. On mushrooms being cultivated at home, mold can range from wispy gray clouds to dense green patches to strange tissue deformations.
Telling mold apart from normal mushroom features can be tricky, since mushrooms themselves are fungi. Here’s what to look for, whether you’re inspecting a package from the grocery store or troubleshooting a grow kit.
Mold on Store-Bought Mushrooms
The mold you’ll find on refrigerated mushrooms from the store usually shows up as fuzzy patches in white, green, blue, or black. These patches sit on the surface and look distinctly different from the smooth skin of a fresh mushroom. Green or blue-green fuzz often comes from Penicillium species, which spread green spores across the surface. Black mold, typically from Aspergillus or Mucor species, forms dark-colored patches that can look powdery or slightly raised.
Mold isn’t the only sign a mushroom has gone bad. Bacterial spoilage is actually more common in the fridge and shows up differently. A slimy, sticky surface is one of the clearest indicators of bacterial breakdown. Sour, ammonia-like, or alcoholic smells mean fermentation has started. Mushrooms that feel overly soft or mushy, or that have darkened extensively, are also past the point of safe eating. Any of these signs, mold or otherwise, mean the mushrooms should go in the trash. Cutting away the moldy part isn’t reliable because mold sends invisible root threads deep into soft, moist foods like mushrooms.
Green Mold
Green mold is one of the most recognizable problems in mushroom cultivation. It’s caused by Trichoderma species and follows a predictable pattern: it starts as white, dense fuzz that’s nearly impossible to distinguish from the mushroom’s own growing tissue. As it matures and begins producing spores, it shifts to a vivid dark green. By the time you see green, the mold is already well established.
This color change is what makes Trichoderma so frustrating for home growers. In the early white stage, it blends right in with healthy mushroom mycelium. The green sporulation typically appears on the growing substrate (the material the mushrooms are planted in) rather than on the mushrooms themselves, and it can spread quickly across the surface. On oyster mushroom substrates, green mold is especially common and is one of the first problems new growers encounter.
Cobweb Mold
Cobweb mold, caused by Cladobotryum species, is the most commonly confused with healthy mushroom growth. Despite its name, it doesn’t look like a spiderweb. It looks more like cotton balls: light gray, wispy, and made of extremely fine strands. It typically protrudes half an inch to an inch above the surface in a cloud-like structure and carries a distinct mildew smell.
The speed of cobweb mold is what sets it apart. It can germinate, grow through the surface material, and begin producing spores in as little as six days. When it reaches a mushroom, it envelops the cap and stem, causing a wet brown rot that destroys the fruit. Over time, the wispy patches become more dense and powdery as spore production ramps up.
Cobweb mold is almost always a problem in cultivation containers, not on store-bought mushrooms. If you’re growing mushrooms at home and see gray, fluffy growth spreading rapidly across the surface, that’s your signal.
Wet Bubble Disease
Not all mushroom mold looks like fuzz. Wet bubble disease, caused by a fungal pathogen called Mycogone perniciosa, distorts the mushroom itself into something barely recognizable. Infected mushrooms develop large, irregular masses of undifferentiated tissue. Caps become deformed, stems swell heavily, and wart-like growths appear on the surface.
The telltale sign is amber-colored droplets that form on the misshapen tissue, eventually leading to rot. In severe cases, white mycelium grows outward onto the surrounding surface and turns brown as spores mature. This is primarily a concern for commercial and home growers rather than something you’d see in a grocery store package.
Mold vs. Normal Mycelium
This is where most people get confused. Mushrooms are the fruiting body of a larger fungal organism, and that organism’s root-like network (called mycelium) is white and fuzzy. So some white fuzz on mushrooms is completely normal, especially near the base of the stem. Home growers often see this “fuzzy feet” phenomenon, where the base of the mushroom develops a ring of fluffy white growth. This is aerial mycelium, and it’s harmless.
Here’s how to tell the difference. Healthy mycelium is bright white and comes in two main forms: a cottony, fluffy type and a stringy, web-like type that looks almost rope-like. Both grow close to the surface and stay relatively compact. Cobweb mold, by contrast, is light gray (not bright white), made of much finer strands, rises well above the surface in puffy clouds, and smells like mildew. If you see vibrant white patches hugging the sides or base of a growing container, that’s almost certainly healthy mycelium. If you see gray, cotton-ball-like growth ballooning upward with a musty smell, that’s mold.
On store-bought mushrooms, the distinction is simpler. A thin white fuzz near the base of the stem or in the gills is often just the mushroom’s own tissue. Patches of green, blue, black, or gray fuzz anywhere on the cap or stem, especially with an off smell, are mold.
How to Prevent Mold on Stored Mushrooms
Fresh mushrooms stay mold-free longest when refrigerated quickly after harvest or purchase. Cornell University’s small farms program recommends getting mushrooms into cold storage within one hour of picking, kept at about 41°F (5°C) in a cool, dry, dark place. Under these conditions, mushrooms can stay fresh and marketable for several weeks.
At home, store mushrooms in the refrigerator in a paper bag or their original packaging with some airflow. Plastic bags trap moisture, and moisture is what mold needs to take hold. Don’t wash mushrooms until you’re ready to use them. If you notice any slimy, discolored, or fuzzy mushrooms in a batch, remove them immediately so the mold doesn’t spread to the rest.