Cobweb Mold: Identification, Is it Dangerous? How to Get Rid of It?

“Cobweb mold” actually refers to several different kinds of molds, but all of them cause the same characteristic problems in mushroom cultivation, and all can be controlled by the same methods[i][ii]. It may make more sense to talk about cobweb as a disease caused by any of several species of mold. If you’re cultivating mushrooms, cobweb disease could cause a real problem, but it’s not as common as you might think, and it IS treatable[iii].

What Is Cobweb Mold Disease?

Various molds cause cobweb disease in mushrooms. Unless you have a special interest in the topic, there is really no need to try to keep track of which is which; identifying molds to species usually requires genetic testing, and the taxonomy of the cobweb molds has been recently reworked, so depending on how old your information source is—and who wrote it (us experts are not all in agreement)–you might find very different names for these molds anyway.

In contrast to many other forms of contamination that might pop up in your grow, where the problem is that something other than your crop is competing for space and nutrition and maybe winning, cobweb molds actually eat your crop themselves. They can attack many different kinds of cultivated mushrooms, and can begin their attack at any stage in the grow. These molds can thus stop colonization, if contamination happens early enough or, if the problem begins later, they can cause lots of aborts. If mushrooms are already reaching appreciable size, the mold can grow over them, and cause them to discolor and then spoil.

Understandably, many people have questions:

  • Is Cobweb Mold Dangerous?
  • What Does Cobweb Mold Look Like?
  • Do I Have Cobweb Mold?

Is Cobweb Mold Dangerous?

It is if you are a mushroom—probably not otherwise. We have been unable to track down a definitive answer to whether mushrooms contaminated—or previously contaminated—with cobweb mold are safe to eat. Some growers claim cobweb is harmless, while others say it’s best not to risk it.

There are molds that can poison the air or cause disease in humans, but cobweb doesn’t appear to be among them.

What Does Cobweb Mold Look Like?

It’s possible for a cobweb mold to be present but not yet visible, but when it does appear it lives up to its name, producing loose, gray material somewhat like the kinds of cobwebs that accumulate in unattended corners of the living room. Another good comparison would be to a grayish version of the synthetic stuffing often used in sewing. A critical field-mark is the fact that cobweb mold produces a thick layer of webby threads above the substrate, rather than making a thin layer of material on the substrate surface only, as most fungi do. If not treated first, the mold may, depending on species, eventually turn its substrate, including your mycelium, pink or yellow.

Not all signs are visual; some species of cobweb mold produce a distinct scent of mildew.

Do I Have Cobweb Mold?

Probably not, no[iv].

Growers often assume they have cobweb mold, since the mycelium they are trying to grow can resemble it, especially to those not familiar with both. But in most cases, these are false alarms. Although there have been outbreaks where cobweb mold became common among the mushroom growers of a given region, under normal circumstances, cobweb mold is rare.

If you think you have cobweb mold, it’s best to double-check so you do not unnecessarily treat, or even throw out, your perfectly healthy crop—as many other growers have, in fact, done.

Cobweb Mold VS. Mycelium

Cobweb mold is mycelium, too, of course; a mycelium is simply the body of a fungus. But when people ask “is this cobweb or mycelium,” what they mean is it cobweb or the mycelium that’s supposed to be there. The question is complicated by the fact that cobweb can attack multiple species, and the mycelium doesn’t always look the same. But we can make some generalizations.

Cobweb is usually grayish. Mushroom mycelium is usually bright white.

Cobweb forms a thick layer, reaching up from the substrate some distance. Mushroom mycelium rests flat on the substrate surface (this distinction won’t be evident in a jar, though).

Cobweb strands make a loose tangle—you can see through the layer to the substrate surface, at least vaguely. It is this looseness that reminds some people of web. Mushroom mycelium is usually much denser—it can remind people of spiderweb, too, but in a very different way.

If mushrooms are present, cobweb will grow over them, rendering them furry. The mushroom’s own mycelium will never do that—some mushroom species do have fuzz, but not loose, webby fur, and in any case you know what the mushrooms you’re trying to grow are supposed to look like.

Cobweb Mold VS. Other Molds and Diseases

Cobweb mold isn’t the only thing that can contaminate mushroom grows, and it’s important to get the diagnosis right because treatments may differ. We’ve already described cobweb, so here are some brief descriptions of some of the other usual suspects.


Trichoderma is a genus of molds, some of which like to eat many of the same mushroom species we do (just as cobweb does). Unfortunately, it is very difficult to treat. The best bet is usually to remove the whole container before the mold can produce spores and spread to all of your other grow chambers.

Its mycelium is white, but it has a fluffier appearance than mushroom mycelium. Once it starts producing spores it turns green and gets much easier to identify, but my then it’s too late.

Pin Molds

Pin molds are a very large and diverse group of fungi, so called because their tiny fruiting bodies somewhat resemble pins with small, black pinheads. Some can attack mushroom grows, and they do stand out from the substrate like fur. But aside from the pinhead dots (which may or may not be present), they are typically white. The visual texture is a little different from cobweb mold, too.

Pin molds are common contaminates and extremely difficult to treat. Many people who think they have battled cobweb and lost have actually lost to a pin mold. Note that some pin molds actually are quite toxic.

Wet Spot

Wet spot is the result of contamination by certain types of bacteria. It does, indeed, look like a slimy, wet patch in the substrate. It doesn’t look anything like mold, but it could confuse someone who didn’t know about the different types of contamination.

Wet spot can occur in sealed grain jars, even if you did everything properly to maintain a sterile work-space after sterilizing the grain. That’s because the bacteria that cause wet spot can form heat-proof spores capable of surviving sterilization. The trick is to soak the grain overnight first, to give the bacterial spores a chance to germinate. Then they lose their heat-proofing and will be killed by sterilization.

How to Get Rid of Cobweb Mold

The best offense is a good defense—using good sterile work practices will reduce the chance of any contamination, including cobweb mold. Also, while cobweb mold can handle a wide variety of growing conditions, it has a definite preference for the casing layer and for wet, warm conditions. Simply by not over-saturating the substrate and by providing for plenty of fresh air exchange, you can reduce your risk of cobweb. Using cultivation methods that don’t involve a casing layer will reduce your risk dramatically, too. Cooler temperatures, provided your mushroom crop can handle it, will slow down cobweb.

Some people regard speed of growth as an identification mark of cobweb, but actually speed depends on growing conditions. Worsen conditions from the perspective of the cobweb, and it won’t grow so fast anymore.

But if you do get cobweb—and you are sure it is cobweb, not Trich or pin mold or healthy mushroom mycelium—spraying it with 3% hydrogen peroxide should work. You may need to repeat the spaying a few times at 12-hour intervals. The hydrogen peroxide shouldn’t harm your mushrooms.


Cobweb mold is among the most worried-about forms of contamination in mushroom cultivation, but it is actually pretty rare and easy to treat. Many people who think they had a cobweb mold problem didn’t—it was either some other mold, or it was healthy mushroom mycelium. Understanding what cobweb mold is and what it looks like is not only key to getting rid of it, but also the best way to be sure you don’t have it and can stop worrying.


[i]      (2021). Mushroom Contamination: How to Spot and What to Do. Fungi Academy

[ii]     Kizzle (n.d.). The Truth About Cobweb. Psilosophy

[iii]    The_Chariot (2006). How to Get Rid of COBWEB MOLD!!!!! Shroomery

[iv]    Shroomscout (2020). LISTEN THE F*CK UP. YOU DO. NOT. HAVE. COBWEB. Reddit

1 thought on “Cobweb Mold: Identification, Is it Dangerous? How to Get Rid of It?”

  1. Great blog. Informative, forceful and light hearted–reminds me of Charlie Papazian who was instrumental in getting the homebrew hobby off the ground. His motto: relax, don’t worry and have a homebrew!


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