What is cobweb mold?

Cobweb mold sits loosely on top of the substrate or mushrooms, while mushroom mycelium grows into the substrate and stays tight to it. Our contamination ID guide has photos.

• Color: Penn State describes "weft-cotton-like mycelium (usually white; sometimes gray, pink, or yellow)" over the casing, mushrooms or pins (Penn State Extension). GroCycle calls it "light gray and not white" (GroCycle).
• Speed: cobweb can cover a jar or monotub in 24 to 48 hours, according to GroCycle, a company that sells growing kits and courses. Fluffy aerial mycelium stays white; fluff that greys or fills the jar within a day or two suggests cobweb, so look again in 24 hours.
• Black dots: grey wisps with tiny black pinheads are black pin mold (Rhizopus), not cobweb. GroCycle says to get rid of it, warning that inhaling its spores or eating mushrooms grown near it "can cause illnesses in some people". Toss those unopened.

Cobweb mold is dangerous to the crop. It is parasitic on other fungi, and left alone it can make pins abort and mature mushrooms rot (GroCycle). AMI says affected mushrooms "eventually turn brown and rot", and cap spotting is often the most damaging symptom on farms (AMI).

What it does to people isn't covered by the Penn State and AMI pages we checked. Its spores are "very readily airborne" (AMI), so handle it as you would any mold: the EPA's "minimum PPE" is an N-95 respirator, gloves and eye protection (EPA). We don't eat mushrooms from a moldy block.

Cobweb mold thrives in still, humid air. GroCycle finds it mostly in grain jars and monotub fruiting chambers with little airflow, often near the end of colonization (GroCycle). Penn State's control advice is to avoid wet caps and 95–100% RH, noting that germination occurs in hours at 60–65°F (16–18°C) (Penn State Extension). Steady fresh air exchange and caps that dry between mistings are the prevention.

Treat cobweb mold only while it's one small, early patch, and switch fans off first: AMI says its spores are most readily airborne during treatment (AMI).

• Hobby method: spray just the patch with 3% hydrogen peroxide. GroCycle says this "will not harm your mushroom mycelium" (GroCycle); no extension page we checked says so.
• Farm method: cover the patch with a damp paper towel, then salt it (Penn State Extension, AMI).

After treating, our practice (not a sourced rule) is to keep the block apart and watch it through one more flush. If the wisps come back or spread, bag the block and remove it.

Hypha's photo journal shows a block's photos side by side, which makes the 24-hour check easy, and its contamination rate per strain shows whether cobweb is a one-off or a pattern.