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White fuzz on a mushroom substrate can be healthy growth or a contaminant. Here's what extension sources say to look for, and where colour alone misleads.

What the usual mushroom contaminants look like, from green mould and cobweb to bacterial blotch and dry bubble, as described by extension sources.

Where mushroom contaminants come from, how spores and bacteria travel, and why wet surfaces and shared equipment raise the risk, with extension sources.

Extension and industry sources on cutting mushroom contamination risk: clean equipment, dry caps, careful watering, fly control and removing dead tissue.

What Penn State Extension and a UK industry body say commercial growers do when mould or bacterial blotch appears, and where that advice stops applying at home.