What is Trichoderma?

Trichoderma gives itself away when it turns green; before that it looks a lot like healthy mycelium. Our contamination ID guide has photos.

• First sign: GroCycle describes a fast-growing layer of bright-white, fluffy growth that seems to rise above the substrate, then bright blue-green spores (GroCycle).
• Extension view: Penn State calls green mold "dense white mycelial growth followed by extensive green sporulation" and warns that "apparently normal spawn runs can give way to large patches of green Trichoderma sporulation" (Penn State Extension). On farms it can start out grey before it turns white and then green (Penn State Extension, AMI).

A white patch that outgrows the mycelium around it, or turns green within a day or two, is likely Trichoderma. Photograph it and check again in 24 hours.

Trichoderma is damaging because it grows fast and attacks the mushroom fungus itself. GroCycle calls it "exceptionally fast-growing" and says it makes enzymes that break down mushroom mycelium's cell walls (GroCycle).

During a green mold epidemic on Pennsylvania button-mushroom farms, crop losses ranged from 30 to 100% from 1994 to 1996. Penn State adds that green mold also causes significant losses on shiitake and oyster (Pleurotus) (Penn State Extension).

Trichoderma lives in soil, and its spores ride indoors on dust (GroCycle). Penn State says it gets into growing rooms mainly on contaminated people and equipment, and also through poorly composted substrate or rooms that weren't steamed off well between crops (Penn State Extension).

GroCycle adds a clue: lots of small, yellowish-brown mites in a grow space usually mean Trichoderma is nearby, and the mites spread it. Blocks may also grow more prone to mold with each flush, Smallhold warns (Smallhold).

Remove a block with Trichoderma right away and clean the area and tools around it (GroCycle). Bag it unopened where it sits so you don't shake spores loose, as our Martha tent guide describes.

• Agar: move a clean wedge from the far side to a fresh plate; see agar.
• Cleanup: wear the EPA's "minimum PPE" for mold, an N-95 respirator, gloves and eye protection, and scrub surfaces rather than only spraying them: "it is necessary to clean up mold contamination, not just to kill the mold" (EPA).

A common mistake is opening a green bag to look closer or to save the clean part. That can release spores into the room you grow in.

Hypha tracks contamination rates per strain, and its family tree links each block to its parent culture, so you can see whether green mold keeps showing up in blocks from one source.