What is contamination?

The contaminants growers meet most are green mold, cobweb mold, bacteria and other molds. Our contamination ID guide has an ID table and photos for each.

Contaminant	Signs	More
Green mold	Dense white growth, then green spores	Trichoderma
Cobweb mold	Grey, wispy tufts above the surface that spread in a day or two	Cobweb mold
Bacteria	Wet, slimy, sour grain; yellow-to-brown spots on wet caps	Bacterial contamination
Other molds	Black, grey, yellow or orange growth or spores	ID guide

GroCycle groups contamination into fungal, bacterial, pest and viral kinds, and calls Trichoderma the most common contaminant by reputation (GroCycle).

Healthy mycelium is usually white and grows into the substrate; most molds turn a color as they make spores, and bacteria look wet and smell sour. Shiitake blocks are an exception that turn brown on their own before they fruit (Field & Forest).

When you're unsure, isolate the jar or block, photograph it, and look again in 24 hours. A spot that changes color or outgrows the mycelium around it is likely contamination. Spores that already show color don't need the wait: bag it where it sits.

Contamination comes in through air, substrate, water, equipment and people, GroCycle says (GroCycle). The pattern tells you which, according to a Cornell Small Farms guide written with Fungi Ally (Cornell Small Farms and Fungi Ally):

• Whole batch: improper treatment or sterilization, or dirty substrate.
• Spotty, one or two places: air, surfaces or tools at inoculation. A still air box or flow hood helps.
• Growing out from the spawn: contaminated spawn.

Heat counts too: Cornell says an overheated bag core can let contaminants in, and keeps incubation at about 70°F (21°C), not above 80°F (27°C) (Cornell Small Farms).

Toss any jar, bag or block showing green, black or orange spores (GroCycle); we bag it unopened where it sits. Cornell's farm guide also says contaminated spawn "needs to be thrown away" (Cornell Small Farms and Fungi Ally). We salvage only agar, by moving a clean wedge to a fresh plate, and one small, early cobweb patch. For cleanup, the EPA's "minimum PPE" for mold is an N-95 respirator, gloves and eye protection, because "mold is allergenic, and some molds may be toxic", dead or alive (EPA).

Hypha tracks contamination rates and compares them across strains, and its family tree links each block to its parent culture, so you can tell one bad block from a bad culture.