Contamination (oh no!)
Firstly, relax. You have a big healthy block of mycelium there and it can largely take care of itself.
The first thing you might encounter is the dreaded trichoderma. This is the fuzzy forest-green mold that grows on onions you’ve forgotten in the pantry for too long.
Shown is surface trichoderma – it didn’t land on an uncolonized food source, and it’s going to have a hard time getting much bigger than it already is. This one is near a fruiting body so we’re going to want to take care of it before it can move over to that. A Q-tip soaked in household bleach and applied to any green spots works great. Make sure to look around, note that TINY green speck above and to the right of the obvious contaminant. Please note this does NOT kill the trichoderma mycelium underneath, which is probably larger than the green spot you see. It just knocks it back a bit and keeps it from spreading in the immediate future.
One other thing you might see is bacterial blotch. This is caused by, usually, Pseudomonas tolassi, or by other Pseudomonas bacteria. It can cause unsightly orange-brown staining and will spread readily between mushrooms. The most common cause is excessive humidity that is causing water to condense and collect on the mushrooms themselves. If you are using active humidification, follow the manufacturer’s instructions on how to disinfect your humidifier weekly. Absolutely do not operate a humidifier with bleach in its water reservoir.
If this happens to you, your best bet is to dry the mushrooms out a little (only for a couple hours) on a table outside of the fruiting chamber, and add bleach to your misting solution. Make sure to label the bottle after you add the bleach. You’re aiming for 150ppm of chlorine, there’s a handy calculator here.
Bear in mind that generally bacterial blotch is usually cosmetic only – the fruits may discolour or the caps may whiten and become slightly tougher (or they may even mutate or deform), but the fruits are not dangerous to eat. The fruits are still safe after an application of dilute bleach as well (once it dries.) If you’re troubled by bacteria, just cut away the affected fruits and try to keep it from spreading.
Discoloured or deformed fruits are excellent in soups, there’s always a silver lining!
Both of these contaminants are rare on a fully-colonized block, and are more likely to happen towards the end of your kit’s life cycle, when the mycelium has exhausted its nutrients and is dry and weak. Developing a small trichoderma contamination out of nowhere after a few flushes is often a good sign a block is ready to be put out to enrich the garden.
Other contaminants may actually render the mushrooms undesirable to eat. So far we haven’t heard of that happening with any of our kits (or anyone else’s), but if something looks very wrong, please contact firstname.lastname@example.org.