A plate is a window. You learn to trust the white edge and not to open anything else.
Edge
What advances matters more than the felt in the centre.
Rhizo
Thin directed strands: often a mycelium in good form.
Sector
A different triangle does not get transferred.
Before the wall
Sample while there is still free agar.
Why it is grown in two dimensions
On a plate, mycelium cannot hide inside a bag. It grows on the surface of a jelly, from the point where it was placed toward the rim. That makes visible the things you discover too late in grain: a glossy bacterial colony, a green ring, a sector running faster than the rest. A plate is not for fruiting. It is for deciding whether that genetic line deserves the grain.
Keep it lid-up, out of the sun. Sun cooks the agar and dries the colony. Label the base: species, date, and where it came from if you know. Lids get swapped the moment you open two of them. A temperature of 20–24 °C is where almost every species in the catalogue becomes readable. Hotter, and fast moulds overtake you. Cooler, and you wait without learning anything.
When the colony has covered the plate, move it to the fridge. A plate left warm after colonisation dries, or it speckles with contaminants that find less resistance on old agar. In the fridge it stays a copy for weeks. It is not a slant. It lasts less, but you can read it better.
What healthy mycelium looks like
White starts at the centre, or at the wedge you transferred, and advances. Two looks are normal and should not be mistaken for disease. Rhizomorphic mycelium makes ropes, almost roots on the surface. Many oysters and some shiitake do this, and it is often a sign of vigour. Tomentose mycelium is a felt, a more even cotton. Lion’s mane is often finer and less spectacular. Neither is “wrong” if it stays white and the edge is continuous.
Sectors are normal. A slice of the colony can run faster, or become denser. If it is white and clean, you can choose to transfer from that sector, because you are selecting a behaviour. If the sector changes colour, turns wet, or has a ragged translucent edge, it is not a selection. It is another organism.
A king oyster plate is a good one to learn on. Pleurotus eryngii makes a thick white, sometimes a little cottony, and the edge should stay dry and defined.
Cardoncello (Pleurotus eryngii) – Piastra Petri Fase Iniziale€17,90
Donko shiitake on a plate is slower. Do not compare it on day three with an oyster on day three. Compare it with itself: the edge is advancing, even if only a little, and it is white.
Shiitake Donko – Piastra Petri Fase Iniziale | Lentinula edodes€17,90
What does not get opened
Green, even a dot, is almost always Trichoderma. By the time you see it, spores may already be ready. Opening the lid to “smell” or to photograph up close releases them into the room. Seal it, bag it, discard it. Powdery black is another group of moulds, same fate. Bright orange or pink on the jelly is not the colour of a Pleurotus.
Bacteria show up as a wet film, often at the margins or where a drop of condensation fell on the colony. The agar around it can turn clear and soft. A sweet or cellar-like smell. Here too you do not cut the stain away hoping the rest is safe, if the film touches the mycelium you meant to transfer. You transfer only from an edge that has not touched that zone, and only if you can already do it in still air. At the first doubt, discard it.
Condensation on the lid on day one is physics: warm agar has wet the air in the plate. It is not contamination. Keep the lid up so drops do not rain on the colony. Repeated drops on the mycelium make it vulnerable to bacteria.
The plate over time, not in one photo
A live edge
White advancing over free agar. That is what you transfer.
A sector
A wedge of different colour or texture. It stays on the plate.
Green
A spot that turns to dust. That plate is finished.
A photo on day three does not say whether the colony is good. It says what it looked like on day three. What matters is the direction of the edge from one day to the next. If it advances, even by a few millimetres, and stays white and continuous, you are looking at mycelium that is working. If the edge stops and cloudy drops appear behind it, it stopped because something else has taken the agar. If the edge turns translucent and runs fast with a wet look, it is often no longer only the fungus.
Keep plates out of direct light and out of the fridge’s draught while they are still growing. The fridge is for afterwards, when the surface is covered. Putting them in the cold halfway slows them and condenses water on young mycelium. When you move them, let them sit for a few hours at room temperature before you transfer: a cold wedge on grain at 22 °C is not a disaster, but agar wet with fresh condensation is.
Do not stack plates that have just been poured or just inoculated. Heat cannot leave, condensation increases, and the lid underneath gets wet. A stack is fine when they are cold, sealed and in the fridge, with a band and a label on the group.
Where you take from
The advancing edge is the young part. The hyphae there are the ones exploring, and they have had less time to be landed on by a spore. The centre is the colony’s past: denser, older, sometimes already a little tired. When you transfer, take a square from the edge, with a bit of agar underneath, so the mycelium does not dry out on the way.
If the plate is already fully white and healthy, an interior point is acceptable, but the edge remains the better choice. If there are two sectors and one is straighter and faster, take from that one, and mark the new plate as a sector transfer. You are doing a small, domestic selection, not a laboratory breeding programme. That is fine, as long as you do not call “a superior strain” something you have only watched run across agar.
Pre-poured MYA plates are where that square lands if you do not pour agar yourself. They open for a moment, receive the wedge, close, and get sealed.
Piastre Petri Agar MYA Mix Pre-Versate + Parafilm – Sterili€17,00
A plate is not a harvest and it is not a medical diagnosis of anything. It is a visual diary. If you learn not to open it when it is green, you have already avoided the mistake that ruins the kitchens of people who grow.
When you compare two plates of the same species, compare the edge, not the thickness of the felt in the centre. A tall felt can be only extra moisture and sugar in the agar, not a better genetic line. An edge that advances straight, white, without a wet halo, is the useful information. Everything else is appearance. If you like a plate, transfer it before it reaches the wall of the dish. There it stops, it ages, and the next day a spore is more likely to land exactly on the zone you wanted to save. The best moment to sample is when a centimetre of agar is still free, not when the plate is “completely full”.