That white fuzz under your humidity dome is botrytis, and the thing that actually stops it is airflow, not a spray. A closed dome is a cool, still, saturated box, which is the exact environment botrytis needs, so the fuzz is rarely a sign that you watered wrong. It is a sign the air inside that dome never moves. Before anything else, read the inside of the dome like a gauge: how much condensation is on the plastic, what film is sitting on the medium surface, and what color the fuzz actually is. Each of those three readings points to one specific airflow correction. None of them points at a bottle.
I lift the dome most mornings before coffee and hold it up against the window, and that is when the reading happens. Beads of water scattered across the plastic, running back down to the tray, means the air is at least turning over a little. A full sheet of water with rivers in it, hanging and dropping onto the leaves, means something different. That is foliage staying wet for hours, and that is what botrytis actually responds to. Not the humidity number. The wet leaf.
There is a smell, too, and I trust it more than I trust most gadgets. Wet potting mix has a smell of its own. Let a dome sit closed for a few days and sometimes you get something stale and faintly sweet underneath it, like a basement floor after a leak. That is not a diagnosis by itself, but it makes me look closely at the stems and the medium line before I put the dome back on.
Fog that drips is the one that matters
Read the walls first. Light beading that drains back into the tray is workable. Leave the dome on, keep lifting it once a day, and stop worrying. A dome you cannot see through, with water sheeting down and dripping on foliage, is the problem state, because every one of those drips resets the clock on how long a leaf stays wet. Botrytis does not need a wet week. It needs a couple of long wet stretches on tissue that is already compromised, in cool, still air. A dome on a cool windowsill in March is exactly that.
The correction is a vent, not a towel. I open the sliding vents a slit and leave them that way, or prop one corner of the dome with a pencil if the dome does not have vents. That gives continuous exchange instead of the on-again, off-again swing you get from pulling the whole dome off and setting it back down. I still lift the entire dome for fifteen or twenty minutes a day so the air inside actually gets replaced rather than just stirred, and I do it at the same time each morning.
There is a tradeoff here worth naming. Opening the dome drops the humidity, and powdery mildew likes fluctuation. Yank the dome on and off all day and you are creating the swing that powdery mildew enjoys while doing very little about the wet leaf underneath. One consistent airing plus a small continuous vent beats a lot of open-close cycles, even when the average humidity ends up about the same.
The film on the medium is usually not disease
The green stuff fools people. If the surface of the potting mix has a slick green film on it, that is algae, and it is there because the medium surface stays wet and lit. It is not eating the plant and it is not botrytis. It is a message about moisture: that surface never dries, which under a dome is normal, but which also means the band of mix right at the base of the stem is permanently damp and permanently available to anything airborne.
The crisp white ring at the pot rim is a different thing again. That is mineral salt left behind when tap water evaporates, and it sits at the rim and along the drainage line. It is dry, it is crusty, and it has no texture you can smear. Fuzz smears. Salt scrapes. Once you have touched both with the same finger, you never confuse them again.
Neither one is a reason to spray. The green film is a reason to ease off misting and to stop leaving the dome fully closed for days on end, because both of those keep the same wet band sitting around the base of the plant where real infections get their start.
White is not one thing
Powdery mildew sits on top of leaves like flour, and it does not care whether the leaf is wet. That is a big part of how you separate it from botrytis at a glance. It turns up on crowded plants under domes because crowding, still air, and swinging humidity are its combination. When I see that dusty white on leaf surfaces, my correction is space and air movement: fewer plants per tray, no leaves touching each other, and the vents opened wider. Wiping it off is temporary. Separating the plants so air can move between them is the change that holds.
Botrytis is gray-brown, softer, and it does not dust the top of a leaf. It starts on tissue that is already dying, damaged, or soaked. A leaf that dropped onto the medium. A cut stem. A leaf tip pressed flat against the dome wall and wet for days. Under a dome, on a leaf resting on wet mix, that fuzzy patch is where you find it. At first it looks like a small gray-brown smear of fibers. By the time it is obvious, it has already been spreading in that still air for a while.
Leaves that touch each other stay wet at the contact point. So do leaves lying on the medium. Those are the two places I check first, every time, because they are the two places the fuzz always starts in my trays.
What I do when I find it
A plant with fuzzy growth comes out of the tray, away from everything else, before I do anything else. Then the affected tissue comes off, back to clean tissue, not just the fuzzy part scraped away. Whatever I pull goes in the trash, not back onto the medium. The plants around it get checked down at the medium line, since that is where it hides.
If it has moved past a leaf or two, a fungicide labeled for botrytis is the reasonable next step, applied strictly according to its label, including the indoor-use directions. A labeled product used off-label indoors is a worse idea than the disease. I would rather pull one plant and lose it than treat a whole tray while guessing at rates.
The other half of it is pulling dead tissue before it becomes a starting point. A dropped leaf sitting on wet mix under a dome is a landing pad. Removing it takes five seconds and takes away the tissue botrytis needs. Same with spent flowers and any cut stem left sitting wet.
The number I ignore
I do not chase a humidity percentage under the dome. The percentage is easy to read, easy to keep high, and not what drives the disease. What drives it is how long a leaf stays wet, whether the air moves at all, and whether there is dead tissue lying around in there. You can hold high humidity with the vents cracked and air moving and be fine. You can hold a lower number in dead still air with wet leaves and a dropped leaf in the tray, and lose the whole flat.
So the order is: lift and look, read the walls, read the medium surface, touch the white patch to see whether it smears or scrapes. Then change one thing, the vent or the spacing or the dead tissue, and look again tomorrow. The dome tells you whether it worked. You just have to hold it up to the light.
