Right on. That sucks. But thanks for the help.
Just my best guesses,
What caused it?
High RH in your largest buds. Room with too high RH? Foliar spray late in flower? Large temp drop when your lights turn off causing high RH?
How do you prevent it?
If you find the answer, spread it around, lol. Keep your RH around 40% late in flower.
And detect it sooner?
First sign is your largest buds dropping a leaf and the leaf turning brown. This is due to the stem where the leaf grows is dying. Once this happens you will have to cut out all the mold, and the surrounding bud from that mold. Me personally, I cut the whole nug from a nod or two below the mold. Anything that isn't moldy is just a early harvest.
What can be done once it's detected?
Cut and dump. Drop your RH in the room to prevent further damage. Once started that late in flower your in trouble.
What is it, a mold or fungus, or what? In what way is it dangerous, specifically?
botrytis - From Wiki
Botrytis cinerea ("botrytis" from Ancient Greekbotrys (βότρυς) meaning "grapes"[1] plus the Neolatin suffix -itis for disease) is a necrotrophicfungus that affects many plant species, although its most notable hosts may be wine grapes. In viticulture, it is commonly known as botrytis bunch rot; in horticulture, it is usually called grey mould or gray mold.
(Not that I want to smoke it, I just want to know what it actually is and why.)
FWIW - The room never got higher than 79% rh and averaged about 65% rh, and the room temp never got above 85*. These are in straight Coir, fed once a week, watered every other day in between, always 25% DTW, and ph is always in check between 5.8-6.2.
Only seems to affect a few top kolas, just the biggest densest top bud, and an adjacent bud or two in a few cases, and we now see it's on about 6 plants of 32. Ruined some otherwise beautiful tops of really nice strains.
We need to stop this from happening again...
Thanks for your assistance.