Botrytis cinerea, the grey mould that hollows out a flower from the inside while the outside still looks finished. Prevention, early detection, and what to do with the rest of a plant once one site has been found.
This is the most expensive disease on the board in a tropical climate, because the conditions that produce a dense flower and the conditions that produce grey mould are nearly the same: high humidity, still air and a canopy that does not dry after dark. It is also the one that is usually noticed too late, since the first sign is a single wilted leaf standing upright in the middle of a bud rather than anything visible on the surface.
Two things are worth fixing in your head before harvest season. It spreads by airborne spores that are already present, so treatment is really humidity, airflow and canopy management rather than a spray. And an infected site is not salvageable: it is cut out with a wide margin and removed from the room, and the rest of the plant is then watched daily.
Where to start
Related elsewhere on the forum: Grow Room Climate Control: VPD, Temperature, Humidity in Indoor Growing.