Looks like bothrytis. Older flowers are very susceptible. Keeping water off the flowers can reduce the chances that they’ll get infected. I have this show up on most of my outdoor plants when the flowers are nearing the end of their life no matter how dry I keep them. I haven’t had any issues with it appearing on new flowers unless they’ve been subjected to something like a day of cold rain.
If it is very humid, even a small temperature drop (like overnight) can lead to condensation on the surface of the flowers - and the dreaded botrytis. If the air is stagnant, a fan might help prevent this sort of of thing, to keep the damp air from settling on the flowers.