100% Bud Rot ! Had it many times and its heart breaking. Always goes for the fattest buds first.
Only thing to do is chuck the bad bits away, and i mean if a cola has it, anything just below and everything above I would chuck out, and watch the rest of the crop like a hawk, it spreads fast and will decimate the big colas.
If your almost done Id cut early, if not, prepare to loose some more.
I bought an ozone generator for this purpose ( and for cleaning up after every run ). Its quite a powerful one too, 5000 to 7000mg.
Id just use it like a hand held blower and go over the entire canopy.
Dont smoke the bad stuff and try not to wave it all over the place either, the spores spread easily.