Hey crossman, from what I can see that looks like budrot. Does the leaves that you have pictured seem quite moist? Typically from what I have seen you will have one or two leaves that are really tucked away close to the bud that will begin dying. That is at the point I have always caught it. I haven't seen it go that far. So here is the deal, if you can harvest it now, my personal opinion, I would do it and do it quickly. Budrot spreads very quickly once established. There may be some that differ with me but once I have the rot, I chop the plant and trim everything at once and then instead of my normal procedure of hanging the entire stems with buds attached to dry, I go ahead and trim all the buds off looking very carefully at each one for rot. If I find one, I toss it. Once all the buds are off I air dry in an OPEN paper bag with only a few buds in each bag along with a dehumidifier in the room I am drying in. You want to get those buds dried as quickly as possible. The problem is, the spores are already there so the quicker you can dry, the less chance you have of more loss. I wouldn't let it grow any longer, some people will say trim the affected buds off but in my experience it is going to spread 90% of the time regardless and it spreads quick. Better to harvest what you have if you can and cut your losses to a minimum. Once everything is harvested, clean the entire grow area up with a weal solution of distilled water and bleach. I normally use 10% solution. The mold spores can live from a while, especially in a moist environment so once it is all cleaned, put a few fans in there to let everything dry off very very good before starting another grow.
I hope that helps.