That's bud rot. Damn, people.... It's caused by mold and spreads. At first is many times white than turns necrotic brown. But, It can be caused by anything that does damage makes a little bit of the plant rot than spreads. It's like gang green but on a plant. If a dead bug decays in a bud, or part of a flower dies, or live bugs eat/shit/piss=kill part of a plant, or mold sets in because the humidity is too high and/or a fan isn't blowing adequately to get the old humid air out of the bud centers it will cause mold and bud rot. keep fans on and vent at night keep humidity under 45% all the time, but especially at night, that's when mold grows the most. Keep your grow area clean. Cut dead shit out now and hang dry separate than your good bud. Also fluctuations of real dry and real moist will make bud rot thrive. If it gets dry it will get brown and when that dead crap gets wet at night it will rot.