that bud rot is far gone in its later stages , dont let the rot get to that stage .. as soon as you see the damp mushy stuff, like fluff or grey cobwebs in the bud cut that bit out
bigger colas tend to get it first, some of the small leaves that pop straight out the colas turn yellow, this is a good sign of bud rot
any moisture or dampness you see on a cola look inside them for rot, you can gently pull the buds apart that make up the cola of many strains, some are just too densely packed to do this , bud rot is a strain aswell as an environment problem, some strains/pheno's never seem to be troubled by it, while others suffer from it in the exact same conditions
although keeping humidity low, with constant air circulation does help to prevent / minimize its effects
just cut out all the rotten bits, if there is enough bud left on it after the rot is removed let it carry on growing so it can fully mature, but makesure you remove all the rot look inside all the colas