but rot is usually caused by the "botrytis" strain of mold, commonly known as "gray mold". infected areas are -- you guessed it -- gray. as a progression, it seems to go from yellowing -> brown/tan shades -> gray "fluff".
the finicky part is spotting it, because the badly infected areas tend to be in the heart of the bud (ie. not visible from the outside). i find that there's an accompanying moldy smell, but it's hard to describe if you've never smelt it.
if you go to pull an out-of-place yellow leaf and it comes off a bit too easy, try inspecting the surrounding area. if you have high ambient rH in flower and a strain that produces dense buds, keep an eye on your top colas.