All though they are hard to tell a part Bud rot is mold, it will have mold and spores with a greyish look.. It will move thru the plant quickly and everywhere. Caterpillar damage is localized and can be picked out and the bud will continue to do fine, you normally will see where the caterpillar has bit the stalk of the infected area causing death to that section of the bud and will most likely be confirmed by finding the worm in that area or other worms on the plant. If bud rot you will remove the infected area see no signs of worms,inspect for mold, and it will continue to over take that plant.