I’d guess Botrytis as it will take down a branch in just a few days. And especially since it’s effecting a specific branch(es), as opposed to the entire plant. Look at the effected stems where it meets the main stalk and see if there is a gray fuzzy mold. If you do have Botrytis the effected stems are toast, you’ll have to cut them off and toss and hope that you can keep it from spreading.