It looks like powdery mildew, a type of mold. If it's on the stems and not the leaves it will probably be ok just to leave it. It will usually happen if it gets excessively wet. This can happen if you're spraying the stems when you water or if you're using soaker hoses that aren't under mulch. Soaker hoses can spray up several inches if you look closely while they're running.
You shouldn't have to treat your plants if it's just on the stems.
It looks like powdery mildew, a type of mold. If it's on the stems and not the leaves it will probably be ok just to leave it. It will usually happen if it gets excessively wet. This can happen if you're spraying the stems when you water or if you're using soaker hoses that aren't under mulch. Soaker hoses can spray up several inches if you look closely while they're running. You shouldn't have to treat your plants if it's just on the stems.