Powdery mildew.

Very possibly Powdery Mildew, although it is more common on plant leaves then on stems. I do see something on the leaves that could indicate PM. As stated in the link below Powdery Mildew is a generic term for a lot of similar fungi but is fairly easily controlled with sprays of (in my experience) a 50/50 mix of fat free milk and water or 1 teaspoon of Baking Soda mixed in 1` quart of water.

Here is a link that might be useful: Powdery Mildew

PM is VERY common on stems.....I've seen it on roses quite often.