Strange powder

On one of the roses I noticed a white powdery substance below the bud union on the stem only. It rubbed off easily enough with my fingers, I suspect something may have layed eggs. it's kind of weird, it's no where else on the plant, so I don't think it's fungus.

It is a fungus, unfortunately. That's classic powdery mildew. On some varieties that aren't overly affected by powdery mildew, they can show symptoms around the lower portion of the bud and down the flower stem a bit. Powdery mildew doesn't just affect the leaves. As you've found out the next most common location is the flower buds. A good soaking in the morning will help tp combat it without the need to spray anything as powdery mildew is generally only a problem when it is dry and the nights are cool with wrm days. It hates water so wetting it will effectively control it.

some times it can be scale which is tiny and white, try some white oil