I am going through this now with my young roses. The leaves don't like being sprayed and systemic is probably not a good idea. I would remove the infected leaves and give the rose plenty of water.

Thank you, but there are no leaves anyhow. I just got those two roses packed in plastic and bareroot. The mildew is on canes, bud union and even on roots. I cut whatever I could and placed them in water to soak. It would be good to kill the virus somehow.
Please help!

Soak them in a diluted bleach solution for a bit and then pot them up and isolate them from your other roses until it clears up. However, the fungus that causes mildew is already present in your garden and only needs the right weather conditions to grow and show up on your roses. You really can't prevent it from showing up if the conditions are right for it.