Hi Becky,

Thank you for your question.  I agree that your rose appears to have powdery mildew. Although Drift roses are relatively problem free and have excellent disease resistance to powdery mildew, they can get it and it can be severe causing defoliation and plant death. It is a cause of bloom failure and drop. It is a fungal disease and prevention is important: good plant hygiene (removing dead leaves, branches and litter, pruning to allow good air flow). Also, proper plant spacing for air flow is very important. In a mature flower bed, it is possible that the plants have grown close enough together that air flow has been compromised and fungal diseases have emerged. I have found references to using abiotic measures such as spraying your roses with a stream of water and neem oil, which I find rarely helps. Most of the time you will need to use a fungicide, choices for which are included in the references below. Or you can go by a nursery or big box store, review the available fungicides and read their directions for roses. If you use a fungicide, choose one that is only a fungicide and does not also include an insecticide as these can be harmful to beneficials. You will probably need to apply the fungicide for a number of weeks at a repeated interval such as weekly. Finally, if the disease is far progressed, it may be easier to dig up and discard the diseased plant before the fungus spreads and get a new, disease resistant replacement. Below are some references from the University of California and the Royal Horticultural Society.

https://hortsense.cahnrs.wsu.edu/fact-sheet/rose-powdery-mildew/

https://www.rhs.org.uk/disease/rose-powdery-mildew

https://ucanr.edu/blogs/blogcore/postdetail.cfm?postnum=30338

If you have further questions or need follow up advice please feel free to contact me.

Bob