In rosy wrote:

Is there anything I can do besides start over? I have over 100 roses.

I haven't had downy mildew as such, but the remedy I've tried this
season (which definitely works on all other fungi) is a milk spray. 1
part milk, 6 parts water.

Apparently it's supposed to work because one of the proteins or fats in
the milk smothers existing spores and stops new ones from settling.
However it works, the results have been spectacular for me.