Sue wrote:
I think the only sure-fire way apart from regular spraying is to stick
to growing the most resistant varieties. Rugosas for instance don't seem
to get affected. Other than that, making sure that the plants are well
fed and watered, as stressed roses are likely to be worst hit.
Christopher Lloyd once recommended mulching thickly underneath with
grass clippings to keep the spores down, but then he dug up his rose
garden so maybe it didn't work all that well after all!

It's indeed hard to admit, but nothing last for ever, even roses. If
these roses are constantly affected, I would dig them out and start
again somewhere else. Perhaps that spot is not the best one for it.