Last year I sprayed the roses, with Rose Clear, which kept it at bay, I find Mahonias are also very susceptible to mildew.
I have decided now that roses are too much trouble, they have to be sprayed constantly, to keep away many diseases. They are too much trouble for me, and look like dried sticks throught the winter, the need pruning every year, they are prickly. I am fed up with them, and have dug them all up.