I have inherited several bushes of roses from the previous house owners. Their flower are gorgeous, but they also developed symptoms of some diseases. On the picture below is Black spot (Diplocarpon rosae) and rose rust (Phragmidium mucronatum).
Cultural practices can help for both, more air circulation and sunlight exposure (e.g., prune off some of the rose branches and overstory and nearby plants), no deliberate wetting of leaves… As in Pests of Landscape Trees and Shrubs book.
Black spot is rare in Davis and rust a problem only during very rainy springs and where people overhead water the plants, or grow them too closely/crowded or heavily shaded. This makes clear problem cause is primarily environment and culture.
In Bay Area I’d grow only rose varieties labeled to be resistant. Extent of problem varies by the yearly and seasonal weather. In combination with culture, repeated spraying is sometimes used. Where it’s often foggy/rainy, fungicide effectiveness can be often unsatisfactory.
Conclusion: I will not spray, just remove the affected leaves. My roses are not crowded and have plenty of sun shine. Let’s see if next spring will be different.