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There are so many variables, it's impossible to give you a really straight answer.
I don't know if you live in a climate that has wet springs and dry summer, or has rains during the summer and that does make a difference.
Black spot spores are active until temps reach 85F. Then the rose will only get bs if the leaf surface is wet for at least six hours.
That tiny bit of information tells you that you don't want to use overhead watering on your roses unless you are positive that the leaves will dry out, otherwise, your roses will be vulnerable. So, choosing a watering method that provides sufficient water for the rose without wetting the leaves is something you can do to reduce the impact of the bs spores.
Bs can defoliate a rose and cause it to die, but most roses can actually be defoliated and come back. Again, it depends on your climate and it depends on the rose.
Garden hygiene is another way you can proactively reduce bs in your garden. Removing old mulch where the spores over-winter and replacing it with fresh mulch helps.
I don't grow plants under or around my roses because they harbor bs spores. Sure it looks pretty, but if it helps keep the rose healthy, I am willing to only mulch under my roses.
Your very best defense is to grow bs resistant roses. If you do a Google search for Earthkind roses, you will find roses listed that have been tested for their resistance.
This is just a short list of things you can do to help your garden have healthier roses.
Feel free to ask more questions on the Rose Forum on NGA.
