It sounds like blackspot fungus. Your profile doesn't list your location. Blackspot is most common in humid areas like the hazy hot and humid northeast, and the just plain humid southeast.
Some roses are disease resistant than others - something to think about if you add new roses.
Cleanup is key - remove dropped leaves, keep mulch away from the drip line of the plant, don't compost rose trimmings or leaves.
A healthy rose is less susceptible to disease. Feed and water your roses regularly. Water at the base of the plant early in the day if possible so any damp leaves have time to dry.
A systemic product like Bayer systemic rose granules applied regularly made a big difference in black spot in my garden. I applied the systemic granules to the vulnerable plants regularly. I marked fertilizing and treatment days on a calendar to keep track.
I garden as naturally as possible. The Bayer systemic rose treatment is the only non natural product I use in my garden. (Bayer sells several types of rose treatments. Shake the bottle. If it rattles, it is the systemic. If it sloshes it is a spray, which I don't use. It is a good product, but I have asthma. I only spray natural products like neems/soap/baking soda)
As I add roses, I look for disease resistant varieties, but I don't want to give up my favorite diva roses. It is a personal dilemma, but I thought that I would share it with the group. People have very strong opinions about organic/natural/chemically assisted gardens. Each gardener makes her/his own choice. This group has always respected divergent opinions.
