If this disease is left alone, it can have a long-term dramatic effect on the overall health of the plant. The primary function of leaves is to serve as the food producers for the entire plant. You can say that the leaves bring home the bacon. So the bottom line is, no leaves, no bacon. Or in the case of the your roses, no leaves, no food, no flowers.
Black spot infects the leaves in the fall.
If the spores of the fungus sit on the leaves for more than seven hours, they can germinate. The spores will overwinter on the leaves as the rose goes dormant. In the spring, pickup any fallen leaves around the base of the rose bush. This will help reduce future spreading of the fungus. Also in the spring, it is a good idea to replace the mulch around the rose and make sure it is in a good area where it can receive good air circulation.