Black spot is caused by the fungus Diplocarpon rosae. It affects some roses more than others and proliferates in areas with high humidity and poor air circulation. The fungus appears as black or dark purple spots with feathery edges. The bottom leaves of the bush are usually the first affected. As the disease progresses toward the top of the plant, the leaves also turn yellow and the plant declines. In severe cases, the spots show up on the limbs, or canes, of the rose bush. Black spot grows best in temperatures between 50 and 80 degrees F. Chemical and organic fungicides are available to treat it.
Prune off dead canes, canes that cross each other, or diseased canes. Cut off the canes where they meet a main branch, or cut them to the ground. Remove all yellow leaves and leaves that have black spots.
Remove all fallen leaves from around base of plant; they harbor the disease. Once water hits on the dead leaves, it splashes the fungal spores back to the plant and the cycle will begin again.
Apply rose fertilizer, following the instructions on the container. Don't add too much; you may kill the plant or cause it to produce too much tender green growth that is susceptible to the fungus.
Spread a 2-inch layer of mulch around base of plant, covering the root zone, to prevent water from splashing and to keep moisture in the soil.
Spray the plant with fungicide made to treat black spot on roses. Follow the directions exactly; the life cycle of the fungus is 10 days, and you need to break the cycle for control.