Black spot on roses is the result of a persistent fungus, Diplocarpon rosae, that invades rose bushes, especially in wet weather. The disease is progressive and will cause all leaves to drop from the plant.
The black spots will enlarge and become surrounded by yellow halos. The leaves then turn a mottled yellowish-brown color before dropping. The fungus lies dormant in winter and begins germinating in spring. High, humid temperatures and long periods of rain lead to rapid spread of the fungus, so it helps to keep the foliage dry. Diseased parts of the bush should be removed and discarded, not composted. Weekly spraying with fungicidal soap or sulfur helps keep the disease under control. Some rose varieties are more resistant to fungus than others.