The main rose diseases are black spot, stem canker and powdery mildew. Black spot symptoms are irregular black spots often surrounded by a yellow halo. As infection progresses, leaves eventually drop. Leaf drop contributes to the spread of the disease by harboring spores and results in weakening the plant and increasing susceptibility to winter injury, drought, dieback and stem cankers. Flower production will be reduced if the disease is allowed to spread.
Prevention is the best control for black spot.
Leaf litter should be removed from the area since the spores will over-winter on infected leaves. Protective fungicidal sprays are advised. The fungus grows beneath the protection of the leaf cuticle, making control difficult once the spores have germinated. Weekly sprays from the time the plant leafs out in the spring until autumn are advised. Spray both the upper and low leaf surfaces.