Problem: Black Spot
Black spot is caused by a fungus (Diplocarpon rosae) that is a severe problem in areas where high humidity or rain is common in spring and summer. The fungus spends the winter on infected leaves and canes. The spores are spread from plant to plant by splashing water and rain. The fungus enters the tissue, forming spots the size of a pinhead. The black spots enlarge, up to 3/4 inch in diameter, as the fungus spreads; spots may join to form blotches. Twigs may also be infected. Plants are often killed by repeated infection.
Apply a fungicide labeled for this disease, following label directions. Avoid overhead watering. In the fall, rake up and destroy the fallen leaves.