Infected leaves characteristically turn yellow. They fall prematurely. This leaf spot can be distinguished from others by the fringed margin and consistently black color. Cane infection produces a reddish-purple spot. In many varieties, pale flower color is also indirectly caused.
Black spot is spread by splashing water.
Infection occurs after leaves are wet for several hours. Therefore, the disease is more serious during periods of rainfall. A preventive program for black spot should begin with a thorough cleanup in the fall. Diseased leaves on the ground should be raked and destroyed. All diseased canes should be pruned off by cutting several inches into good wood.