Shrub type roses bloom beautifully with few chemical controls needed, while the more susceptible varieties such as hybrid teas, require an effective spray program to be in place before the growing season begins. The disease is caused by the fungus, Diplocarpon rosae . It is most severe after long wet, warm periods in the spring. Symptoms occur on rose leaves as circular, black spots surrounded by a yellow area. Infected leaves often drop from the plant.
Infection continues throughout the summer months.
The immature wood of first year canes develops raised, purple-red irregular blotches. Plants become stunted and produce fewer, paler flowers. By mid-summer severely infected plants may have lost all of their leaves. In the fall remove all old leaves on the ground, and any mulch that has been contaminated with infected leaves. Severely infected plants should be pruned back in the fall or early spring to within 1 to 2 inches of the bud union, according to variety and cultivar.