Answer: I'd do a little pruning, both to remove the canes with the worst spotting and to increase air circulation. Increasing air flow around rose plants will reduce the severity of the disease by promoting rapid drying of the foliage. The spores that spread the fungus are carried by water as it is splashed onto other areas of the plant. Because the disease survives in fallen leaves and the canes, these parts, if diseased, should be removed and destroyed in the fall to help prevent a heavy resurgence of the disease next spring. Protectant fungicides can be used; application should begin in the spring when the new leaves appear. From then on, apply weekly well into the fall on highly susceptible varieties.
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