Downy mildew's initial symptoms are dark purple or brown spots on new foliage. As the disease progresses, the spots get larger. The infected part of the rose's foliage will turn brown and develop a grayish-white fuzz, and leaves may drop. The fungus is spread easily from plant to plant. However, with prompt and thorough treatment, downy mildew can be controlled. If your rose bushes are repeatedly infected, thin its foliage and avoid overhead watering. If infections still persist, consider purchasing downy mildew-resistant rose varieties.
Prune all infected rose leaves and blossoms. Bag and discard them far from your rose bushes. Wipe your pruning shears with an alcohol swab after each cut to prevent inadvertently spreading the fungus.
Clear away plant debris or mulch from the base of the rose bushes. Bag it and trash it far from your rose bushes.
Spray the rose bush with a fungicide in the dormant season. Strip all of the leaves and dead blossoms from the plant. Coat the plant's tissue with the fungicide, according to the manufacturer’s instructions. The fungicide does not kill existing fungi but will prevent it from taking hold the next season.