Downy mildew can appear on any type of plant but the cause of this disease is different depending on the type. When downy mildew forms on roses it is caused by the fungus "peronospora sparsa." This type of fungus forms best when the humidity is high and the temperatures are below 80 degrees. White and powdery, it causes rose bush leaves to have a reddish-black discoloration which starts at the top portion of the bush causing the leaves to die and fall.
Remove all infected parts of the rose bush by cutting away damaged stems, leaves and flower buds with a pair of pruning shears.
Gather the rose bush cuttings and place in a garbage bag. Remove the top layer of dirt surrounding the rose bush by scraping the area with a garden claw and placing into the garbage bag.
Survey the area to make sure every leaf, bloom or stem infected with the fungus has been removed. Discard the garbage bag in an area far from your garden.
Spray the rose bush with a fungicide specifically formulated for downy mildew. Mix and apply per the manufacturer’s instructions. Some common fungicide formulas are fosetyl aluminum and dimethomorph among others. This step may need to be repeated every seven days for the fungus to be completely treated and avoid its return. Always defer to the fungicide manufacturer’s guidelines.