Downy mildew is a common fungal disease affecting roses (and other plants). It causes colored spots on the leaves (red, brown, yellow or purple) and is easily distinguished from powdery mildew, which shows up as a light, powdery dust over foliage. As with all fungal diseases, the sooner you treat this, the less chance there is that it will devastate your roses or spread throughout your garden.
Wear disposable gloves when dealing with the diseased roses--this is easier than disinfecting your garden gloves (especially leather gloves), which is necessary to prevent spreading the spores. If you wear contaminated gloves and touch a healthy plant, it’s possible the plant will become infected with the disease.
Trim dead foliage from the roses, using garden shears, and dispose of the foliage in garbage bags. Diseased plant material should never be composted.
Use a garden rake to remove all debris from around the rose plants, including mulch, which might be infested with downy mildew. As with the dead foliage, this should be discarded in garbage bags.
Spray a fungicide on the foliage. Sacto Rose recommends using systemic fungicide containing metalaxyl. Systemic fungicides continue to work despite frequent rainstorms, unlike fungicides which remain on the surface of the plant and are easily washed away.
Alabama Cooperative Extension suggests that fungicides containing azoxystrobin or fosethyl-Al will also work on downy mildew. Re-spray at intervals advised by the product label (you may not have to re-spray roses if you catch this disease early).
Disinfect tools that contacted the diseased plants. Plant Path recommends using a bleach solution. Fill your kitchen sink with 1 cup of bleach to 9 cups of water. Place your garden sheers to soak in this for 15 minutes before rinsing and drying. Do the same with the head of your rake. If the head is not removable (some are) to leave in the sink, just place it so that the head is nevertheless submerged in the bleach water for 15 minutes.