Downy mildew in roses is caused by Peronospora sparsa fungus. Affected leaves yellow and drop; young shoots wilt and die. Left to its own devices, downy mildew eventually kills the rose plant. If you typically avoid using strange chemicals, you'll be relieved to learn that they're really not helpful for eradicating downy mildew, anyway, and they're not much better than more natural control measures. First and foremost, exercise good cultural practices and focus on prevention as your major line of defense against the fungus. Keep an eye on your garden, because it only takes about one to three days for this beast to get away from you.
Treat your rose plants with homemade fungal preventive spray early in May, when weather begins to warm but is still a bit cool. Combine 1 tbsp. of baking soda with a gallon of water. Add two to three drops of dishwashing liquid and mix well. Apply to plants once weekly. The fungus typically raises its ugly head in temperatures below 75 degrees F and when humidity rises over 80 to 85 percent.
Inspect rose plants carefully each day of the growing season for evidence of downy mildew. Youngest growth is the most likely to be attacked first. Look for dark red, burgundy or black stains on stems and leaves. Be particularly diligent when atmospheric conditions are cool and moist.
Water your rose plants as usual throughout the growing season. Take care not to wet the stems or foliage if they won't have time to dry thoroughly before the temperature drops for the evening. Downy mildew spreads easily in moist conditions and can defoliate a plant in as little as a single day.
Combine 1 part household bleach with 10 parts hot water in a bucket. Dip clean, sharp pruning shears in the solution to sterilize them, and wipe dry with an old rag. Drop the rag into the bucket, pull on some disposable gloves and head for the rose garden.
Aggressively prune any affected stems and foliage from your rose plant. Drop the clippings into a plastic garbage bag--don't pile them up or toss them on the ground, and don't add them to the compost heap. You don't want to be dropping any fungus spores, because they'll immediately find themselves new victims and renew their assault.
Wring out the rag in the bleach bucket and wipe all surfaces of your gloves and your shears thoroughly when you have finished pruning each rose plant. This will go a long way in preventing the spread of live spores from one plant to the next.
Spray rose plants that have a history of downy mildew infections with weekly applications of fungicidal soap throughout the growing season, as per the packaging instructions.
Treat roses with Neem or other horticultural oil during dormancy if they have suffered downy mildew infections during the summer. Be sure to follow the manufacturer's recommendations carefully.