Powdery mildew, indicated in its early stages by crinkling young leaves, is a common disease of roses. As the disease progresses, powdery white patches appear on leaves, stems and buds. Use this treatment if powdery mildew appears on your roses.
Remove affected parts of the plant
Remove all affected leaves and stems. Bag and seal them for disposal or burn them. Never compost leaves or stems affected by powdery mildew, because the mildew spores persist in the compost.
Use a fungicide to prevent additional outbreaks
Mix 1 tablespoon of baking soda, 1 teaspoon to 1 tablespoon of dishwashing soap, 1 teaspoon to 1 tablespoon of vegetable oil, soybean oil or horticultural oil and 1 gallon of water. Water the roses well by watering at the roots, and then spray the baking soda mix on the plants. Be sure to spray all the stems and the undersides of the leaves as well as the tops. Spray the soil beneath the rose to knock out spores there. Repeat this procedure every week or two.
Replace plants with resistant varieties
If all else fails, replace roses killed or severely weakened by powdery mildew with varieties bred to be resistant to the disease, such as Knockout or Carefree Beauty roses.