Answer: Sounds as though your rose has powdery mildew, a fungal disease. It is quite common on all types of roses. Try using a homemade baking soda/oil spray: mix 1 tsp. baking soda and 2.5 tsp. summer oil (a lightweight horticultural oil) in 1 gallon of water. Spray this once per week, covering both upper and lower leaf surfaces. Also, keep your roses healthy through proper care, since they'll be more resistant to disease. A sunny location, good air circulation all around the plant, and ample moisture are essential. You can remove the leaves affected with powdery mildew so it won't spread to healthy leaves.
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