Hi Suzanne, Thank you for asking. The powdery appearance on the rose leaves is likely caused by a fungus disease (appropriately) called powdery mildew. It could also be a similar disease called downy mildew. These and other fungi are around us and will infect plants when the conditions are suitable (temperature and humidity). The disease usually does not kill the plant but may reduce the vigor and is unsightly. The treatments are similar for powdery and downy mildew. You can prepare and spray various solutions of for instance, baking soda and soap to reduce the impact. Mix 1 tablespoon of baking soda + ½ teaspoon of liquid soap (such as ivory soap) in a gallon of water. Another suggestion is to use 3 parts of milk in 10 parts of water. Other environmentally friendly control substances like neem oil may also work. Hope that answers your question, Meriam Karlsson