This starts first on new shoots as crinkled leaves and then a soft white powdery fungus (Podosphaera pannosa) appears, if left alone it will rapidly spread across the plant and infect other roses. The problem is worsened by poor airflow, poor light, soil drying out and poor nutrition. So keep under planting to a minimum, plant in the sun and feed and water roses well. Try to water really well in the morning, so the foliage dries out and to reduce night time humidity. Feed with a balanced rose fertiliser (highly nitrogenous fertilisers produce too much soft growth which is prone to fungal attack) and add in sulphate of potash for added cell wall strength. Unlike many other fungal diseases powdery mildew doesn’t necessarily need wet conditions and can thrive over a hot dry summer.
Powdery mildew over winters on the stems and in some dormant buds. This is a good time to spray with a fungicide to get a jump start on reducing the problem. Then keep an eye on new growth in spring and spray as soon as the mildew is seen. Use a good commercial fungicide or make up your own.