It's powdery mildew. Usually as a result dry conditions, so roses planted against walls, or not adequately watered can quickly be attacked. Very common. Sprays are moderately effective, but by far the best treatment is healthy vigorous growth and plenty of water at the roots. Roses are best when they are well watered, their tops in full sun. Watering over the plant can result in downy mildew, which is often mis-diagnosed as blackspot. Last year was dreadful for it, and even one of the top rose growers (I shan't mention the famous name), were spraying twice weekly to counter it! Plenty of time this year. Cut the roses back, and give them a good feed, soak them, and wait for slightly later blooms on good growth