Answer: It sounds as though your roses are really struggling, not only with the moisture in the air, but the lack of sunshine. Roses really need a minimum of 8 hours of full sunshine to perform well. Four hours just isn't enough to keep them healthy and they are showing their stress by developing mildew. It may be that roses will never be successful in that site and that you'll have to choose another plant to grow in that area. How about camellias? They will grow in partly shady spots without developing powdery mildew. They come in a wide variety of flower colors and provide lots of interest in the garden. Best wishes with your landscape!
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