On sedum, brown scab-like lesions develop with little powdery growth. On some plants, like in this picture, infected leaves turn a purplish color. From a distance, powdery mildew could easily be confused with a leaf spot disease. Look closely for the fine, white fungal threads and chains of powdery mildew spores.
Fact Sheet: Powdery Mildew Diseases of Ornamental Plants