Received this photo of what appears to be powdery mildew on Sedum from Bobby Fletcher, Jr. This disease is quite common in nurseries and unlike most powdery mildews on other hosts, very little mycelium is present and is usually only visible with the aid of a hand lens. As with other powdery mildews, disease development is favored by conditions that creat a humid environment (i.e., overcrowding, shade, etc.). Fungicides containing either triadimefon (e.g., Bayleton) or myclobutanil (e.g., Eagle) are labelled for use on Sedum to control powdery mildew, but they must be applied preventatively to protect the foliage from damage.
Donald M. Ferrin, Ph.D., Louisiana State University Agricultural Center, Baton Rouge, LA