Keep an eye on plants like yarrow, asters, azaleas, coreopsis, dogwoods, euonymous, gaillardia, gerbera daisy, honeysuckle, hydrangea, lilac, monarda, phlox, pulmonaria, roses, rudbeckia, scabiosa, spirea, verbena and zinnias for powdery mildew. Check out this link for more information on powdery mildew.