Powdery MildewPowdery mildew is a fungal disease that affects foliage of several landscape plants and is commonly found on lilac, flowering dogwood, and sycamore.  It appears as a powdery white or grayish white covering on leaf surfaces. This can be unsightly, but usually does not do serious damage to the plant. Control with fungicides is generally successful. Improvements to air circulation via pruning can reduce advance of the fungus.