These often appear to be droppings of insects or specks of plant debris. Removing fallen leaves from around infected plants prior to spring may help reduce infections the following year. Powdery mildew also overwinters under the bud scales of the terminal buds on some woody plants. These buds will be unthrifty appearing and less plump than non-infected buds. Many powdery mildew fungi in warm regions do not produce cleistothecia but survive from one year to the next as mycelium on stems and leaves and in buds of perennial plants.
Fungicides are also available for the control of powdery mildew.
Additional information about powdery mildew is available On-Line . Additional References Boyce,J.S. 1961. Inc. Forsberg, J.L. 1975. Diseases of Ornamental Plants. University of Illinois Press.