A picture of affected "Cathedral" live oak leaves.
Determine how much damage is acceptable before treatment is necessary. I have seen defoliation of this cultivar later in the growing season that might be attributable to this pathogen growing unchecked. This potentially took the affected trees out of availability for sale until new leaves could be grown in the spring. Here is a link to a publication on treatment for powdery mildew.
Professional Disease Management Guide for Ornamentals http://edis.ifas.ufl.edu/pp123
and here is a publication on powdery mildew on gerbera daisy with a list of fungicides that that can be used for ornamentals. http://edis.ifas.ufl.edu/pp257
Make sure that if you decide to spray you read the label for proper rates!