Powdery Mildew on Laurel Hedge

At the front of my house is a laurel hedge that was planted and well established from when the house was built in 1995.  The hedge is kept to a well defined height, width and depth, and is trimmed back every year in the autumn to keep it in shape to a strict rectangle shape.  I moved into the house in 2010.  Every year about now as it starts to put a growth spurt on with new leaves, I notice the hedge starts to develop powdery mildew on the leaves and the hedge starts to look a bit sorry for itself, slowly looking worse until I trim it back come the Autumn.  Is there anything I can do to treat the mildew?  The hedge is about 25 feet long, 8 feet tall and around 2 foot deep.