Posted: 01/07/2014 at 12:54
Good deduction Edd
That would've been my guess too.
Powdery mildew affects plants that are susceptible due to stress of one sort or another - often under-watering, particularly with something recently planted which has a lot of greenery to support, when its roots haven't really had time to establish.
I would make sure that there's no grass or weeds growing around the base of your hedge, and give it a good watering once a week until the autumn. If we get a prolonged hot dry spell do this twice or more a week.
With a bit of care I'm sure your hedge will recover.
Good choice by the way - I love a hawthorn hedge