Powdery mildew, caused by dryness at the roots usually. It will die down soon and the leaves drop off anyway. Next year, water more.
You won't get rid of it now - the plants will start to go over very soon as the weather cools and that'll be it until next year.
Mildew won't infect the tuber Aym, so it will be ok whne dried off for the winter and planted next year.
Powdery mildew attacks plants which are susceptible because they've been weakened by something in the past - the most common cause is that they've been too dry at the roots.
The best way to avoid this happening in the future is to give each plant the optimum conditions for its needs - easier said than done and sometimes weather and soil conditions conspire against us.