Can they be composted in a hot composter ? This is at 60 degrees C.
Outside United States compost
Soil scientist Dr. Lee Reich, himself an organic gardener, keeps a record of the temperatures in every part of his compost heaps (25 of them). He concluded that it is impossible for all parts of a compost pile to be at the required temperature to definitely destroy weed seeds and infected plant material unless the heap is commercial size. The short answer is that you don't want to run the risk of the powdery mildew surviving in your compost. It's best to bag the leaves and put them out to trash.