There are several chemicals which you can use. Benomyl and dinocap are two options. I suggest that you go to chemical seller and tell him which type of fruit tree you want to treat. And take a sample of the diseased leaf with you.
Apple powdery mildew is called Elma Kullemesi (in Turkish Elma küllemesi)
Peach powdery mildew is called Seftali Kullemesi (in Turkish &Seftali küllemesi)
If you do not want to use chemicals the try and google powdery mildew and you should get some suggestions.