There are seven genera of powdery mildews and several hundred species. The powdery mildew that attacks roses is not the same one that attacks apples, and the one on grapes is different than the one on oak trees, etc.
Fortunately, the treatment for powdery mildews is all the same. On plants, which do not develop systemic infections, sprays will help control powdery mildew. Usually, once powdery mildew is present, several sprays throughout the summer and fall may be necessary to keep the mildew under control.
For your apple tree and for other plants that develop systemic infections (oaks), it is necessary to prune off the diseased terminals and then spray for continued control. Ideally, the pruning should be done in winter (dormant season). However, since the infected terminals on your tree will never return to “normal,” I suggest pruning them off now and then protect new shoots with fungicides.
Always use plant protection products according to label directions for best results.