That white powder looks like powdery mildew. This fungus is encouraged by moisture on the leaves, poor air circulation and humid weather. I suggest you collect any really infected leaves that you can see and rake any that fall to the ground. The good news is that with fall coming soon, the tree will get a fresh start as the mildew will not survive the winter. To keep it from coming back, you can use a dormant spray on your magnolia when it is leafless in winter, following the directions on the package. Your local nursery can recommend a dormant spray for your area. My second tip is to add a fresh mulch of wood chips or bark dust or compost all around the base of this tree in the spring. This will help to seal in mildew spores that dropped to the ground to overwinter. Do not wet the foliage with a sprinkler, and thin out weak or small branches by pruning them to the trunk, as this will increase air circulation. That said, the tree could be stressed by too much pruning. Perhaps you could thin branches rather than shorten them to give the tree a break from pruning. The good news is that your tree will most likely survive and look great next summer. Keep growing, Marianne Binetti