Thanks for writing. If you've found black powder, it may be sooty mold, a common problem. This fungus doesn't actually harm the plant, but is an indicator that your plant is under attack by insects such as scale or aphid. So I'd take a good look at your tree for signs of those insects.
If you don't see any signs of insects, your best bet is probably to contact your county extension office. They should be able to help more if you can bring them a sample of the affected leaves. Unfortunately, without that benefit, all I can do is guess!
---Justin, Senior Garden Editor, BHG.com