Answer: A powdery white substance on foliage almost always means a fungal disease called powdery mildew. Be sure to rake up all the fallen leaves; next spring, just before the leaves unfurl, spray your tree with a preventative bordeaux (lime-sulfur). This is the same product used to protect apple trees from fungal diseases. Spray as the leaf buds begin to swell and it should protect your tree from further attacks.
Best wishes with your landscape!
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