Answer: Sunscald is the most common reason for brown areas on Japanese maple leaves. Sometimes they are tan rather than brown. Sun can also bleach the color from the leaves. Because both your maples are affected, I suspect the problem is sunscald. On the other hand, fungal and bacterial diseases can also cause brown spots. These are defined as spots rather than blotches though and they tend to enlarge as the disease develops. Without knowing whether the problem is caused by the sun or by a disease, I can't really suggest a remedy. You might want to take a sample of the affected leaves into a local Master Gardener clinic or cooperative extension office so they can examine the leaves and diagnose the problem. Best wishes with your maples.
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