Answer: A white powdery substance on the surface of the leaves could be a fungal disease called powdery mildew. If your rhodie is in a very shady spot where air circulation is poor, it can develop this disease, especially if daytime temperatures are warm and nighttime temperatures are cool. There are fungicides you can use to control powdery mildew but you'll need to improve the growing conditions or the disease will return. Best wishes with your rhododendron!
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