Answer: What you're describing sounds like powdery mildew, especially if the undersides of the leaves have a brown, feathery looking discoloration. Eventually the upper sides of the leaves will develop yellowish discoloration, followed by brown spots. Black spots on the undersides of leaves are usually the fruiting bodies of a fungus called Phyllosticta. Rhododendrons are susceptible to both of these fungal problems. You can prune to remove affected plant parts and stop the spread. If this is not practical, use a fungicide such as Funginex or Safer's Garden Fungicide. Be sure to follow label directions.
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