Answer: Sounds like sooty mold which is actually a byproduct of insect feeding. As aphids, scale insects or mealybugs feed on the fluids within the leaves of your rhodies they exude a sticky substance called honeydew. When this dries it becomes a black powdery substance that you can scrape off with your fingernail. You'll need to inspect your plant for insect pests. Look under the leaves and where the stems and leaves join. Once you control the insect population, the sooty mold will weather away. Good luck with your rhododendrons!
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