Answer: Things don't sound good for your rhododendron. Black spots or blotches accompanied by a rusty fungus sounds like rust disease and/or powdery mildew. While not fatal in themselves, they do compromise the health of a rhododendron and make it susceptible to other problems. I suspect you are seeing either whiteflies or the cast skins of aphids on the undersides of the leaves. Both insects can be controlled with chemical sprays. Before trying to treat the problems, I'd suggest taking samples of the affected leaves to your local university extension office for positive diagnosis. Or, take samples to your nursery (be sure to place them in sealed plastic bags). In either place trained professionals can determine exactly what the problems are and will give you some control options. Best wishes with your rhodie!
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