Answer: Unfortunately, it is very difficult to diagnose rhododendron problems long distance. They can deteriorate due to pest or disease issues, winter damage, poor planting, unsuitable soil conditions, improper watering, exposure to chemicals such as herbicide, or overfertilizing for example.
I would strongly suggest you consult with your local Cornell extension staff to obtain a specific diagnosis of the problem and based on knowing that, decide what to do for them.
In the meantime, keep them mulched with two to three inches of organic mulch spread out in a flat layer over the root area and water as needed to supplement rain so the soil stays evenly moist like a wrung out sponge. It should be damp, not sopping wet/saturated and not dried out. Also, remove any discolored or fallen foliage and dispose of it in the trash in case this is a disease problem. Do not try fertilizing them, this would just increase their stress level.
I'm sorry I can't be more specific long distance.
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