Answer: Congratulations on keeping a rhododendron for so long in a container! I am sorry you are having trouble with it now. Wilting can be a sign of over or underwatering; if your plant has grown a lot, it may need repotting into a larger container to prevent it from wilting quickly due to loss of moisture.
Or, it might be related to several diseases such as root or crown rot or dieback. These tend to affect rhododendrons already stressed by being grown in poorly drained soil. Repotting might help, especially if the old soil has deteriorated and lost its ability to drain freely. If well advanced, some of these disease problems are not very treatable while others such as phomopsis, can sometimes be treated and part of the treatment includes removing the affected branches.
It is very difficult to make this type of diagnosis long distance. I would suggest you also consult with your Cornell cooperative extension to see if they can give you a more specific diagnosis of the problem. Based on knowing that, you can determine how to proceed. If it is something that requires a chemical control, they will have the most up to date recommendations on what to use and how/when to apply it for maximum results.
I hoep your plant can be saved!
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