Originally Posted by NFPPlants

Dear Thorny Rose,
It is likely the plant has suffered with a Phytophthora (foot rot) fungal pathogen last year. It will appear to still have roots but it is the fine root hairs that are affected. One plant can be affected as some factor has allowed a harmful pathogen to gain dominance at the microbiological level.
Rhododendrons carry quite a reserve of energy and the visual sypmtons don't appear until they start trying to grow again in the spring and are unable to get sufficient moisture and food.
Two options: -
1 drench with a product like Murphy Traditional Copper Fungicide
2 root and shoot prune and replant in a smaller pot
Drenching knocks the fungal pathogen back while the fine root hairs regrow and the plants natural defences can establish again.
Pruning in this way saves using chemicals but is more drastic. You can always prune out any dead branches later in the summer.
I suggest trying a drench and giving it a chance to recover through the early summer. Rhodos are surprising for their ability to come back from a bad time especially with a bit of help and TLC.