@akamaingrower, the filtered bark has no pieces smaller than about 1/8 of an inch, so the roots are breathing very well in that soil. My experience with Al's gritty mix has taught me how to structure a soil.

What are some of the possible causes for the black stem damage?

Assuming it is not overwatering, you would expect the drooping leaves that are new growth to straighten out as they mature? That plant is under a large tree, so it gets only minutes of sun at a time through the shade of that tree.

The necrotic stem (if it is really dead) could have suffered physical damage, but there seem to be seed pods and what's left of some blossoms hanging on. This suggests some sort of fungal infection that spread from decaying leaves/blossoms to the woody parts of the plant. Having someone look at the plant would be better than any diagnosis of mine by photograph. I think this is definitely worth doing, as I agree that the plant does not look healthy. Under trees does provide good shade, but often allows caterpillars, some of which feed only after dark, to drop down from the branches. New rhododendron growth is soft and floppy until it has time to harden, but lots of the leaves are damaged by some sort of insect attack and I still believe too much watering may be contributing to the plant's troubles. See what a good local nursery, your county extension service or state ag department has to say after examining it.

@akamainegrower, you were right about the overwatering. I reached down into the root ball and it is really moist compared to the douglas fir bark in which it is planted. I'm not sure why the other plants are thriving whereas this one is over watered, but maybe the material in the root ball was different for this one.

The bigger problem is the infection. A local nursery is telling me they think it is systemic because multiple stems are showing that black necrotic color, starting from the top down. They sold me a systemic anti-fungal. My plan was to let the plant dry out completely, then water with the anti-fungal and let it dry out again, hoping that the thirsty plant will take as much of the anti-fungal into its roots as possible.

Does that sound like a reasonable plan? The plant is in trouble so I don't know if I can save it or not.

I don't think I would let the rootball dry out completely. This would simply add another level of stress. Instead, I think I would repot it into a mixture with larger particles that would hold more air - something along the lines of the 5-1-1 mix maybe with an extra portion of coarse perlite. Planting with the rootball slightly above the surface should also help. From your description, I assume the fungicide is intended to be used as a drench rather than a spray. Follow the manufacturer's recommendation. You won't gain anything by trying to apply it to a dry rootball (this would actually impede absorption) or repeating the treatment twice. What did the nursery say about pruning out the necrotic stems back into healthy tissue? You also have to face the fact that fungal infections like this one are very difficult to eliminate once they have become established. Fungicides are far better as preventative rather than curative agents. Certainly worth trying, but be prepared for failure.

@akamainegrower, it is already planted in fir bark that is filtered. There are no fines in there. They did not think I needed to trim off the dead sections, but I'll revisit that if they continue to grow.