Have you checked for or already dismissed powdery mildew? Is the plant in a humid or coastal location? Does it get fertilized in July or later?

It is in a fairly narrow (12yrds) space between two houses and yes, it's the most humid part of the yard. No, I've never fertilized, I have a bag in the garage but have never applied as the plants are so old. In other words, they've soldiered along for 60 yrs. with no attention to speak of so I've been prioritizing away from them. I don't know how to check for powdery mildew. How would I do that?

The comment about fertilizing in July or later was an obtuse comment, that is a bad time to fertilize. There are special rhododendron fertilizers such as HollyTone, but they should only be fertilized in May or early June and at half the rate on the package.

Powdery Mildew, Microsphaera azaleae, causes azalea and rhododendron leaves to discolor and become coated with a white powdery fungal growth. I don't see any sign of that in your photo.

What I see is a general lack of vigor. There can be many causes for that:

It looks like there may be too much shade. Perhaps in the 60 years, besides being between two houses, some trees have grown up and given it too much shade.

It looks like there may be hash winds blowing between the two buildings. In any case moist soil is a very bad condition. Perhaps any moisture could be diverted from this area.

I would suspect that the roots are not mulched. The mulch not only protects the roots, it also feeds the roots as it decomposes. It is very important to the health of rhododendrons.

The link below lists 10 steps to growing a rhododendron.

Here is a link that might be useful: How to grow rhododendrons and azaleas

Thank you for your detailed reply. After researching powdery mildew (which I occasionally DO get on squash plants) I agree that that is certainly not the problem. There are no harsh winds and they are not in shade. This is not a particularly wet area. What troubles me is that the remaining bushes are very healthy and only this one is suffering. The only discernible difference (as they are planted close together in a row) is that this bush has a rotting Sassafras stump mingled with it's base (we did not plant or cut down the tree but inherited the stump) could the decomposing stump be part of the problem?

In general, rhododendrons flourish when they are near rotting stumps or rotting wood of any kind, so it's very unlikely that the sassafras has anything to do with the trouble your experiencing. Unless, of course, some sort of chemical was poured on the stump to hasten its decomposition.

It would be helpful (maybe) to know what species you're dealing with. Are all the rhododendrons in the row the same species with only this one declining?

If you live in a part of the country where rhododendrons are commonly grown, your extension service, agricultural college or a local American Rhododendron Society chapter should be able to help.

I also wouldn't totally dismiss the powdery mildew (or other fungal infection) possibility. Powdery mildew in rhododendrons does not look the same as what you'd find on squash or on deciduous azaleas. If, however, all the rhododendrons are the same species, it's pretty unlikely that only one plant would be infected over a number of years.

Rotting conifer stumps are often beneficial. In fact i have seen rhododendron seedlings grow in a rotting Doug Fir stump. An old sassafras stump could create a slimy mess that could cause your problem. If practical, it may be worthwhile removing the sassafras slime and replanting. The Rhododendron Species Foundation had to do this after the sawdust they used when they planted hundreds of rhododendrons turned to slime.

We live in Western Pa, rhododendrons grow wild here all over the place. These are quite different from the hybrid varieties I see at nurseries and in front yards in my neighborhood and I've always believed them to be species Rhododendron. They have a 'looser' growth habit somewhat like a Serviceberry and all top out around 20'. The flowers (which will not bloom for another couple of weeks at least unlike most of the others I see) are white with pink and yellow highlights.

The stump is in no way slimy.

The height plus the late bloom time indicates that they are most likely rhododendron maximum, a native species commonly found growing wild in PA.

As noted before, the only way I can see the sassafras stump being part of the problem is if it were treated with one of the chemicals sold to speed up decay.

Diagnosis at a distance is often very difficult. You should be able to find someone nearby - maybe a local nursery - to look at the plant's leaves, etc. and give a more informed opinion.

For Rhododendron maximum growing in Western Pa, the problem looks like some late opening shoots or shoots that tried to open during our loooong winter and were frozen back. I get this fairly often on a number of varieties in Eastern PA.