The middle photo shows a number of buds which did not bloom and very desiccated stems. Did this part of the plant look ok in May and then turn brown and dry? When a rhododendron exhibits these symptoms it's almost always a problem with the roots. This could be excessive dryness (or poor drainage) or one of the root rot diseases that begin to appear with warmer weather in the South. Without your explanation and with just the top photo to go by, this would look like typical damage from winter sun, wind and cold. Is that a possibility? In any case, survival of this rhododendron is not very likely.

There were some buds that didn't bloom but nothing that led me to believe that they were really unhealthy. We had a very wet, cool winter but May was unusually dry here so it may be a combination of cold and not enough water in May. For the most part though the decline has been rapid and severe since the bloom. I may need to just break up the soil more and try again with a new plant. Would there be something I could do to prevent root diseases in the future? I planted some native azaleas in another part of our yard under some pines and they did great.

Rapid severe decline most likely means one of the root rot diseases. These are caused by any of a number of water molds and become active once soil temperatures rise above 50 degrees or so. Poor drainage is a contributing factor, so improving drainage and planting with the top of the root ball slightly above grade are important. The best thing you can do, however, is to find a resistant variety. The Southgate series of rhododendrons uses the highly resistant species hyperythrum as one of the parents and was bred specifically for the South. Most other varieties such as Nova Zembla have no built in resistance and are likely to fail in hot climates. Native azaleas are not immune, but do have a measure of resistance. If you do plant another rhododendron, avoid putting it in the exact same spot.

If 'Nova Zembla' was sold by a Piedmont NC nursery, you should stop patronizing that nursery.

For gardeners not exceptionally skilled in the art of keeping difficult plants alive, all standard old-school cultivars/"ironclads" like this one should be considered ungrowable south of the DC area. As mainegrower mentions, one should only attempt the hyperythrum hybrids down there. (and/or a handful of other specifically recognized, root rot resistant cultivars.)

I actually bought this at Sampson Nursery near Godwin, NC. However, it was late in the season and they didn't have much stock left. I'm interested in native plants so if anyone can recommend a rhododendron catawbiense cultivar that's root rot resistant and produces an interesting colored flower, I'd appreciate it.

davidrt28 knows more about this than I do (and I was completely wrong about the source of the Nova Zembla), but I don't think you are going to find any cultivar of r. catawbiense that is resistant to root rots. Catawbiense has been used in 100's of hybrids at this point because of its excellent hardiness. The native r. catawbiense, however, is not a common plant and inhabits primarily high mountain areas or other places such as stream sides where soil (and air) temperatures remain cool. Brought down to the southern coastal plain or any other hot and humid environment and it will pretty quickly succumb to phytophthora and other fungal maladies. A great deal of effort has gone into breeding resistant varieties using r. hyperythrum. If you want a broad leafed rhododendron in your location, one of these is about all that's likely to succeed.

I couldn't have said it better myself.
IIRC, years ago some allegedly lowland form of Catawba rhododendron made a brief appearance on nursery lists but it didn't really prove to be notably tougher in most southern gardens. You just have to look elsewhere for the genes to resist modern strains of root rot. (and, perhaps it isn't surprising that Asia is the source of such genes because some of the more virulent Phytophthora strains, which are now widespread around the world, probably came from there)

Look at it this way: R. 'Charles Loomis', a non-patented R. hyperythrum hybrid...is still 25% native LOL. I have mine in almost full sun, not very good drainage by my garden's overall standards for rhododendrons (not sloped, etc.) and it is growing like a champ, and sailed through the treacherous summers of 2010, 2011, & 2012.