Here are some possibilities:

If a leaf has brown areas with white spots, it probably has a local fungal infection of Pestalotia rhododendri. This is seldom controlled with fungicides and is best mitigated by good sanitation and avoiding excessive moisture.

There are a number of leaf spots or burns caused by fungi such as Botrytis cinerea, Pestalotia rhododendri, Phyllosticta, Septoria and others. Many are secondary infections happening after mechanical damage or environmental stress, such as sunburn, drought, winter damage or windburn. They generally occur during wet weather and many times are self limiting with drier weather. Good sanitation is helpful, so remove brown and fallen leaves. Also provide good air circulation. Spraying with Benomyl or similar fungicide can be useful, but is frequently not necessary.

Leaf spotting can also be caused by chemical injury, such as drift from cleaners, paints, or chemicals used to kill moss on roofs as shown in the photo on the right. Sometimes the results of such injury may not show up for weeks or months.

Some leaf spots are caused by a virus thought to be a potexvirus, the most common ailment being called Necrotic Ring Spot. The symptoms are reddish-brown rings or spots on the leaves. It generally occurs only on the two year leaves of a few rhododendron cultivars such as R. 'Unique', or on Kalmia latifolia. It also appears on the first year foliage of some R. 'Loderi' clones. Little is known about the disease and a does not seem to spread from one cultivar to another. No control is known or generally necessary.

Here is a link that might be useful: Problems and fixes for rhododendrons and azaleas.

It has been exceedingly wet here for the last week or two, so hopefully now that it's drying out things will start looking up, and I'll make sure to keep dead leaves cleaned up and maximize air circulation. Thank you so much for your help, and keep your fingers crossed for me!

Matt