There may be too much reflected heat from the brick wall. This would explain why the growth closest to the wall is affected while the rest of the plant looks fine. If you've been experiencing hotter than normal temperatures this year, that would explain why it's happening now and did not in the past.

Root problems frequently cause these symptoms, but dosen't seem likely since Cunninghams White is one of the varieties most resistant to fungal root attack.

"since Cunninghams White is one of the varieties most resistant to fungal root attack"

True but it isn't impossible I suspect; and this one looks like it might have been planted too deeply. A wall also can reflect water when it rains, and/or there could be a drain spout nearby, and/or land might slope down towards this corner of the house. At any rate where it is planted needs to be corrected...further from the house (whoever invented the concept of "foundation planting", I hereby spit on your grave) and not in a low spot.
There is still time to save it. Act now.

All 3 rhodies are doing this same thing. This summer has actually been cooler than usual and these are under a big oak tree. The top of the plant is actually slightly above ground, so they aren't planted that deep into the ground. When we planted them, we also did some regrading, so the water slopes away from the house. If we pull one out and inspect the roots, is it possible to determine if it's a fungal issue that way? What should I be looking for to determine the cause.

"is it possible to determine if it's a fungal issue that way"

No, not really. What rhododendron collectors call root rot fungus isn't a fungi in the true sense. It isn't going to look like little mushrooms, although such organisms can set in very near the end of a plant's life.

OK, so, even with some shade from an oak, if you are sure it isn't a water innundation problem it probably is the reflected heat. I see sun in the picture... Move it away from the wall as mainegrower suggested. We had a cooler summer but that actually meant more clear spells with low humidity and higher solar irradiance.

What if moving them out isn't an option? Is there a product I can apply?

You need to give us an idea of how much shade they are getting from the oaks, if it is only an hour or two the problem is never going to go away and the plants aren't going to tolerate it forever.

They only get sun from 4 or 5pm until sunset.

With the further information, it does seem likely that the plants are suffering from so-called stem dieback (also called twig blight). This is usually caused by botryosphaeria infection. Usually you can see a dark brown discoloration just under the bark on one side of the stem if this is the case. The best thing you can do at this point is to prune out the dead and dying parts back to sound, undiscolored wood, being sure to disinfect the pruners with a clorox solution or alcohol between cuts. Remove and dispose of any fallen leaves and, if you want to be really thorough, replace the mulch which may harbor fungus spores. There are no curative or preventative fungicides for botryosphaeria that I am aware of. The good news is that Cunningham's White is a tough and vigorous variety and should be able to, as davidrt28 suggests, outgrow the problem.

I agree, it is dieback caused by drought. I see it frequently during dry summers.

Drought can cause entire branches or entire plants to fungal dieback. We have had several years of drought here and we observe that if rhododendrons and azaleas are not watered during a drought some plants will die, but others will just have one section of the plant die. It seems to be the plants way to conserve what little moisture it has. Prolonged drought weakens plants and often results in the appearance of fungal cankers on the branches of older azaleas. Look for branches that wilt in hot, dry weather in late summer and be sure to water azaleas if drought drags on more than a few weeks. Prune out the affected branches to stop the spread of fungal canker diseases.

Botryosphaeria Dieback is caused by the fungus Botryosphaeria dothidea. Typically, dying branches (stem dieback) begin to appear on an otherwise healthy plant. The leaves die and can remain attached to the plant until late summer. Usually a single branch on an established plant is affected. Scraping away the bark with a knife reveals a reddish-brown discoloration under the bark on dying branches of rhododendron. Dieback is difficult to control on rhododendrons in the landscape. The following rhododendron varieties are considered resistant: 'Boursalt,' 'Chionoides White,' 'Cunningham's White,' 'English Roseum, 'Le Barr's Red,' 'Roseum Two' and 'Wissahickon.' Reduce stress to the plants by planting in partial shade and watering during dry periods. Avoid wounding the plant. Prune infected branches well below all discolored wood and dispose of dead plant material. Clean pruning tools between cuts with a dilute solution of household bleach (1 part bleach to 9 parts water).