Answer: You might have a problem with midges. These tiny insects roll the edges of young leaves and cause leaf edges to turn brown. Adults are two-winged flies, less than an eighth of an inch long. You did the right thing in pruning off affected foliage. In early spring, spray plants with a dormant oil to kill eggs and overwintering nymphs.
Another possible cause is leaf blight, caused by any number of fungi. It would be a good idea for you to gather up all fallen leaves and debris, and remove them from the bed, to remove overwintering spores. Keep a close eye on the plant come spring.
Another factor might be intense sun. Rhododendrons prefer light, dappled shade, and placement in direct sun on the south side of the house could be causing some damage to new, tender leaves. Sunscald on young leaves can also make them more susceptible to invasion by disease. Rhododendrons also will not tolerate dry soils. It sounds like you did some good soil prep., but this could be a factor if you have very sandy soil. On the other hand, they won't tolerate saturated soils either.
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