The two primary reasons that rhododendron leaves turn brown are cold weather and rhododendron wilt. The former is a naturally occurring phenomenon that takes place as the result of changing weather patterns and doesn't necessarily indicate a dead plant. The latter is the result of a soil fungus.
If decreasing temperatures aren't the culprit, attention should be turned toward the plant soil. Phytophthora, the formal name for rhododendron rot, breeds in very wet soil and often occurs in locations that are naturally wet or in areas of poor drainage. Aside from browning leaves, the roots of infected plants are often described as mushy and dark in color.
There is no way to rid a plant of phytophthora. However, a plant may continue to live if the soil is aerated and drainage improvements are implemented. In some cases, a rhododendrun may need to be transplanted to a different location in order to save it. Soil contaminated with phytophthora will continue to infect or kill rhododendron until steps are taken to eliminate the conditions that are causing the fungus to grow. Combat the fungus growth by planting drain tiles, implementing a formal irrigation system or constructing a raised bed for better soil drainage.