Q: One of my rhododendrons has brown on the edge of the leaves, and it seems to be going further into the leaf.
Lack of water is usually the reason for that type of water. Rather than daily watering for a few minutes deep water it with 3/4 of an inch of water twice a week. Muclhing with 3 inches of organic mulch such as wood chips will help soil to retain moisture, Do not fertilize until you see signs of healthy new growth - say next spring, and do no use chemical fertilizers. Holly Tone by Espoma is a good choice, Best And Happy Yardening, Nancy