Answer: Rhododendron dieback is caused by a number of different fungi, of which Phytophthora is one. Many fungi are quite specific, meaning they will attack only one type of plant or family of plants. (For example, the powdery mildew on lilacs won't spread to squash plants, because they are different strains.) I'm not sure how specific Phytophthora is; however, I do know that there are a number of different strains. I suggest you avoid planting rhododendrons and their relatives, the azaleas.
I have a feeling the stress of the excess water, heat and sun all took their toll on the plant. Since the drainage problem is fixed, remember that you'll probably need to apply supplemental water this first season to anything you plant, and make sure that overhanging eaves don't prevent rainwater from reaching the plants underneath.
Here are a few suggestions for reliable plants for sun to part shade:
Evergreen: glossy abelia, hollies, Japanese andromeda, Japanese yew
Deciduous: spirea, sweet pepperbush, oakleaf hydrangea, viburnum
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