This looks like the typical damage done by one of the fungal leaf diseases of Rhododendron. When the fungus kills an area of the leaf the dead tissue drops out, making it look like something is eating the leaf. But when you can see brown spots where the leaf tissue hasn’t fallen away yet, you know that it’s a fungal problem not an insect problem. Ask at your local garden center for a fungicide labeled for leaf-spot on Rhododendron and use according to directions. Most garden centers carry organic fungicides as well as conventional ones, so you’ll be able to find a product that meets with your gardening philosophy.
In seasons to come, try to water your plants so that the foliage isn’t frequently splashed with water. Frequent water, which sometimes comes from drift off lawn sprinklers, is the cause of many leaf-spot diseases. For this reason it’s best to water all plants deeply but less often.
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