Answer: One thing to note is that rhodendrons have very shallow roots, so if you've done anything to disturb the top few inches of soil, that could be a factor in weakening the plants.
It's possible that your plants are suffering from an infection of rust, a disease caused by the fungus Pucciniastrum vacinii. Symptoms include rust-colored pustules on the undersides of the leaves, which eventually release golden or brownish spores. This fungus requires an alternate host--hemlock--to complete its life cycle, so if there are hemlocks nearby, suspect rust.
Rust can be difficult to control once an infection has taken hold. You can try spraying with a sulfur-based fungicide every 10 days or so during July and August. Or you could try spraying a neem-based product. Neem is a botanical extract that has fungicidal properties. Also, prune and destroy all affected foliage.
I can't say for sure if you are dealing with rust, but I hope this offers some help.
Q&A Library Searching Tips