I planted 16 Rhododenrons along my deck and in a bed nearby. Nearly all of them died this year. Some turned yellow first. Some just withered and leaves dropped. Only 4 still remain on the north side of the house. They were planted in the shade (mostly) as are the "healthy" ones. Is there a blight or some disease going around for Rhododendrons? Thanks
You did not mention when you planted the shrubs. If they were planted within the past year most likely they had transplant issues and were not able to establish their root systems. If so, this is not a disease problem.
Some reasons for decline include planting too deeply; poor drainage; if container plants, were the roots teased apart so they could grow into the native soil; excessive mulch; if you added a lot of organic matter into the planting hole you may have created a bathtub effect as water sits in the planting hole and drowns the tree. We also had a very rainy season and many shrubs were affected especially if the soil is clay and poorly drained.
Take a look at our website for the planting process, video (explains the process), and aftercare.
Here is our publication on these problems http://extension.umd.edu/sites/extension.umd.edu/files/_docs/programs/hgic/HGIC_Pubs/TreesandShrubs/...
Rhododendrons grow best in morning sun and afternoon shade in an acidic well drained soil.
All you can do is look for the above issues and it is possible you may need to replace in the spring.