Rhododendrons or Rhodos turn yellow and brown for several reasons but most often dry soil, being planted too deep or a really hard winter will cause the lower leaves to drop. If they turn brown and dry it could be lack of water. If the leaves are mostly yellow and soft it could be poor drainage and if just one side of the plant is suffering that could be wind damage from a hard winter. Too much fertilizer (rhododendrons are not heavy feeders) can make the leaves turn brown at the tips. Bark or compost piled on top of the shallow roots or touching the trunk can encourage stem rot or suffocate the feeding roots making yellow leaves that fall from the plant in large numbers. Leaving the roots exposed can also burn them so the plant droops. Apply a wood chip mulch two to three inches thick around the dripline of the shrub and feather the mulch so that it less deep as you move toward the center of the plant. Rhodies are thirsty plants so don't let the soil dry out. Hope this helps, Keep Growing, Marianne Binetti