It looks like your  rhododendron is being eaten by the lace bug. This is a tiny flying insect with wings that look like they are made of lace. The larvae crawl up from the ground where they overwinter and feed on the undersides of the leaves. You can remove the most infested leaves and destroy them or try using a strong jet of water on the back of the leaves to dislodge them. Lace bugs like the sun so moving the plants to more shade can also help. You can also spray the back of the leaves with Neem Oil  or Horticultural oil. There are also some stronger insecticides labeled for use to control lace bug.  The good news is the lace bug rarely kills the rhododendron but some years this insect is worse than others and most damage shows up in the month of June. Good luck and Keep Growing, Marianne Binetti