Appears to be typical lacebug damage. The dark material on the leaf undersides is excrement from the lacebug nymphs. The white colored stippling on the upper surfaces is a result of their feeding.

Agree completely - lacebugs.

Thanks so much! Now I have to see if I can get rid of them...

Some of my rhododendrons were hit particularly hard last year. I'm thinking our unusually extra dry summer had something to do with it. I'm not able to water most of my plants, but I do site them with that in mind. It seems that some varieties with the smaller leaves were attacked more than the large leaved ones.

It's gonna be interesting to see what happens this year.

mike

In my experience a continent's width away from jasons74 and miketann, once a lacebug infestation begins it only grows worse. Severe damage as shown in miketann's photo can seriously weaken a rhododendron when repeated over a a second or third year. County Extension Services are usually the best source of advice on controls and especially the timing of any sort of spray. Timing is very important. Catch the first generation at the right stage and the battle is made much easier, at least for that year. I do think some systemic action from sprays or drenches is necessary unless you have just a few plants and can use a contact spray. I have had very good results using an imidacloprid drench at about half the recommended dilution. Rhododenrons with thick indumentum/tomenum are pretty immune. Those with thinner leaves are much more vulnerable. Some varieties also seem particularly prone to damage. Boule de Neige is a well known example.