Hot, wet weather provides ideal conditions for fungal infections in rhododendrons prone to them. There are many, many different possibilities for the precise type of fungus causing the leaf spotting shown in the photos. Normally, such spotting is much more an aesthetic issue than a threat to the plant's overall health, but in this case it is pretty severe and spraying with a fungicide is probably warranted. The whitish residue on the leaves may be left over from a previous fungicide application. With no clue as to your location, it is impossible to recommend a particular fungicide. A nearby garden center or the place where you bought the rhododendrons should be able to help.

Ok, thank you! I'm located in Western Washington. I'll contact the place where I purchased.