Mayfirst, that looks like petal blight.

Caused by a fungus, the disease develops during periods of moist weather at flowering time. You'll want to practice good sanitation and remove any spent flowers, litter under your plants. Spraying buds with fungicide (Bayleton recommended) just as they begin to show color but before they open is a good means of control...since you've had petal blight in your garden this season, you might want to consider treating your rhododendrons early next year.

Thank you very much. It sounds serious. Is it too late to do anything this year? I would hate to loose these beauties!

Hello, mayfirst. Sorry but, no amount of spraying will cure this year's infestation. However, after all of the flowers have dropped, practice the sanitation techniques that morz8 recommended. Then, next year, begin a fungicide application routine just as the buds begin to swell and show a little color. Spray up to bloom time.

Good luck,
Luis