Answer: Well, this could be a number of things, but lets try the easiest first. It if doesn't work...come back to http://www.garden.org
Since there are white spots as well as dead leaves, I'm going to assume that this is NOT wind-burn or sun-scald. If it's possible that either of those are the culprit, move the rhodies to a shaded place or protect them in the winter with lathe or burlap. In the summer be sure they don't dry out during hot weather.
But, it could be Powdery Mildew. Here's a spray that rosarians use with great success on their roses. Maybe it'll work on your rhodie. It surely won't hurt! Use 1 Tbsp. Baking Soda; 2 1/2 tbsp. ultrafine horticultural spray oil; 1 gallon water. Mix all this and spray on a cloudy day. Be sure to cover the leaves thoroughly-even the bottoms. Also, rake up and dispose of all fallen leaves.
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