My rhododendrons have a problem. What appears to be a white powder covers the buds and spreads up the leaves. What is it, and what can I do to stop it?
I cannot be absolutely certain without seeing the plants, but it sounds as if your rhododendrons could have powdery mildew.
Here is an article from the Washington State University Cooperative Extension which describes this disease. One preventive measure you should certainly take is to clean up all the fallen leaves and twigs under your rhododendrons, because the fungus which causes powdery mildew can overwinter there.
You could bring in a sample to a Master Gardener Clinic, and ask if they can diagnose the disease as well (they are at the Center for Urban Horticulture and other locations–see their website.