I have a potted rhododendron. It was healthy but some leaves have now started to shrivel and on the upper area of the leaf there is what looks like a sprinkling of fine sawdust. The leaves affected are starting to shrivel at the edges, but the rest of the plant looks healthy.
Rhododendron Bush Disease?
Certified GKH Gardening Expert
Looks like you have mealybugs. You can treat the plant with a pesticide to get rid of them. I personally like neem oil as it is organic and beneficial bug safe.
Here is some information that might help: