Hello Jill,

There may be several reasons for the damage to your rhododendron. The first is insects chewing on the leaves, and the second is a fungal disease. So, we must rule out whether it is an insect or fungal disease. Examine the underside of the leaves to look for anything crawling on the leaves or chewing the leaves. If you see anything like that, take a picture of the insect and submit it back to us so we can determine which insect it is and help you control the pest. It could be fungal if you see no pests, but I am leaning toward insects.  It could be a lace bug, thrip, black vine weevil, aphids, or spider mites.

resources:

11 Common Problems With Rhododendrons You Shouldn't Ignore (epicgardening.com)

Azaleas and Rhododendrons: Identify and Manage Problems | University of Maryland Extension (umd.edu)

rhododendron leaves are getting holes in them - Ask Extension