Thank you for your question, Stanley.  Rhododendrons suffer from a variety of nutritional problems which cause yellowing (chlorosis) in their leaves.  It may be an issue of pH (which should be between 4.5 and 6.0), a lack of either iron or manganese, or a combination of nutrient deficiency of the three main nutrients:  nitrogen, phosphorus and potassium.  I would suggest that you have the soil around your plant and ask the testing lab for recommendations to cure the problem(s).  Here is an OSU article with a list of labs in Oregon:  Analytical Laboratories Serving Oregon | OSU Extension Catalog | Oregon State University

Here is another general guide to rhododendron care that you might find helpful:  Azalea and Rhododendron Care and Culture | OSU Extension Catalog | Oregon State University

I hope these are helpful.  Good luck!