It looks like your rhododendron may have two problems. Many rhododendrons last summer suffered from leaf scorch because of the unusually hot and dry weather. While direct sunlight can cause this problem, so can sun that reflects off a house or other building. Be sure your rhododendron is protected from direct sun by providing partial shade with trees or move it to a shady area. Make sure to keep your rhododendron well-watered during hot weather and use mulch around the base to help retain moisture in the soil. When your rhododendron finishes blooming this year, prune out the damaged leaves. This will get rid of the unappealing leaves and help avoid future problems for your rhododendron. More information on hot weather gardening can be found in: Heat Wave in the Garden.

The yellowing of the leaves can be caused by a lack of nitrogen. Nitrogen helps leaves maintain their green color. Lack of nitrogen in the soil may be only part of the problem. Rhododendrons like an acidic soil, between 4.5 and 5.5. If the soil is outside of that range, the rhododendron will not be able to properly uptake nitrogen from the soil. A pH test would be advised before adding nitrogen. This year the soil has been cold longer and it has been wetter than usual, this will also cause the roots not to be able to uptake nitrogen properly. As the weather continues to become warmer, nitrogen will again become available to the rhododendron. The article, Rhododendrons and Azaleas Need Strong Acidic Soil, should be helpful.