The brown spots are most likely the result of fungal infection. Rhododendrons are subject to a great many of these, but most are are more of an aetsthetic problem rather than a threat to the plant's continued health. Using a fungicide, either systemic soil drench or a spray, can halt its spread protect newly emerging leaves.

The unhealthy yellowish color indicates a nutritional problem. The place to begin is a soil test, including the soil's pH.