Rhododendron leaf damage
Our plant pathologist looked at your photos. This looks like the decline of older leaves, not a disease. This symptom also occurs because of winter injury (which affects all types of broadleaf evergreens). Winter injury can also damage developing buds. Eventually, these leaves will drop out and new growth will develop later this spring. https://extension.umd.edu/sites/extension.umd.edu/files/_images/programs/hgic/Publications/HG51_IPM_...