I don't know what the issue is but I'd really hesitated to kill it off. Plants have good and bad years and can come back from problems. They can't be perfect all the time.

The new growth looks healthy. The blemishes are on foliage which will drop soon anyway. The browned flowers will be gone soon too. I'd clear away any fallen material, take care with watering, leave it alone and give it a chance.

Well, Floral, thank you! I sometimes do get around to taking that approach, but not usually my initial reaction. [g] I do grow organically, so my toolbox for addressing any kind of disease is limited. But I did take that approach with a rose that had a significant problem and I was actually reminded of that yesterday when I was looking at how healthy it looks this year. After about 4 years of limping along.

What do you mean, 'the blemishes are on foliage which will drop soon anyway' ?

I thought all the foliage on Rhododendron's was 4 season? I've been growing azaleas but not Rhodies.

Even evergreens drop leaves, just not all at once like deciduous plants. There is a constant natural turnover of leaves.