Leaf spot diseases appear when the weather is wet and there is poor air circulation around a plant. Here is a link to list of potential diseases on rhododendron. Scroll down to Cercospora leaf spot) and follow the directions for treatment.

Thank you, Eileen. Two more questions:
Can this spread to my other rhododendrons?
Since it is crowded between two other shrubs , does it make the most sense to  take it down, if it does spread? Or can it be saved?
It also sits right under an area where the house gutter gets clogged and may be getting extra irrigation, but we already have had so much rain.
I really appreciate your assistance.

Fungal diseases spread by making tiny spores that go airborne so they can spread easily. However, the spores are always in the air because other people have the same plant. You can't eliminate fungal diseases for that reason.
The best way to control (not cure) a fungus is hygiene. Remove all infected leaves and leaf debris. Bag tit up and put it in the trash. That gets rid the spore source nearby.
Then prune crowded shrubs to increase the air flow. Eliminate overhead watering because it wets the leaves. Water at the soil level with soaker hoses or drip irrigation so the leaves stay dry. Many microorganisms evolved originally from water and need drops of water on the leaves to successfully invade the plant.
As to the nearby gutter, that would be a good place for a rain barrel. The downspout would empty into the barrel instead of getting the plant wet. Those kind of spots are best for wetland plants because the extra water can reduce the oxygen available to the roots.
Here is a link to the website of the rhododendron society. They have lots of tips on keeping your plants healthy.