I would worry less about the spotted leaf than the overall health of the shrub. Should be darker green leaves and many more than it does. (no bare bottoms)

How are you watering and/or fertilizing? If at all.

The plant looks healthy. The leaves only last 2 or 3 year and then look like what you are seeing when they fall off. Nothing to worry about. Actually fertilizing causes more problems than not fertilizing unless you use a slow release organic material such as Hollytone. They seldom need fertilizing. Their most urgent need is good drainage.

It looks like you have a poor clay soil and very little mulch. They like to be planted in a raised bed with lots of organic matter, excellent drainage, and a good mulch layer.

Here is a link that might be useful: How to grow rhododendrons and azaleas.

Thanks very much for your replies. It's a relief to know the plant is relatively healthy. The soil tends to be sandy, which I tried to dig out and replace with compost before I planted. I have placed a little peat moss on top and keep the roots protected with pine bows. I think the drainage is OK. It hasn't rained in a few days, though we've had a wet and cool spring. As you can tell, this is my first rhododendron.

With sandy soil, drainage is probably not a problem. A good mulch layer will help retain moisture. Peat moss is a soil amendment and not a mulch. A mulch will help keep the roots cool in summer, the soil moist, and prevent frost heave in the winter.