I would think it's a mineral deficiency.
Make sure you feed it ericaceous plant food as it's an acid loving plant.

Hi Glienberg and welcome to GoY rhododendrons only grow well in acidic soil. If you have alkaline soil you will need to either grow in a container of acidic soil or, if you are growing in the ground, create a large planting hole filled with acidic soil/compost. You will also need to feed with sequestered iron.

In the UK, sequestered iron is sold under the product name Sequestrene, don't know if you have that brand over there. Note that its not a feed, simply an iron tonic to correct chlorosis on acid loving plants. Can be used six monthly for acid loving plants growing in slightly alkaline soil.