Although one leaf is not a lot to go on, my opinion from your photo is that your rhododendron is in heavy soil and with our wet spring, it is waterlogged.  The fine absorbing roots are probably rotting and so cannot do their job of supplying water to the plant.  (If the problem were a leaf spot fungus, we should be able to see little black dots of the spore producing bodies on the leaf spots.)  Depending on how affected the plant is, you may not be able to save it, but you can try moving it to a site with better drainage.  Rhododendrons can be pruned back to about 6 inches above the ground if necessary, and cutting it back and replanting it in better soil means the remaining healthy roots may be able to supply the water needed.  Trim off any blackened rotting roots before you replant.  However far you decide to cut it back, make the pruning cuts just above a leaf node so that new growth will start near the cut.  Good luck with your rhododendron.