brown leaves on rhododendron
There are a number of foliar fungal leafspots that can affect rhododendron. They are usually worst in wet conditions. You can apply protective fungicide sprays, but they won't cure leaves that are already infected. If the wet weather continues, it may be beneficial to spray to protect new growth that develops. But if it dries out, new infections will likely stop on their own. Healthy rhododendrons can withstand quite a bit of defoliation and still come back well. Since your rhododendrons are defoliating this year, consider using a protective fungicide spray next spring.