Answer: Unfortunately, there are a number of possible problems that can beset rhododendrons with a number of similar symptoms, and a drought year stresses the plants and makes them more susceptible in general. Since you have so many plants and they are exhibiting different symptoms, I would suggest you take a sample and possibly some photos and consult with your county extension and/or professional nursery staff to see if they can make a more precise determination of what is happening.
In the meantime, do not fertilize them.
Keep in mind that watering should be done to supplement rainfall only as needed to keep the soil evenly moist but not sopping wet. These plants are shallow rooted and like to have a humusy, evenly moist yet well drained soil, so you need to water enough to keep about the top six to eight inches of soil moist. Typically in hot weather this might mean watering about every five days or so, dig down with your finger and see if and when you need to water. Then water, wait a few hours, and dig down to see how effective your watering actually is -- sometimes it is surprising.
To help keep the soil moist, use several inches of organic mulch (no more than about three inches deep or you will cause the roots to grow upward for more air) over the root zones but do not place it up against the stems.
To reduce the chances of foliar infection, it is best to avoid watering the foliage; to reduce reinfection opportunities of all kinds, remove any fallen leaves promptly and destroy them.
I would suggest you check your watering system to make sure it is not over or underwatering any specific areas; sometimes a clog or leak can interfere with the amount of water being delivered.
I would also strongly suggest you run some basic soil tests to check for any imbalances or deficiencies. Your county extension should be able to help with the tests and interpreting the results with an eye toward growing rhododendrons.
I'm sorry I can't tell you specifically what is wrong. Good luck with your plants!
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