Answer: Based on your description I am not certain what is happening to your rhododendrons. First off, you may be overwatering or underwatering. The soil should be evenly moist like a wrung out sponge, not sopping wet and not dried out. Feel into it with your finger to see if you need to water, or not. Check that the old and new soil are both moist, sometimes they can drain differently with the original soil where the roots are drying out.
Also, the drain hole in the bottom of the pot must be working to allow excess water to drain out, and any saucer under the pot should be emptied so the pot is not sitting in water. So not use water that has been through a water softening system, the added chemicals are damaging to the plants.
The soil mix should be a good quality soilless mix with a texture similar to what was in their original pots. They should be planted no deeper than they were in the original pot, if anything a quarter inch higher than originally set.
Next, they should be in morning sun or bright dappled light all day, not in all day sun and not in afternoon-only sun. They should also be protected from wind. If the pots are sitting on a paved surface, raise them up a few inches to allow for air circulation underneath them. This will help reduce heat stress on the roots caused by heat reflecting onto the pots.
If all of the above seem to be in order, there may be a disease problem. I would suggest you contact your local county extension for help in diagnosing the problem. If it is something that can be treated, they will have the most up to date information on what to use and how/when is best to apply it to maximize results. Since they are new plants, you might also want to check back with the retailer where you purchased them.
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