What is your soil type...clay? That would do it considering how much rain you had. Roots would probably be toast or nearly so.

Agree with Smivies. Unless you have near-perfect drainage, these can be a struggle. My soil is thick, heavy clay in most of my yard, and I finally gave up on these.

No, I should have mentioned, it's sandy soil, with some compost mixed in. It's sandy enough that even after a heavy downpour, it soaks right in. The area is very flat though and for three months the rain was so frequent the soil never got a chance to dry out.

I've seen mention on the local news about rhododendrons carrying sudden oak death disease due to suppliers shipping infected plants to Indiana. It may not be what's wrong with your plants, but it might be worth looking into just in case.

survived a severe winter with little damage.

==>> with the long cool spring many of us had.. it could be minor winter damage ... just finally showing... when the heat of summer starts to build ...

i suspect a number of problems... heaping one upon another ...

did your winter include strong very cold wind ...

ken

Yes it did.

FWIW: https://cbs4indy.com/2019/05/22/dnr-oak-tree-killing-fungal-pathogen-found-in-rhododendrons/

It's kind of disappointing that the nation's wholesale nursery industry hasn't nipped this problem in the bud, for good. No pun intended!

I suspect that the roots are drowning. Being an experimenter, I would try two things:

1) One plant I would plant up in a mound, so that the roots get above the current perched water table. I would be tempted to put the top of the rootball about eight inches above the surface level of the soil and build your mound based on that height.

2) The second plant I would put into a giant cut wine barrel and clean the rootball and plant in a soil that is either 100% sized bark (filter out sawdust) or 50% sized bark and 50% turface (helps to rehydrate dry bark and helps to keep soil more evenly hydrated away from watering).

I think both plants will do pretty well but if the plant in the container takes off and the plant in the mound does not, then either you have a soil problem or some other drainage issue.

I don’t know where you are located but my experience with newly planted rhodies in the northeast may apply to you. I planted a dozen rhodies last year. Since then we have had record rain. So much that I really forgot about watering and rejoiced at my good luck. So much that I didn’t pay attention to the mini drought we had in August .. which turned out to be enough to do in three of them! It is easy to be complacent about the need new plants have for supplemental watering until their roots are really established. So I don’t know about your location, but were you checking their root balls throughout the whole time? The lesson for me is that many plants cannot survive even temporary drought if they aren’t firmly established, which can take several years.