Hello,  I planted 6 arborvitaes this spring. I have one that recently has started to turn brown in one section.  I have been trying to make sure they get watered, but not too much.  I have tried to self diagnose by reading online, but have found myself becoming more confused.

I read about the different diseases and root rot as well as fungicides(copper vs multi-purpose).  I plan to take a look at roots tomorrow to try to determine if it is root rot, but as this is my first time planting arborvitaes, I am not sure how easily I will be able to tell.  I am including two pictures taken a day or two apart, to show the progression and a 3rd that is a closeup.

The weather has been very hot and humid for a couple of weeks, does that help promote diseases?

Thanks for any assistance,
Paul