Poor drainage? Planted too deeply?

Well the thing was, when I dug or excavated the holes(with excavator) there were so many rocks and roots less than 50% of native soil was left to backfill so I had to use topsoil for that. Not sure if that affects drainage but would they be showing symptoms only a month into planting? And is this disease reversible?

One should not need an excavator to plant rhododendrons!! They are very shallow rooted and only need a very flat, shallow planting hole.

Both poor drainage and deep planting can lead to root rots. And root rots are rarely ever reversible.

Well it was in the woods full of tree roots and big rocks so otherwise I would’ve been digging forever with the 20 huge rhododendrons I had to plant. So it is definitely root rot and not just stem rot? Also would I risk spreading the disease to my azaleas at home? I don’t wear my work boots back home but the fungus could’ve splashed on my lower pants or skin and I could spread like that? I don’t know if that’s only what phytophthora does or even any form root rot caused by poor drainage

Heruga this discussion is practically meaningless unless you disclose what variety this is. Alas, is is still very common for east coast nurseries to sell varieties they have no business selling*. I saw scads of 'Blue Baron' for sale at the fancy 'Terrain' nursery in SE PA. I can guarantee established ones in gardens south of coastal New England are 'rare as Hen's Teeth'. https://www.gardenlady.com/i-hate-rhododendron-blue-baron/

* - or perhaps I should say, they have no ethics selling!

That being said, just eyeballing it, this site looks WET to me. And the plant maybe planted too deeply.

I don't think backfilling a planting hole with topsoil helped.

Looks like a combo of stress from transplanting and waterlogging.