Any corresponding fertilizer or other chemical applications to the potting soil? The second picture shows interveinal chlorosis so that plant at least was having a problem with soil minerals before the sudden calamity. Otherwise: any recent very high temperature episodes?

plants to start dying literally in a 48 hour period.

==>>> odd that it would happen this fast.. and literally turn brown and shrivel up that fast ...

tell us more about the 6 weeks of drought.. how high was heat.. how did you water.. any fert'g? .... that one plant is in a rather shallow not very wide planter ....

ken

Ken, I think you’re referring to the Joe Pye in the green planter? They were small plants from Bluestone received this year in early May. I was tiding them over until we finished regrading our yard and installing drain tile and a dry bed to put them in the ground. They looked lovely last Monday morning lol.
Re the weather, it’s been hot and dry, for MI anyway. Very dry. I was watering deeply ~3x per week.
For fertilizer, I haven’t added anything since maybe the Fourth of July weekend. I’ve read you shouldn’t fertilize after July as it encourages new growth that may not have time to “harden off” before the frost comes.
Prior to that, the blueberries, azalea, viburnum and hydrangeas got some HollyTone in April. Blueberry and azalea also had some Ericoid mycorrhizae watered in around April and May.
Viburnum had a foliar iron spray in June as it was looking chlorotic.
Otherwise, they’ve all had a few waterings with diluted orchid fertilizer.
I sprayed most all of my plants with pre-diluted copper fungicide in May or June.
I can’t think of anything any of them, healthy or the currently unhealthy/dying/dead, have had since late June/early July.
Prior to these two rain showers, some looked a bit droopy but seemed to perk up after a good watering.
The only plants I had concerns about prior to this last week were the blueberry bushes. They all had some dark spots developing and 1-2 canes of the dry silvery-bronze foliage that’s overtaken all the leaves now. Those canes were pruned out roughly 3 weeks ago.

Also, I have Tardiva, Limelight and White Diamonds paniculata hydrangeas in the ground, all in the same bed, same exposure, same soil conditions, etc., and the Tardiva looks great, white the others look wilted, have brown foliage, and blooms are sparse with some of the blooms also having browning.

Timing continues to suggest it was due to the heat, for one reason or another. With for instance the potting soil perhaps being infested with something (or some things) that started showing their full effects then because of the high temperatures - start by reading up on phytophthora root rot.

Phytophthora is my fear and I’m really hoping this is not the cause. I’m just bewildered as the potting/planting mediums are not consistent across affected plants and I’m careful to sterilize equipment. I’m wondering if there could be a local pest of some sort that could’ve been triggered by the high heat and sudden influx of water.

Due to the nature of how this water mold travels and spreads, it would be unusual for phytophthora to be an issue with containers. Unless they rested on soil or other solid surface where rainwater would facilitate that. It can certainly be an issue in nurseries where containerized plants are laid out on landscape fabric or plastic sheeting or soil where a slight depression can allow both surface water movement and puddling but far less of an issue if on concrete, paving, gravel or decking.

I'd suspect it is some sort of reaction to weather conditions, although it seems a bit too uniform and extreme for that as well.

Any chance there was some sort of chemical sprayed or applied in the area recently?

The only I can think of is that my neighbor had a landscaping crew over last weekend. I haven’t noticed anything on our shared fence line dead/dying, though.
I also have a 5 year old, so I suppose anything is possible in that regard, but I keep our chemicals out of reach and it’s rare that he tries to go outside by himself.
I don’t mean to keep poo-pooing ideas, I really appreciate the brainstorming, it just seems like nothing I am aware of fits

I have an endless summer hydrangea that I potted earlier this summer. After days of rain, it died even though the planter had good drainage. All of the stems were dead and snapped easily. I was too lazy to dump it and last week I saw new growth at the crown. So maybe the plants are stressed and not entirely gone.

Surface water movement and puddling is exactly the thing that happens on impervious surfaces like paving or decking. And one of the means by which phytophthora gets onto plant tops is through splashing - it doesn't have to be through container bottoms. Phytophthora ramorum has even said to fall onto plants from out of the sky, via rain drops.

"Surface water movement and puddling is exactly the thing that happens on impervious surfaces like paving or decking."

Not at all!! Unless you have a very uneven surface to start with. Or receive rainfall in torrential amounts. And most paving is graded for a slight slope and decking is far from impervious.

As one who grows extensively in containers for more than 30 years, I have never had any issues with any soil or water borne pathogens with any of my container plants. Issues can arise from over or underwatering but they rarely manifest in just 48 hours.