In ground Seiju Elm- mysterious death...

[GreatBasinBonsai]

I have been growing a dozen or so Seiju Elms in the ground the last five or six years, with great results, but in the last 24 hours one of my Seijus just up and died, looking almost as if it had dried out, although it's been consistently watered by a sprinkler system, I'm just a little baffled?? Does anyone have experience with Seiju Elms in the ground, and perhaps fungal or pest or other issues, something that could rapidly kill a tree, going from lush green foliage to brown crispy dead foliage within a matter of a couple days? Here are a couple pictures, you can see two healthy ones and one crispy one all growing close together, I'm a little worried that whatever it was might spread to the healthy trees. Thanks so much for whatever light anyone can shed! (I looked high and low for any sign of pests but I can't see anything)

 

[BonsaiManic]

Do you have nearby neighbors, who maybe have a grudge?
Seriously though, my city planted a number of lacebark elm around town a few years ago. Last year, I noticed a few of them die suddenly, as you are describing. None of them were in the same neighborhoods, just different parts of town. There were several in a park or along the road, and only a single tree was affected. I don't know what the cause was, but these were significant sized trees (15 - 25 feet tall). It was less than a week from when I noticed the decline until they were crispy, as if they were poisoned.
This isn't much help for you, but it makes me curious all over again.

 

[River's Edge]

I have been growing a dozen or so Seiju Elms in the ground the last five or six years, with great results, but in the last 24 hours one of my Seijus just up and died, looking almost as if it had dried out, although it's been consistently watered by a sprinkler system, I'm just a little baffled?? Does anyone have experience with Seiju Elms in the ground, and perhaps fungal or pest or other issues, something that could rapidly kill a tree, going from lush green foliage to brown crispy dead foliage within a matter of a couple days? Here are a couple pictures, you can see two healthy ones and one crispy one all growing close together, I'm a little worried that whatever it was might spread to the healthy trees. Thanks so much for whatever light anyone can shed! (I looked high and low for any sign of pests but I can't see anything)View attachment 568393View attachment 568394

Seiju elm typically develop issues from roots that stay to wet with poor drainage. They require frequent root work to prevent drainage issues from developing and often show quick response to poor conditions within a short time frame. based on my experience I would consider poor candidates for ground growing unless planted in soil that drains very well.
Some commonly used insecticides such as malathion can be very toxic to certain species and cause rapid decline of the foliage.
Two aspects you might consider. Bringing up the tree and examining the roots and soil conditions would help to clarify some aspects/

 

[GreatBasinBonsai]

Thank you Frank. I was mulling over the possibility that perhaps the ground could have become too wet, and possibly compacted, so yes for now that's going to be my guess. I'm hesitant to dig anything up right now but I think in the spring I will reassess the soil where they are all planted…