Got to thinking I could have replanted the plant too deep. Was thinking that could cause the stem rot.
Done a search before I went out.
Went out and checked the neck of the plant to see if I replanted it too deep which I did. About one half inch or so.
Checked the other plants I did and they were replanted without potting mix up around the plant neck. Those plants are doing fine so far.
I took a trowel and removed the sick plant from the container and washed of the potting mix from the plants root.
The roots were healthy and no sign of black root.
How to Plant Pansies in Containers
"Place pansies at the same level they were in the flat or seedling pot, and then firm the soil around the plants."
Have some material in the garage for more containers to build 16"x16"x16"deep for the two squash and Siberian pepper.
Will start on that this afternoon.