Q.Why Did My New Cuke Die?
Everything was looking good then they started to (stem) get white up around the leafes. See pictures. Thanks

Certified GKH Gardening Expert
Damping off is usually caused by fungal or bacterial infections. Typically, this originates from wet soil conditions, so if it was accidentally overwatered at one time, then this could be the source. Soils that are too compact will have this issue, as well.
https://www.gardeningknowhow.com/garden-how-to/propagation/seeds/what-is-damping-off.htm