Running a 110 v pond pump wiring underground. I will use plastic
conduit underground. Can I continue the plastic conduit above ground to
box?

A conditioned "yes". If it is marked sunlight resistant it is designed to be
run above ground. The good news is I have not seen any grey PVC (Rigid
Nonmetalic Conduit) that was not listed for above ground and underground at any
of the home stores. It should be fine.
Provide a post or other support when it leaves the ground and to hold the box.
Your burial depth is a minimum of 12" for a residential GFCI protected 120v 15
or 20a circuit. Otherwise RNMC should be 18".