My Ures have survived unscathed for about ten years now in
zone three. What kind of rootstock do you use? I would consider using
ussuriensis and burying the graft union. (If you use communis this should
help also)
Also, if your trees are already on ussuriensis and they are dying, it
could be due to a shallow water table, ussuriensis can't stand poorly
drained or wet soils. (although two feet has been ok in southern WI for
ussuriensis but the drainage sensitivity may be affected by stress
brought on by other factors.)
Kevin B