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)
Occassionally, maybe one out of fifty or less, they will die for no
apparent reason after several years. I believe this is pear decline.
Burying the graft union should alleviate this 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 beenok in southern WI for ussuriensis but
the drainage sensitivity may be affected by stress brought on by other
factors.)
Kevin B
www.edibleforestnursery.com