Left: Improper planting. Soil is above the graft union, the hole is too small, and excessive roots were not trimmed. Right: Proper planting with the graft union above the soil; plenty of space for roots in the hole, and excessively long roots have been removed.
Improper planting with soil above the graft union and a small hole with crowded roots vs. proper planting with graft union just above the soil and space around roots.

NC State, North Carolina Extension Gardener Handbook, Figure 15-18