I've read when planting apple rootstock with an interstem (such as M26 on M111) it is preferable to bury the tree's trunk with only a couple inches of the interstem remaining above ground. What are the benefits of burying a portion of the interstem verses leaving the entire interstem above ground?

Greg
Sorry this has taken so long, but just found your message.
M.26 rootstock has a tendency to form many burrknots. The are ugly looking almost tumor-like growths on the side of the stem. They are root primordia (roots looking to get a start) and wiil allow bacterai and virus to enter the plant.
Burying the M.26, ledaving jus an inch or two will lessen the probability of burrknots.
If you have not already buries your M.26 scion, suggest mounding soil arount the M.26 section of you tree.

Ted