I've got a pear question for those familiar with pear rootstocks. I'm interested in bud grafting a particular cultivar of Pyrus ussurensis and don't know if I'm limited to using Pyrus ussurensis as the rootstock. If I'm not limited to this, what are my rootstock options? I've currently got 90% of my pears on either OHxF 97, OHxF 87 or Pyrodwarf and would like to stick to these if they're compatible with the P. ussurensis.