Re: your most difficult fruit?
Last that I know of only Dave Wilson nursery in California sells cherrys like that. But you can only get the "z-stem" plum as the interstock and "citation" as the rootstock which is another Zaiger developement in itself which has 50% peach x 50% myrobalan plum breeding in it. I think the citation would be a great one to stand in place of z-stem because it seems resistant like z-stem based on eveidence about my first (frost) peach I bought grafted on citation by Biringer wholesale nursery since it was still under a patent at the time. I was not very good in some years covering my peach for the winter months so the disease had taken only everything above the graft.
That happened a long time ago and I then ordered more citation stocks from Raintree. They grew for 10 years with red peach looking shaped leaves. They were healthy all that time up to the 10th year I decided to graft beach plum at six feet. Beach plum bloomed for a few years and would have had plums but they were not self fruitful like my frost peach was. It then died but like my old peach the citation was always good right up to the graft.
I tried to establish more because the 17 year plant patent expired and because I was having trouble getting more of citation from Raintree at the time. So I hacked the citation but no root shoots started when I decided to pull up root sections to graft and never got around to that.