Not all Malus fusca is fire blight resistant...though
there are certainly very resistant individuals,
susceptible trees are not terribly uncommon as well.
If I recall, Jim Cummins (or maybe some one else)
tried M. fusca in the Geneva rootstock breeding
program, and the results were not especially
inspiring. I wish I could remember, but I think he
said 25% of seedlings were FB susceptible.