Combo Mirabelle Plum (4 Varieties)

Does this tree have any resistance to black knot fungus?

No, this tree does not have black knot resistance.

Will this tree be affected by the rust on surrounding Bradford pear trees?? In what size pot does the tree arrive?

Plums are not susceptible to pear rust so this tree should be unaffected. This product ships bareroot.

I notice on the combo mirabelle plum tree it says self fertile, but when i check out the varieties that make up the combo most of them say they require a pollinator. is the combo tree self fertile because of the 1 or 2 varieties that can be self fertile? Would it still be best to also have another tree specifically for pollination?

This a multi-grafted tree, with several varieties of Mirabelle plums on a single tree. They will cross pollinate each other, so the tree is functionally self fertile. If the tree were to lose too many varietal limbs it would no longer be self fertile.

Will these do well in California Bay Area? Planting zone 9B? I’m thinking about surprising my sister with one of these as a gift

They should do very well in the Bay Area of California. Make sure you get another European plum, such as Victoria to ensure good pollination.

I live in Minnesota in zone 4(b). Would I be able to grow these trees, and if so, what special care would give them the best chance to survive our winters.

These plums are hardy to zone 5. With a lot of care, including heavy annual mulching and even covering, you could grow them, but with a very slim margin for error.

Are these resistant to root rot (Armillaria) fungus?

Not specifically no.