Is this a healthy rootstock? It's a tree that come with the house

[Rain]

Hello every one. I'm very clueless about citrus trees. I don't know if I should get rid of it or keep it. It was already there when I got the house but it look like the rootstock took over a long time ago.

The base of the tree have flaky skin peeling of upon touching which I'm not sure if that's some kind of disease or it was from animals chewing on it. The leaves have inconsistent color. Some part have light small green leaves. Some part have big green leaves. And the trunk is loaded with this fussy growth. I appreciate any advice. Thanks.

[dytandme]

Looks to be a poncirus, which happens to be a common choice for cold hardy citrus rootstock. It's often known as a bitter orange.

That hole could be from varmints or from a lost limb. The black stuff looks like old tree sealant to me.

[Rain]

So it is a fine tree to be top work? Thank you 😊

[Millet]

The trifoliate leaves & branches are from the rootstock.    However, you can cut out the trifoliate branches and retain the monfoliate branches from the original tree.  Then top work

[citrange]

All the leaves in your photos appear to have three leaflets, so, as dytandme says, this is either Poncirus or some other trifoliate rootstock. If you follow the main trunk upwards, is there no part which is still producing single leaves? If so, the original top variety could still be rescued by hacking off all the branches that have trifoliate leaves back to the main trunk. This could re-invigorate the top variety if no trifoliate shoots are allowed to grow.
However, the black hole at ground level and split trunk above it, are signs of phytophthera root rot. That's not a good sign. It may not spread or it may eventually girdle the whole trunk and kill everything above. If you want to risk it, you should choose one or two of the trifoliate shoots for grafting and remove the rest.
If you don't want to take the risk, then dig the whole thing up and buy a new tree!

[Rain]

I check the main trunk and there no graft line or single/different type of leaves to be found. It's  like 10 ft tall pure rootstock. The previous owner doesn't even remember what type of citrus it is and they never seen flowers or fruit from it.

So I'm thinking if it is healthy then I want to try grafting orange and mandarin. I play around and try grafting as practice with Mayer lemon but none of the graft took. I assume they failed because it was too hot in the middle of the summer lol.

[dytandme]

citrange is definitely a little more experienced than myself so I would heed the warning about the potential for the tree passing due to root rot. If it were me, I'd pull back the pine back chips from the trunk to reduce the amount of rotting wood near it's wound and increase airflow. Again, I'd defer to citrange the others here who know a bit more if they feel differently.

But so long as it lives, it should make a great stock for multi-grafting. If you do cut the tree back to graft onto, you'll have a lot of good cuttings that could be potentially be rooted, producing a bunch more trifoliate rootstock.

[vnomonee]

Pull the mulch back and expose the root flare. The wound will stay damp under the mulch and soil, inviting more rot as well as insects to bore into it.

[Rain]

Hey every one. I decided to get rid of it after I learn more about phytophthera root rot. It's hard to let go of my multi graft tree potential lol.

Thanks for all the great advice!