contact your local COUNTY extension office.. and see what advice they may provide.. report back ...

im not sure that a topical application of a fungicide will help what is probably a systemic problem .. but i know nothing about citrus.. and why i recommend the local peeps...

frankly.. your tree is very far gone.. and you will probably be better off removing it.. and starting over .. yes.. i understand the history .. and that you really dont want to to do that .. but the alternative is to spend the next few years.. watching it die.. and that can be harder than making the call ...

its all up to you.. ken

ps: i never heard of exposing the cambium layer of the tree and slathering product on it ... it just seems so wrong... to clog up the vascular system of the tree .. perhaps that is why the branches above have died???

Since the tree isn't grafted, presents gummosis, and has portions of dead bark, there's a good chance the underlying pathogen is phytophthora. It's probably late now, but if you're dead-set on saving the tree, try inarching. It saved the parent Washington navel orange in the ~1940s. Use trifoliate rootstocks.

Though since you live in Florida, I'm not sure it's worth the effort due to rampant HLB.

I had a Meiwa kumquat on Kuharske citrange fro harris citrus tha I aproache grafted to Flying dragon because Meiwa is incompatible to Kuharske

@ken_adrian Adrian MI cold Z5, thank you so much for your comment and especially for suggesting that I get in touch with my local county extension! I did just that and heard back from a Master Gardener in my county. He was extremely informative and confirmed that my tree is, in fact, pretty much dead from citrus greening, AKA HLB (which is a terminal disease). I was relieved to finally have confirmation as to whether I should keep my tree or remove it before it infects my other healthy citrus plants, but am of course sad that it has to go. You were right, too, that the tree IS too far gone.

@tom1328732, this is good info even if you do say it might not work against HLB, which my tree is in fact suffering from (or...was, as it seems quite dead now). Thank you!

So it seems that there is absolutely no saving my tree. I'm planning on cutting it down and removing it this week.

Yes, it hurts, because it's been there for forever, it feels like--but I guess I should have known that Murcott tangerine grown from seed can be trouble, and this is why they sell grafted trees--less disease susceptibility!

Of course, if anyone else has any comments or ideas, I still welcome them! For example--can I plant a new citrus tree in the same spot this one is? Will it get sick, too? And how can I prevent my other citrus babies from getting HLB? ;_;

Australian finger limes are tolerant to HLB, so if you want to still grow citrus, give that a shot. I'm not sure how grafting plays into that susceptibility though. They are thorny little monsters, however!

FYI, they were able to isolate a peptide from it that seems to control HLB:

https://www.pnas.org/doi/10.1073/pnas.2019628118