Avocado Trunk Rot

Col starts with ...
I've just discovered that one of my two large (5m+/- tall) backyard avocados (about 25 y.o.) has some serious rot in the first fork of the trunk (see pic 4), about 1.8m above ground level. I've not noticed this before because I'm not that tall :-(, and use an extendable cutter to retrieve fruit or prune branches.
The only remaining branch has also suffered (see pic 3) though whether from the same cause I don't know.
Whilst is rot looks to have affected most of the trunk, there is still quite a reasonable canopy with new growth, and over the last couple of years there has been lots of fruit, although all were quite small - larger ones were the size of duck eggs, but many were smaller than a chicken egg (see pic 2) - even though I removed 1 in 3 when they were small grape size.

I don't know the variety of this tree (see pic's 1 & 2 of fruit and leaf)

Questions:
1. Would this wood rot explain the undersized fruit?
2. What should I do now:
2.1 Try to cut out and/or treat the rot with chemical?
2.2 Cut the tree down to a 1m high stump (like the Californians did with their orchards when they were experiencing severe drought)?

Pictures - Click to enlarge
Picture: 1	Picture: 2	Picture: 3
Picture: 4

jakfruit etiquette says...
I think you would need to treat it to lessen the chance of reinfection, before you cut it back to a stump.