I'm resident in Laos and 18 months ago was lucky enough to move into a house that has a fantastic old avocado tree. When I moved in it was July and the tree was absolutely teeming with fruit. It must've produced 60 + fruits in 2013.
This year there was no fruit. I wasn't too worried, as after a quick search online I saw that these trees can have a year off now and again. But recently I noticed more and more dead branches present, and today I set about clearing some of them. The amount of them, and condition, tells me something is up with this poor tree.

I haven't given it any special treatment as I didn't think a tree as mature and established as this required it. No additional water as it rains a lot during rainy season here and no fertilizer (maybe a mistake).

It's full of leaves and has numerous new green shoots all over, but the amount of dead branches is staggering. There are ants in the dead branches but I think that these are taking advantage of the problem, rather than causing it?

I can't find these symptoms listed online anywhere so am hoping that someone could assist me.

Not sure what these spots are?

Very few branches look as healthy as this top one.

Harmless lichen?

I cut this one by accident. It had 2 new shoots growing off of it but doesn't look good inside

Branches removed today

Please let me know if you require any further information or pictures.

Thank you in advance for any help offered.