I would be very grateful for any help that you kind folk could offer me.

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.

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Not sure what these spots are?

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Very few branches look as healthy as this top one.
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Harmless lichen?
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I cut this one by accident. It had 2 new shoots growing off of it but doesn't look good inside
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Branches removed today

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

Thank you in advance for any help offered.