Avo's are very suseptable to root rot. I'd guess that 90% or more of tree loss, especially new trees, is from that. Too much water. once you have lost one tree to it in a planting area, the next tree will have a very high chance of dying from it to (it's prevalent in the soil now), so be sure to try a spot several feet away if you try again. good luck

There is a virus called 'Laurel wilt' that is killing bay trees, avocados and others that are in the Lauraceae family. There is no known cure that I have heard of. This tree should not disposed of with your other garden trash.

Well the Voodoo Garden Guy who has grown 100s of plants of different varieties of plants he had a Die off of leaves in his Garden room , there's a YouTube Video of it , anyway he said as his last resort he d try Miracle Grow , and in a month or so they all got new healthy growth , He doesn't like to use it he's All Natural but he was out of Ideas , I've used it over the years a couple times its very potent so make sure you read the directions then maybe dilute by half just try a little bit. Also I see Your pit is Buried the avocado famers and growers always leave that half exposed , go to WisconsinGarden.com Video Blog 353 she explains everything about growing avocados well good luck . :)

I am admittedly not an avocado expert, but I do have two fully mature trees and live in a huge avocado farming area. Avocados need a LOT of water, but they don't like to sit in the water, which is why most of them here are planted on hillsides. We water twice a week (on irrigation), but our trees are on a somewhat steep incline/hill, so that the water that isn't absorbed can run away. kristincarol's advice about wanting a deep leaf litter (6" or so, I mean deep) around their roots is also spot on - if you move the leaf litter, you'll even notice little feeder roots growing in the bottom layer of the leaves. They do not like to be in bare soil that is exposed to the sun to turn into cement, as they take up most of the water/nutrients very close to the surface. You also want to be fertilizing every three months or so if they are putting out fruit (follow the directions carefully since those are such little guys you have there), in case the nursery didn't mention it. Anyway, I personally would suggest frequent shallow/short watering so that they're not sitting in water as it looks like you don't have them on mounds/hills, but enough so that the feeder roots can uptake the water. Don't water if the soil is soggy. I'd also try to get some more leaf litter in there - if you have other trees to steal dead leaves from it should work until your guys build up their own, although I would imagine you could substitute something like wood chips or deep straw, but I would avoid anything that would mat down like grass clippings. The mulch should help somewhat with keeping the surface soil moist and you should be able to avoid watering as often. I bet you can save them. I think I'd find another expert to help in your area outside of that nursery, what they're telling you is so all over the map that it's kind of scary.